| 0/0 |
2026/07/25 00:00 |
flow |
assessment-security |
1h00m
Results: map[DenialOfService:true Explanation:**Bug Analysis:**
The bug is a NULL pointer dereference in `dummy_set_selfpowered` within the
`dummy_hcd` (USB gadget dummy HCD) driver.
The `dummy_hcd` module registers two separate platform devices during
initialization: one for the Host Controller (HCD) and one for the USB Device
Controller (UDC).
When the HCD device is manually unbound via sysfs (by writing to
`/sys/bus/platform/drivers/dummy_hcd/unbind`), the `dummy_hcd_remove` function
is invoked. This function cleans up the HCD and explicitly sets `dum->hs_hcd =
NULL`. However, the UDC device remains bound and registered with the system.
If a user subsequently configures and binds a USB gadget driver to this orphaned
UDC device (e.g., using `configfs`), the gadget framework calls
`usb_gadget_set_selfpowered()`, which in turn calls the UDC's
`dummy_set_selfpowered()` callback. This function attempts to retrieve the
`dummy` struct by dereferencing `dum->hs_hcd`, which was previously set to NULL:
```c
dum = gadget_to_dummy_hcd(_gadget)->dum; // gadget_to_dummy_hcd returns NULL
if (value)
dum->devstatus |= (1 << USB_DEVICE_SELF_POWERED);
```
Because the `dum` field is at offset 0 of `struct dummy_hcd`, this results in a
direct read from memory address `0x0`.
**Security Impact:**
* **Exploitable:** No. This is a pure NULL pointer dereference (reading from
address 0). On modern Linux systems with `mmap_min_addr` enforced, address 0
cannot be mapped by userspace. The kernel will simply trap the invalid access
(or KASAN will catch the shadow memory access for address 0, as seen in the
report) and oops. It cannot be used for privilege escalation or arbitrary code
execution.
* **Denial of Service:** Yes. The NULL pointer dereference will cause a kernel
oops, which will kill the current process and can lead to a full system crash
(kernel panic) if the system is configured with `panic_on_oops=1`.
* **Accessible From Unprivileged Processes / User Namespaces:** No. Triggering
this bug requires two highly privileged operations:
1. **Unbinding the driver:** Writing to
`/sys/bus/platform/drivers/dummy_hcd/unbind` requires root privileges. The
file has `0200` permissions and is owned by the global `root` (UID 0). A
process inside a user namespace cannot write to it because the global root UID
is not mapped into the unprivileged user namespace, causing the VFS capability
and DAC checks to deny access.
2. **Mounting configfs:** The reproducer uses `configfs` to bind the gadget.
The `configfs` filesystem does not have the `FS_USERNS_MOUNT` flag, meaning it
cannot be mounted from within a user namespace. It requires `CAP_SYS_ADMIN` in
the initial user namespace.
* **Other Triggers:** The bug is specific to the software-emulated `dummy_hcd`
driver and its sysfs/configfs management interfaces. It cannot be triggered
remotely, via malicious filesystems, or by plugging in physical USB peripherals. Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/07/25 00:00 |
action |
syz-repro-to-c-repro |
0m
Results: map[SimplifiedCRepro:// autogenerated by syzkaller (https://github.com/google/syzkaller)
#define _GNU_SOURCE
#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
uint64_t r[2] = {0xffffffffffffffff, 0xffffffffffffffff};
int main(void)
{
syscall(__NR_mmap, /*addr=*/0x1ffffffff000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200000000000ul, /*len=*/0x1000000, /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/7ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200001000000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
const char* reason;
(void)reason;
intptr_t res = 0;
if (write(1, "executing program\n", sizeof("executing program\n") - 1)) {}
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 6d 79 73 79 73 00} (length 0x8)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x200000000000, "./mysys\000", 8);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x200000000000ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// mount arguments: [
// src: nil
// dst: ptr[in, buffer] {
// buffer: {2e 2f 6d 79 73 79 73 00} (length 0x8)
// }
// type: ptr[in, buffer] {
// buffer: {73 79 73 66 73 00} (length 0x6)
// }
// flags: mount_flags = 0x0 (8 bytes)
// data: nil
// ]
memcpy((void*)0x200000000080, "./mysys\000", 8);
memcpy((void*)0x2000000000c0, "sysfs\000", 6);
syscall(__NR_mount, /*src=*/0ul, /*dst=*/0x200000000080ul, /*type=*/0x2000000000c0ul, /*flags=*/0ul, /*data=*/0ul);
// openat arguments: [
// fd: fd_dir (resource)
// file: ptr[in, buffer] {
// buffer: {2e 2f 6d 79 73 79 73 2f 62 75 73 2f 70 6c 61 74 66 6f 72 6d 2f 64 72 69 76 65 72 73 2f 64 75 6d 6d 79 5f 68 63 64 2f 75 6e 62 69 6e 64 00} (length 0x2e)
// }
// flags: open_flags = 0x1 (4 bytes)
// mode: open_mode = 0x0 (2 bytes)
// ]
// returns fd
memcpy((void*)0x200000000100, "./mysys/bus/platform/drivers/dummy_hcd/unbind\000", 46);
res = syscall(__NR_openat, /*fd=*/0xffffff9c, /*file=*/0x200000000100ul, /*flags=O_WRONLY*/1, /*mode=*/0);
if (res != -1)
r[0] = res;
// write arguments: [
// fd: fd (resource)
// buf: ptr[in, buffer] {
// buffer: {64 75 6d 6d 79 5f 68 63 64 2e 30} (length 0xb)
// }
// count: len = 0xb (8 bytes)
// ]
memcpy((void*)0x200000000200, "dummy_hcd.0", 11);
syscall(__NR_write, /*fd=*/r[0], /*buf=*/0x200000000200ul, /*count=*/0xbul);
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 00} (length 0x9)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x200000000000, "./config\000", 9);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x200000000000ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// mount arguments: [
// src: nil
// dst: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 00} (length 0x9)
// }
// type: ptr[in, buffer] {
// buffer: {63 6f 6e 66 69 67 66 73 00} (length 0x9)
// }
// flags: mount_flags = 0x0 (8 bytes)
// data: nil
// ]
memcpy((void*)0x200000000080, "./config\000", 9);
memcpy((void*)0x2000000000c0, "configfs\000", 9);
syscall(__NR_mount, /*src=*/0ul, /*dst=*/0x200000000080ul, /*type=*/0x2000000000c0ul, /*flags=*/0ul, /*data=*/0ul);
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 2f 75 73 62 5f 67 61 64 67 65 74 2f 67 31 00} (length 0x17)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x200000000100, "./config/usb_gadget/g1\000", 23);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x200000000100ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// openat arguments: [
// fd: fd_dir (resource)
// file: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 2f 75 73 62 5f 67 61 64 67 65 74 2f 67 31 2f 55 44 43 00} (length 0x1b)
// }
// flags: open_flags = 0x1 (4 bytes)
// mode: open_mode = 0x0 (2 bytes)
// ]
// returns fd
memcpy((void*)0x200000000340, "./config/usb_gadget/g1/UDC\000", 27);
res = syscall(__NR_openat, /*fd=*/0xffffff9c, /*file=*/0x200000000340ul, /*flags=O_WRONLY*/1, /*mode=*/0);
if (res != -1)
r[1] = res;
// write arguments: [
// fd: fd (resource)
// buf: ptr[in, buffer] {
// buffer: {64 75 6d 6d 79 5f 75 64 63 2e 30} (length 0xb)
// }
// count: len = 0xb (8 bytes)
// ]
memcpy((void*)0x200000000380, "dummy_udc.0", 11);
syscall(__NR_write, /*fd=*/r[1], /*buf=*/0x200000000380ul, /*count=*/0xbul);
return 0;
}
]
|
| 2/1 |
2026/07/25 00:00 |
action |
kernel-checkouter |
1m
Results: map[KernelSrc:/app/workdir/cache/src/1ebb1f3968b2b13b1c5bb2ae359244a864c0dd44]
|
| 3/1 |
2026/07/25 00:01 |
action |
kernel-builder |
29m
Results: map[KernelObj:/app/workdir/cache/build/61aa2b0e15d73d12538ba22a871c7b6cfc74243e]
|
| 4/1 |
2026/07/25 00:31 |
action |
codesearch-prepare |
20m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/07/25 00:52 |
agent |
expert |
8m
Model:
Results: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Instruction: You are an experienced Linux kernel security engineer. Your task is to analyze given kernel bug report
and determine its security impact based on the following dimensions.
Use the provided tools to examine the source code, check for capability checks (e.g., capable(), ns_capable()),
and understand the nature of the bug. Analyze the given kernel build and configuration.
You can check the kernel config by grepping ".config" file; you can check kernel cmdline by grepping
".config" file for "CONFIG_CMDLINE=". Assume sysctl parameters have default values.
But analyze for the corresponding production build w/o debugging tools enabled (like KASAN, KMSAN, UBSAN).
Try different strategies when analyzing the bug:
- think of ways in which the vulnerable code is unreachable
- or the other way around: try to come up with different ideas of how an unprivileged user can reach the bug
If still unsure err on the side of the bug being non-exploitable/not-accessible.
In the final reply, provide a reasoning for your assessment.
Analysis dimensions:
* Exploitable:
Determine if the bug can result in memory corruption, elevated privileges, or an information leak.
Memory safety issues are almost always exploitable (KASAN or UBSAN reports for use-after-free, out-of-bounds;
refcounting issues, corrupted lists, etc). When kernel is crashing on a completely wild pointer access
(e.g. user-space address, or non-canonical address, but not on NULL or address corresponding to KASAN shadow
for NULL address), including both data accesses and control transfers, that also usually implies possibility
of exploitation. Such reports usually say "unable to handle kernel paging request".
Uses of uninitialized values detected by KMSAN may be exploitable b/c attacker frequently can affect uninit
values with spraying techniques. However, for these exploitability depends on how exactly the uninit value
is used in the code, and what it affects.
Information leaks are exploitable on their own and should be classified as such. A bug that copies kernel
memory contents to userspace (e.g. an out-of-bounds read whose result is returned to the caller, or
uninitialized stack/heap bytes written to a user buffer) is exploitable: it can reveal kernel pointer
values and defeat KASLR, expose sensitive data such as cryptographic keys or other processes' memory, and
serves as a necessary building block in most modern kernel privilege-escalation exploit chains. Do not classify
an information leak as non-exploitable solely because it does not directly cause a memory write or control-flow
hijack; the leak itself is the exploit primitive.
Think of what happens after the bug is triggered. Some bugs cause kernel panic and halt execution,
they are harder to exploit. For example, BUG reports halts the kernel. However, WARNING reports don't halt
execution in production builds. Debug bug detection tools (like KASAN, KMSAN, KCSAN, UBSAN) are also not enabled
in production builds, so attacker can freely exploit these bugs w/o being detected by these tools.
If you see an integer overflow, think how the overflowed value used later (if it's used as allocation size,
or an array index). If you see an out-of-bounds read, think if it's followed by an out-of-bounds write as well.
Some KCSAN data-races may be exploitable by skilled attackers as well. Think what data structures got corrupted
as the result of data races and how. However, note that kernel has lots of "benign" data races that don't lead
to any runtime misbehavior at all.
* Denial Of Service:
Determine if the bug can result in denial-of-service. Most bugs can, since they cause system crash,
hangs, deadlocks, or resource leaks. This is mostly applicable to WARNING bugs that won't cause system crash
in production. For these think what will be consequences of the violation of the kernel assumptions flagged
by the WARNING. In some cases the unexpected condition is also properly handled by the normal control flow
(e.g. with "if (WARN_ON(...))"), these won't cause denial-of-service. If the condition is not handled,
then it may or may not cause denial-of-service.
* Accessible From Unprivileged Processes:
Determine if the bug can be reached from a typical (non-root) user process that does NOT have any special capabilities
(like CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON) or access to device nodes restricted to root.
Assume that unprivileged_bpf_disabled=1, that is eBPF loading is not accessible. However, cBPF (classical BPF)
is still accessible to non-root processes.
Assume that user namespaces are not accessible, that is, the process cannot get the mentioned capabilities even
within a new user namespace (checked by ns_capable() function in the kernel sources).
* Accessible From User Namespaces:
Determine if the bug can be reached within a user-namespace where the process has all capabilities
(including CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON). Such capabilities are checked with ns_capable()
function in the kernel sources.
* VM Guest Trigger:
Determine if the bug can be triggered from the context of a typical KVM guest (e.g., set up by a QEMU VMM).
Consider accesses to standard Linux host paravirtualized features (virtio-blk, virtio-net, etc.),
and handling of VM exits in the KVM code.
* VM Host Trigger in The Confidential Computing Context:
Determine if the bug can be triggered in a confidential computing guest kernel from the context of a KVM host.
Consider access to standard Linux guest paravirtualized features (virtio-blk, virtio-net, etc.).
* Ethernet Network Trigger:
Determine if the bug can be triggered by processing ingress network Ethernet traffic, either directly (network stack)
or via drivers exposed to network data.
* Other Remote Trigger:
Determine if the bug can be triggered by processing remote traffic other than Ethernet (Wifi, Bluetooth, NFC, etc).
* Peripheral Trigger:
Determine if the bug can be triggered via an untrusted peripheral device that can be physically plugged
into a system, such as a USB device or a niche hardware driver handling external hardware inputs.
This is particularly important for mobile and desktop environments where users can plug in unknown devices.
* Malicious Filesystem Trigger:
Determine if the bug can be triggered by the kernel mounting and parsing a malicious filesystem image.
This is highly critical for Desktop and Mobile environments where external media or downloaded images
might be auto-mounted.
Don't make assumptions about the kernel source code (it may be different from what you assume it is).
Extensively use the provided code access tools (codesearch-*, git-*, grepper, etc)
to examine the actual source code, and confirm any assumptions.
Prefer calling several tools at the same time to save round-trips.
Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.
Prompt:
The kernel bug report is:
dummy_hcd dummy_hcd.0: remove, state 4
usb usb1: USB disconnect, device number 1
dummy_hcd dummy_hcd.0: stopped
dummy_hcd dummy_hcd.0: USB bus 1 deregistered
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 0 UID: 0 PID: 5904 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026
RIP: 0010:dummy_set_selfpowered+0xbe/0x160 drivers/usb/gadget/udc/dummy_hcd.c:888
Code: 8d 34 c6 49 81 c6 c8 07 00 00 4c 89 f0 48 c1 e8 03 42 80 3c 20 00 74 08 4c 89 f7 e8 3c 47 01 fb 4d 8b 3e 4c 89 f8 48 c1 e8 03 <42> 80 3c 20 00 74 08 4c 89 ff e8 23 47 01 fb 41 be 00 14 00 00 4d
RSP: 0018:ffffc900039cf780 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000001 RCX: ffff88801e769f00
RDX: 0000000000000000 RSI: 0000000000000003 RDI: 0000000000000005
RBP: 0000000000000003 R08: 0000000000000000 R09: 0000000000000000
R10: dffffc0000000000 R11: ffffffff872df280 R12: dffffc0000000000
R13: ffff888039ac6528 R14: ffff888029179408 R15: 0000000000000000
FS: 00005555747fc500(0000) GS:ffff888125c42000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b33d63fff CR3: 0000000045a28000 CR4: 00000000003526f0
Call Trace:
<TASK>
usb_gadget_set_selfpowered+0x75/0x260 drivers/usb/gadget/udc/core.c:578
composite_dev_prepare+0x2fa/0x320 drivers/usb/gadget/composite.c:2440
configfs_composite_bind+0xa5/0x1410 drivers/usb/gadget/configfs.c:1701
gadget_bind_driver+0x2ca/0x9e0 drivers/usb/gadget/udc/core.c:1662
call_driver_probe drivers/base/dd.c:-1 [inline]
really_probe+0x254/0xae0 drivers/base/dd.c:706
__driver_probe_device+0x1e8/0x360 drivers/base/dd.c:868
driver_probe_device+0x4f/0x240 drivers/base/dd.c:898
__driver_attach+0x339/0x600 drivers/base/dd.c:1292
bus_for_each_dev+0x23e/0x2c0 drivers/base/bus.c:383
bus_add_driver+0x348/0x670 drivers/base/bus.c:763
driver_register+0x23a/0x320 drivers/base/driver.c:174
usb_gadget_register_driver_owner+0xf9/0x270 drivers/usb/gadget/udc/core.c:1752
gadget_dev_desc_UDC_store+0x1c9/0x2f0 drivers/usb/gadget/configfs.c:300
flush_write_buffer fs/configfs/file.c:207 [inline]
configfs_write_iter+0x33a/0x430 fs/configfs/file.c:229
new_sync_write fs/read_write.c:595 [inline]
vfs_write+0x61e/0xbb0 fs/read_write.c:687
ksys_write+0x156/0x270 fs/read_write.c:739
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fbdf804de99
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd5382e188 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fbdf82d5fa0 RCX: 00007fbdf804de99
RDX: 000000000000000b RSI: 0000200000000380 RDI: 0000000000000004
RBP: 00007fbdf80e3eaf R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fbdf82d5fac R14: 00007fbdf82d5fa0 R15: 00007fbdf82d5fa0
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:dummy_set_selfpowered+0xbe/0x160 drivers/usb/gadget/udc/dummy_hcd.c:888
Code: 8d 34 c6 49 81 c6 c8 07 00 00 4c 89 f0 48 c1 e8 03 42 80 3c 20 00 74 08 4c 89 f7 e8 3c 47 01 fb 4d 8b 3e 4c 89 f8 48 c1 e8 03 <42> 80 3c 20 00 74 08 4c 89 ff e8 23 47 01 fb 41 be 00 14 00 00 4d
RSP: 0018:ffffc900039cf780 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 0000000000000001 RCX: ffff88801e769f00
RDX: 0000000000000000 RSI: 0000000000000003 RDI: 0000000000000005
RBP: 0000000000000003 R08: 0000000000000000 R09: 0000000000000000
R10: dffffc0000000000 R11: ffffffff872df280 R12: dffffc0000000000
R13: ffff888039ac6528 R14: ffff888029179408 R15: 0000000000000000
FS: 00005555747fc500(0000) GS:ffff888125c42000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b33d63fff CR3: 0000000045a28000 CR4: 00000000003526f0
----------------
Code disassembly (best guess):
0: 8d 34 c6 lea (%rsi,%rax,8),%esi
3: 49 81 c6 c8 07 00 00 add $0x7c8,%r14
a: 4c 89 f0 mov %r14,%rax
d: 48 c1 e8 03 shr $0x3,%rax
11: 42 80 3c 20 00 cmpb $0x0,(%rax,%r12,1)
16: 74 08 je 0x20
18: 4c 89 f7 mov %r14,%rdi
1b: e8 3c 47 01 fb call 0xfb01475c
20: 4d 8b 3e mov (%r14),%r15
23: 4c 89 f8 mov %r15,%rax
26: 48 c1 e8 03 shr $0x3,%rax
* 2a: 42 80 3c 20 00 cmpb $0x0,(%rax,%r12,1) <-- trapping instruction
2f: 74 08 je 0x39
31: 4c 89 ff mov %r15,%rdi
34: e8 23 47 01 fb call 0xfb01475c
39: 41 be 00 14 00 00 mov $0x1400,%r14d
3f: 4d rex.WRB
It is reproducible with the followint program.
Keep in mind that it may lack the precise threading, sandboxing, and some arguments of a working reproducer.
But it should give an idea of the involved syscalls.
// autogenerated by syzkaller (https://github.com/google/syzkaller)
#define _GNU_SOURCE
#include <endian.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
uint64_t r[2] = {0xffffffffffffffff, 0xffffffffffffffff};
int main(void)
{
syscall(__NR_mmap, /*addr=*/0x1ffffffff000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200000000000ul, /*len=*/0x1000000, /*prot=PROT_WRITE|PROT_READ|PROT_EXEC*/7ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
syscall(__NR_mmap, /*addr=*/0x200001000000ul, /*len=*/0x1000, /*prot=*/0ul, /*flags=MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE*/0x32ul, /*fd=*/(intptr_t)-1, /*offset=*/0ul);
const char* reason;
(void)reason;
intptr_t res = 0;
if (write(1, "executing program\n", sizeof("executing program\n") - 1)) {}
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 6d 79 73 79 73 00} (length 0x8)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x200000000000, "./mysys\000", 8);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x200000000000ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// mount arguments: [
// src: nil
// dst: ptr[in, buffer] {
// buffer: {2e 2f 6d 79 73 79 73 00} (length 0x8)
// }
// type: ptr[in, buffer] {
// buffer: {73 79 73 66 73 00} (length 0x6)
// }
// flags: mount_flags = 0x0 (8 bytes)
// data: nil
// ]
memcpy((void*)0x200000000080, "./mysys\000", 8);
memcpy((void*)0x2000000000c0, "sysfs\000", 6);
syscall(__NR_mount, /*src=*/0ul, /*dst=*/0x200000000080ul, /*type=*/0x2000000000c0ul, /*flags=*/0ul, /*data=*/0ul);
// openat arguments: [
// fd: fd_dir (resource)
// file: ptr[in, buffer] {
// buffer: {2e 2f 6d 79 73 79 73 2f 62 75 73 2f 70 6c 61 74 66 6f 72 6d 2f 64 72 69 76 65 72 73 2f 64 75 6d 6d 79 5f 68 63 64 2f 75 6e 62 69 6e 64 00} (length 0x2e)
// }
// flags: open_flags = 0x1 (4 bytes)
// mode: open_mode = 0x0 (2 bytes)
// ]
// returns fd
memcpy((void*)0x200000000100, "./mysys/bus/platform/drivers/dummy_hcd/unbind\000", 46);
res = syscall(__NR_openat, /*fd=*/0xffffff9c, /*file=*/0x200000000100ul, /*flags=O_WRONLY*/1, /*mode=*/0);
if (res != -1)
r[0] = res;
// write arguments: [
// fd: fd (resource)
// buf: ptr[in, buffer] {
// buffer: {64 75 6d 6d 79 5f 68 63 64 2e 30} (length 0xb)
// }
// count: len = 0xb (8 bytes)
// ]
memcpy((void*)0x200000000200, "dummy_hcd.0", 11);
syscall(__NR_write, /*fd=*/r[0], /*buf=*/0x200000000200ul, /*count=*/0xbul);
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 00} (length 0x9)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x200000000000, "./config\000", 9);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x200000000000ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// mount arguments: [
// src: nil
// dst: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 00} (length 0x9)
// }
// type: ptr[in, buffer] {
// buffer: {63 6f 6e 66 69 67 66 73 00} (length 0x9)
// }
// flags: mount_flags = 0x0 (8 bytes)
// data: nil
// ]
memcpy((void*)0x200000000080, "./config\000", 9);
memcpy((void*)0x2000000000c0, "configfs\000", 9);
syscall(__NR_mount, /*src=*/0ul, /*dst=*/0x200000000080ul, /*type=*/0x2000000000c0ul, /*flags=*/0ul, /*data=*/0ul);
// mkdirat arguments: [
// fd: fd_dir (resource)
// path: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 2f 75 73 62 5f 67 61 64 67 65 74 2f 67 31 00} (length 0x17)
// }
// mode: open_mode = 0x1ff (8 bytes)
// ]
memcpy((void*)0x200000000100, "./config/usb_gadget/g1\000", 23);
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x200000000100ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
// openat arguments: [
// fd: fd_dir (resource)
// file: ptr[in, buffer] {
// buffer: {2e 2f 63 6f 6e 66 69 67 2f 75 73 62 5f 67 61 64 67 65 74 2f 67 31 2f 55 44 43 00} (length 0x1b)
// }
// flags: open_flags = 0x1 (4 bytes)
// mode: open_mode = 0x0 (2 bytes)
// ]
// returns fd
memcpy((void*)0x200000000340, "./config/usb_gadget/g1/UDC\000", 27);
res = syscall(__NR_openat, /*fd=*/0xffffff9c, /*file=*/0x200000000340ul, /*flags=O_WRONLY*/1, /*mode=*/0);
if (res != -1)
r[1] = res;
// write arguments: [
// fd: fd (resource)
// buf: ptr[in, buffer] {
// buffer: {64 75 6d 6d 79 5f 75 64 63 2e 30} (length 0xb)
// }
// count: len = 0xb (8 bytes)
// ]
memcpy((void*)0x200000000380, "dummy_udc.0", 11);
syscall(__NR_write, /*fd=*/r[1], /*buf=*/0x200000000380ul, /*count=*/0xbul);
return 0;
}
Reply: **Bug Analysis:**
The bug is a NULL pointer dereference in `dummy_set_selfpowered` within the `dummy_hcd` (USB gadget dummy HCD) driver.
The `dummy_hcd` module registers two separate platform devices during initialization: one for the Host Controller (HCD) and one for the USB Device Controller (UDC).
When the HCD device is manually unbound via sysfs (by writing to `/sys/bus/platform/drivers/dummy_hcd/unbind`), the `dummy_hcd_remove` function is invoked. This function cleans up the HCD and explicitly sets `dum->hs_hcd = NULL`. However, the UDC device remains bound and registered with the system.
If a user subsequently configures and binds a USB gadget driver to this orphaned UDC device (e.g., using `configfs`), the gadget framework calls `usb_gadget_set_selfpowered()`, which in turn calls the UDC's `dummy_set_selfpowered()` callback. This function attempts to retrieve the `dummy` struct by dereferencing `dum->hs_hcd`, which was previously set to NULL:
```c
dum = gadget_to_dummy_hcd(_gadget)->dum; // gadget_to_dummy_hcd returns NULL
if (value)
dum->devstatus |= (1 << USB_DEVICE_SELF_POWERED);
```
Because the `dum` field is at offset 0 of `struct dummy_hcd`, this results in a direct read from memory address `0x0`.
**Security Impact:**
* **Exploitable:** No. This is a pure NULL pointer dereference (reading from address 0). On modern Linux systems with `mmap_min_addr` enforced, address 0 cannot be mapped by userspace. The kernel will simply trap the invalid access (or KASAN will catch the shadow memory access for address 0, as seen in the report) and oops. It cannot be used for privilege escalation or arbitrary code execution.
* **Denial of Service:** Yes. The NULL pointer dereference will cause a kernel oops, which will kill the current process and can lead to a full system crash (kernel panic) if the system is configured with `panic_on_oops=1`.
* **Accessible From Unprivileged Processes / User Namespaces:** No. Triggering this bug requires two highly privileged operations:
1. **Unbinding the driver:** Writing to `/sys/bus/platform/drivers/dummy_hcd/unbind` requires root privileges. The file has `0200` permissions and is owned by the global `root` (UID 0). A process inside a user namespace cannot write to it because the global root UID is not mapped into the unprivileged user namespace, causing the VFS capability and DAC checks to deny access.
2. **Mounting configfs:** The reproducer uses `configfs` to bind the gadget. The `configfs` filesystem does not have the `FS_USERNS_MOUNT` flag, meaning it cannot be mounted from within a user namespace. It requires `CAP_SYS_ADMIN` in the initial user namespace.
* **Other Triggers:** The bug is specific to the software-emulated `dummy_hcd` driver and its sysfs/configfs management interfaces. It cannot be triggered remotely, via malicious filesystems, or by plugging in physical USB peripherals.
|
| 6/2 |
2026/07/25 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 11840
output: 46
thoughts: 929
Thoughts: **Analyzing a Kernel Bug Report: A Deep Dive**
Okay, so I've got this kernel bug report, and it's a doozy β a general protection fault, likely due to a non-canonical address. The trace points to a KASAN null-pointer dereference in `dummy_set_selfpowered`, specifically within `drivers/usb/gadget/udc/dummy_hcd.c`. The address it's complaining about is `0xdffffc0000000000`. That looks like a straightforward memory access issue, but the context is key.
The call trace provides a critical breadcrumb trail. It looks like we're deep within the USB gadget subsystem, with calls leading from `dummy_set_selfpowered` through `usb_gadget_set_selfpowered`, into the composite device preparation and binding within `configfs`. This is interesting; we're dealing with USB gadget configuration through `configfs`, which is how these things are generally set up. We see `configfs_composite_bind`, `gadget_bind_driver`, and eventually, the registration of the driver itself: `usb_gadget_register_driver_owner`. It looks like it's a driver binding issue, likely related to the interaction with `configfs`.
The reproducer steps give a great clue: unbinding the `dummy_hcd` driver, then creating a USB gadget in `configfs`, then *binding* `dummy_udc.0` to the UDC. Specifically, it unbinds the driver from the platform bus then attempts to create the gadget in configfs. I'm guessing that something is going wrong when the driver is unbound and then subsequently, bound.
So my initial thought is, `dummy_set_selfpowered` is being called with an invalid pointer or context. Now I'm going to jump into the `dummy_hcd.c` code to see exactly what's happening in `dummy_set_selfpowered`. I'll also check the adjacent code to ensure that the logic is correct during the unbinding and re-binding process. I want to see how this function is called, what data it operates on, and whether there are any race conditions or improper pointer handling that could explain the null-pointer dereference given that this seems to be a classic memory access bug. This one seems like it will be fun.
|
| 7/2 |
2026/07/25 00:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/usb/gadget/udc/dummy_hcd.c Name:dummy_set_selfpowered]
Results: map[SourceCode: 883: static int dummy_set_selfpowered(struct usb_gadget *_gadget, int value)
884: {
885: struct dummy *dum;
886:
887: _gadget->is_selfpowered = (value != 0);
888: dum = gadget_to_dummy_hcd(_gadget)->dum;
889: if (value)
890: dum->devstatus |= (1 << USB_DEVICE_SELF_POWERED);
891: else
892: dum->devstatus &= ~(1 << USB_DEVICE_SELF_POWERED);
893: return 0;
894: }
SourceFile:drivers/usb/gadget/udc/dummy_hcd.c]
|
| 8/2 |
2026/07/25 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 13548
output: 49
thoughts: 47
Thoughts: Okay, let's break this down.
**Debugging a Kernel Panic: Line 888 and the `gadget_to_dummy_hcd` Function**
Alright, the system is down, and the panic log points directly to line 888: `dum = gadget_to_dummy_hcd(_gadget)->dum;` Hmm, that's a pretty specific point of failure. My first instinct, as always, is to follow the breadcrumbs. So, `gadget_to_dummy_hcd`. That function call looks like the key to understanding this. I need to figure out precisely what that function does; it seems crucial for interpreting the state the crash occurred in. This is likely an abstraction layer, probably converting a higher-level gadget object or a peripheral descriptor into something the hardware driver, or a lower-level function, understands. The `->dum` is interesting too; it looks like we're accessing a member called `dum` within the returned structure from the function, which is likely a data structure holding the actual low level representation of the peripheral being accessed.
|
| 9/2 |
2026/07/25 00:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/usb/gadget/udc/dummy_hcd.c Name:gadget_to_dummy_hcd]
Results: map[SourceCode: 314: static inline struct dummy_hcd *gadget_to_dummy_hcd(struct usb_gadget *gadget)
315: {
316: struct dummy *dum = container_of(gadget, struct dummy, gadget);
317: if (dum->gadget.speed == USB_SPEED_SUPER)
318: return dum->ss_hcd;
319: else
320: return dum->hs_hcd;
321: }
SourceFile:drivers/usb/gadget/udc/dummy_hcd.c]
|
| 10/2 |
2026/07/25 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 14049
output: 59
thoughts: 85
Thoughts: Okay, let me think this through.
**Analyzing Potential Null Pointer Dereference in USB Gadget Code**
Alright, so the function `gadget_to_dummy_hcd` is returning either `dum->ss_hcd` or `dum->hs_hcd`. My main concern is, if `dum->hs_hcd` happens to be NULL, then any subsequent call to `gadget_to_dummy_hcd(_gadget)->dum` would immediately lead to a null pointer dereference, and we really don't want that kind of crash. I need to understand how the code is structured, but I feel like I'm close to a good analysis. The next logical step is to track down where this `dum->hs_hcd` member is being set and, importantly, where it might get cleared. Understanding those set/clear points will reveal the conditions under which this potential vulnerability exists. That should help me understand the full implications and what needs to be fixed.
|
| 11/2 |
2026/07/25 00:52 |
tool |
codesearch-find-references |
0m
Args: map[ContextFile:drivers/usb/gadget/udc/dummy_hcd.c IncludeSnippetLines:2 Name:dummy::hs_hcd SourceTreePrefix:]
Results: map[References:[map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:dummy_hcd_init SourceFile:drivers/usb/gadget/udc/dummy_hcd.c SourceLine:2868 SourceSnippet:2866: }
2867: for (i = 0; i < mod_data.num; i++) {
2868: if (!dum[i]->hs_hcd ||
2869: (!dum[i]->ss_hcd && mod_data.is_super_speed)) {
2870: /*
] map[ReferenceKind:writes ReferencingEntityKind:function ReferencingEntityName:dummy_hcd_probe SourceFile:drivers/usb/gadget/udc/dummy_hcd.c SourceLine:2732 SourceSnippet:2730: put_usb2_hcd:
2731: usb_put_hcd(hs_hcd);
2732: dum->hs_hcd = dum->ss_hcd = NULL;
2733: return retval;
2734: }
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:dummy_hcd_remove SourceFile:drivers/usb/gadget/udc/dummy_hcd.c SourceLine:2747 SourceSnippet:2745: }
2746:
2747: usb_remove_hcd(dummy_hcd_to_hcd(dum->hs_hcd));
2748: usb_put_hcd(dummy_hcd_to_hcd(dum->hs_hcd));
2749:
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:dummy_hcd_remove SourceFile:drivers/usb/gadget/udc/dummy_hcd.c SourceLine:2748 SourceSnippet:2746:
2747: usb_remove_hcd(dummy_hcd_to_hcd(dum->hs_hcd));
2748: usb_put_hcd(dummy_hcd_to_hcd(dum->hs_hcd));
2749:
2750: dum->hs_hcd = NULL;
] map[ReferenceKind:writes ReferencingEntityKind:function ReferencingEntityName:dummy_hcd_remove SourceFile:drivers/usb/gadget/udc/dummy_hcd.c SourceLine:2750 SourceSnippet:2748: usb_put_hcd(dummy_hcd_to_hcd(dum->hs_hcd));
2749:
2750: dum->hs_hcd = NULL;
2751: dum->ss_hcd = NULL;
2752: }
] map[ReferenceKind:writes ReferencingEntityKind:function ReferencingEntityName:dummy_setup SourceFile:drivers/usb/gadget/udc/dummy_hcd.c SourceLine:2567 SourceSnippet:2565: hcd->self.sg_tablesize = ~0;
2566: if (usb_hcd_is_primary_hcd(hcd)) {
2567: dum->hs_hcd = hcd_to_dummy_hcd(hcd);
2568: dum->hs_hcd->dum = dum;
2569: /*
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:dummy_setup SourceFile:drivers/usb/gadget/udc/dummy_hcd.c SourceLine:2568 SourceSnippet:2566: if (usb_hcd_is_primary_hcd(hcd)) {
2567: dum->hs_hcd = hcd_to_dummy_hcd(hcd);
2568: dum->hs_hcd->dum = dum;
2569: /*
2570: * Mark the first roothub as being USB 2.0.
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:find_endpoint SourceFile:drivers/usb/gadget/udc/dummy_hcd.c SourceLine:1590 SourceSnippet:1588: int i;
1589:
1590: if (!is_active((dum->gadget.speed == USB_SPEED_SUPER ?
1591: dum->ss_hcd : dum->hs_hcd)))
1592: return NULL;
] map[ReferenceKind:reads ReferencingEntityKind:function ReferencingEntityName:gadget_to_dummy_hcd SourceFile:drivers/usb/gadget/udc/dummy_hcd.c SourceLine:320 SourceSnippet: 318: return dum->ss_hcd;
319: else
320: return dum->hs_hcd;
321: }
]] TruncatedOutput:false]
|
| 12/2 |
2026/07/25 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 15744
output: 46
thoughts: 316
Thoughts: **Analyzing the Reproducer of the Null Pointer Dereference**
Okay, let's break down this reproducer. First, I see the process attempting to unbind the `dummy_hcd` driver from the `dummy_hcd.0` device. This is done with an `openat` call targeting a specific file path, followed by a `write` operation. The critical part is that this unbinding triggers the `dummy_hcd_remove` function. I know this function, and I recall that it explicitly sets both `dum->hs_hcd` and `dum->ss_hcd` to NULL. This is the key.
Now, the reproducer moves on. It tries to bind the gadget driver to `dummy_udc.0`, again using `openat` and `write`. This should trigger `usb_gadget_set_selfpowered`, and I can immediately see where things will likely go wrong. This function will call `dummy_set_selfpowered`.
Inside `dummy_set_selfpowered`, a key conversion happens. `gadget_to_dummy_hcd(_gadget)` is called, which returns `dum->hs_hcd`, which we know is NULL. And then it dereferences that NULL pointer, `->dum`. This is textbook null pointer dereference, and exactly what the reproducer is designed to trigger.
I need to confirm my hunch by reviewing the `dummy_hcd_remove` function and understanding how the UDC gets unregistered. I'll need to double check the code path for gadget unregistration. I'm guessing there's a missing synchronization or an incorrect assumption about the state of the `hs_hcd` pointer after the unbinding operation. This feels like a classic race condition or improper cleanup scenario.
|
| 13/2 |
2026/07/25 00:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/usb/gadget/udc/dummy_hcd.c Name:dummy_hcd_remove]
Results: map[SourceCode:2736: static void dummy_hcd_remove(struct platform_device *pdev)
2737: {
2738: struct dummy *dum;
2739:
2740: dum = hcd_to_dummy_hcd(platform_get_drvdata(pdev))->dum;
2741:
2742: if (dum->ss_hcd) {
2743: usb_remove_hcd(dummy_hcd_to_hcd(dum->ss_hcd));
2744: usb_put_hcd(dummy_hcd_to_hcd(dum->ss_hcd));
2745: }
2746:
2747: usb_remove_hcd(dummy_hcd_to_hcd(dum->hs_hcd));
2748: usb_put_hcd(dummy_hcd_to_hcd(dum->hs_hcd));
2749:
2750: dum->hs_hcd = NULL;
2751: dum->ss_hcd = NULL;
2752: }
SourceFile:drivers/usb/gadget/udc/dummy_hcd.c]
|
| 14/2 |
2026/07/25 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 16816
output: 46
thoughts: 64
Thoughts: **Analyzing the HCD/UDC Removal**
Okay, so I'm processing the code, and this `dummy_hcd_remove` function... it's striking me that it's removing the HCD, the Host Controller Driver. But I'm immediately thinking, "What about the UDC, the USB Device Controller?" The architecture's got me thinking there's more to the picture.
And right, now it's coming back to me: the `dummy_hcd` module... it actually registers *both* a host and a device controller, doesn't it? So, removing the HCD in isolation isn't the complete story. I need to understand how the UDC is handled in this scenario. Therefore, the next logical step is to dive into the `dummy_udc_remove` function to see how the device controller is unregistered. That'll provide a more complete understanding.
|
| 15/2 |
2026/07/25 00:52 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/usb/gadget/udc/dummy_hcd.c Name:dummy_udc_remove]
Results: map[SourceCode:1119: static void dummy_udc_remove(struct platform_device *pdev)
1120: {
1121: struct dummy *dum = platform_get_drvdata(pdev);
1122:
1123: device_remove_file(&dum->gadget.dev, &dev_attr_function);
1124: usb_del_gadget_udc(&dum->gadget);
1125: }
SourceFile:drivers/usb/gadget/udc/dummy_hcd.c]
|
| 16/2 |
2026/07/25 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 17278
output: 49
thoughts: 319
Thoughts: **Analyzing a Potential NULL Pointer Dereference in the Dummy HCD Module**
Okay, so here's what's rattling around in my head. I'm looking at this `dummy_hcd` module, and it's got these two platform drivers: `dummy_hcd` and `dummy_udc`. Now, the problem arises when `dummy_hcd.0` gets unbound, which triggers the `dummy_hcd_remove` function. The kicker is, `dummy_udc.0` might *still* be bound at this point! Meaning the UDC is still registered.
The scenario unfolds like this: We unbind the HCD, the UDC *remains* bound. Then, the user decides to bind a gadget driver to `dummy_udc.0`. That gadget driver's bind function then calls `usb_gadget_set_selfpowered`, which ultimately calls `dummy_set_selfpowered`. Inside `dummy_set_selfpowered` is where the trouble begins. It makes this assumption that `dum->hs_hcd` is *not* NULL. But guess what? `dummy_hcd_remove` previously set it to NULL!
Now, the question becomes: is this exploitable? Looking at the code:
```c
dum = gadget_to_dummy_hcd(_gadget)->dum;
if (value)
dum->devstatus |= (1 << USB_DEVICE_SELF_POWERED);
else
dum->devstatus &= ~(1 << USB_DEVICE_SELF_POWERED);
```
I see that `gadget_to_dummy_hcd` returns `dum->hs_hcd` which can be NULL. That's a NULL pointer dereference! We're dereferencing `NULL->dum`. Let me see... `gadget_to_dummy_hcd` actually returns a pointer to `struct dummy_hcd`! That's correct. Now, the potential NULL dereference becomes much clearer. Let me just quickly check the layout of `struct dummy_hcd` to confirm my understanding of the memory model...
|
| 17/2 |
2026/07/25 00:52 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:drivers/usb/gadget/udc/dummy_hcd.c FieldOffset:<nil> Name:dummy_hcd]
Results: map[Fields:[map[Name:dum OffsetBits:0 SizeBits:64] map[Name:rh_state OffsetBits:64 SizeBits:32] map[Name:timer OffsetBits:128 SizeBits:640] map[Name:port_status OffsetBits:768 SizeBits:32] map[Name:old_status OffsetBits:800 SizeBits:32] map[Name:re_timeout OffsetBits:832 SizeBits:64] map[Name:udev OffsetBits:896 SizeBits:64] map[Name:urbp_list OffsetBits:960 SizeBits:128] map[Name:next_frame_urbp OffsetBits:1088 SizeBits:64] map[Name:stream_en_ep OffsetBits:1152 SizeBits:32] map[Name:num_stream OffsetBits:1184 SizeBits:120] map[Name:timer_pending OffsetBits:1304 SizeBits:1] map[Name:active OffsetBits:1305 SizeBits:1] map[Name:old_active OffsetBits:1306 SizeBits:1] map[Name:resuming OffsetBits:1307 SizeBits:1]]]
|
| 18/2 |
2026/07/25 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 18428
output: 28
thoughts: 334
Thoughts: **Analyzing a Null Pointer Dereference**
Okay, so I'm looking at this code and immediately see a potential issue. `dum` is at offset zero within the `struct dummy_hcd`, which means `gadget_to_dummy_hcd(_gadget)->dum` is essentially trying to read from address zero. Classic null pointer dereference. The code reads a pointer from address zero and then tries to dereference that pointer to modify `devstatus`.
Initially, my instinct is that this will simply cause a crash due to reading from NULL, and therefore, it's likely not exploitable. However, wait a minute, the `gadget_to_dummy_hcd(_gadget)` function returns `dum->hs_hcd` which is itself NULL. So, the `->dum` access *directly* reads the first 8 bytes of NULL (address 0). That's a definite fault. This is a pure null pointer dereference, right?
The next crucial question is: is this accessible from unprivileged processes? The provided reproducer uses `sysfs` and `configfs`, which immediately makes me think about user namespaces. Can an unprivileged user mount `sysfs` and `configfs` within a user namespace? The answer, I believe, is yes. But, I'd need to verify this assumption.
I need to quickly determine whether the `dummy_hcd` driver is loaded by default or if an unprivileged user can load it. It's a test module, so it's probably not loaded in a production environment. However, if it *is* loaded, the unprivileged user should be able to unbind the driver within a user namespace. The unbind process involves writing to `/sys/bus/platform/drivers/dummy_hcd/unbind`, which in turn involves checking the permissions of the `unbind` file in sysfs. Those permissions determine whether an unprivileged user can trigger this flaw.
|
| 19/2 |
2026/07/25 00:52 |
tool |
grepper |
0m
Args: map[Expression:unbind PathPrefix:drivers/base/bus.c]
Results: map[Output:drivers/base/bus.c=230=static const struct kset_uevent_ops bus_uevent_ops = {
--
drivers/base/bus.c-234-/* Manually detach a device from its associated driver. */
drivers/base/bus.c:235:static ssize_t unbind_store(struct device_driver *drv, const char *buf,
drivers/base/bus.c-236- size_t count)
--
drivers/base/bus.c-250-}
drivers/base/bus.c:251:static DRIVER_ATTR_IGNORE_LOCKDEP(unbind, 0200, NULL, unbind_store);
drivers/base/bus.c-252-
--
drivers/base/bus.c=675=static int __must_check add_bind_files(struct device_driver *drv)
--
drivers/base/bus.c-678-
drivers/base/bus.c:679: ret = driver_create_file(drv, &driver_attr_unbind);
drivers/base/bus.c-680- if (ret == 0) {
--
drivers/base/bus.c-682- if (ret)
drivers/base/bus.c:683: driver_remove_file(drv, &driver_attr_unbind);
drivers/base/bus.c-684- }
--
drivers/base/bus.c=688=static void remove_bind_files(struct device_driver *drv)
--
drivers/base/bus.c-690- driver_remove_file(drv, &driver_attr_bind);
drivers/base/bus.c:691: driver_remove_file(drv, &driver_attr_unbind);
drivers/base/bus.c-692-}
]
|
| 20/2 |
2026/07/25 00:52 |
llm |
expert |
0m
Model:
Tokens:
input: 19661
output: 48
thoughts: 552
Thoughts: **Analyzing `unbind` File Permissions in User Namespaces**
Okay, so I'm looking at this `unbind` file, and its permissions are set to `0200`, meaning it's write-only by the owner. The owner, in this case, is `root`. My question is, can a user *within* a user namespace actually write to it?
When a user namespace is created, the system automatically mounts the sysfs. However, sysfs files are generally owned by the global root, right? Unless they're specific to a network namespace. So, I need to verify whether a user inside a user namespace can, in fact, modify this `sysfs unbind` file.
Typically, in user namespaces, sysfs is either mounted as read-only, or if it's writable, the files are still owned by `nobody` or `root` (UID 0 in the initial user namespace). This, in turn, often maps to `nobody` within the user namespace itself, so the user namespace's root user *shouldn't* have write access. Unless... wait a second... if the user namespace is created first, and *then* they mount sysfs, the files are owned by the global root.
Okay, so let's check if we can even mount sysfs inside a user namespace. Yes, we can, *if* the user has `CAP_SYS_ADMIN`, either in the network namespace (for network devices) or in the user namespace itself. However, the kernel ultimately determines the ownership of these sysfs files. Most of them are owned by `GLOBAL_ROOT_UID`.
So, from what I've considered, a user in a user namespace shouldn't be able to write to `/sys/bus/platform/drivers/dummy_hcd/unbind`.
Wait, the reproducer uses:
```c
syscall(__NR_mkdirat, /*fd=*/0xffffff9c, /*path=*/0x200000000000ul, /*mode=S_IXOTH|S_IWOTH|S_IROTH|S_IXGRP|S_IWGRP|S_IRGRP|S_IXUSR|S_IWUSR|0x100*/0x1fful);
syscall(__NR_mount, /*src=*/0ul, /*dst=*/0x200000000080ul, /*type=*/0x2000000000c0ul, /*flags=*/0ul, /*data=*/0ul);
```
It mounts `sysfs` and *then* opens `unbind`. So if syzkaller is running as root (which it often does in its VMs), then it should have no problem writing to `unbind`.
But, what if it's running as an *unprivileged* user inside a user namespace? Can it write to `unbind` in that case? I need to double-check if `unbind` is truly writable in that scenario.
|
| 21/2 |
2026/07/25 00:53 |
tool |
codeexpert |
6m
Args: map[Question:Can a process running as root in a user namespace (but unprivileged in the initial user namespace) write to `/sys/bus/platform/drivers/*/unbind`?]
Results: map[Answer:No, a process running as root in a user namespace (but unprivileged in the initial user namespace) **cannot** write to `/sys/bus/platform/drivers/*/unbind`.
Even if the sysfs filesystem is mounted as read-write inside the user namespace, the write attempt will be denied with **`Permission denied` (`-EACCES`)** due to standard Linux VFS discretionary access control (DAC) and capability checks.
### Detailed Step-by-Step Analysis
#### 1. File Definition and Permissions
In `drivers/base/bus.c`, the `unbind` attribute is defined with write-only permissions for the owner:
```c
static DRIVER_ATTR_IGNORE_LOCKDEP(unbind, 0200, NULL, unbind_store);
```
The mode `0200` means only the owner of the file has write permissions; group and "other" have no permissions.
#### 2. Inode Ownership in Kernfs/Sysfs
Sysfs is built on top of `kernfs`. In `fs/kernfs/inode.c`, default attributes for kernfs nodes are initialized to the host's root UID and GID:
```c
/* assign default attributes */
ret->ia_uid = GLOBAL_ROOT_UID;
ret->ia_gid = GLOBAL_ROOT_GID;
```
These attributes are applied to the VFS inode's `i_uid` and `i_gid` fields during inode initialization and refresh. Therefore, the `unbind` file is owned by `GLOBAL_ROOT_UID` (UID 0 in the initial user namespace).
#### 3. VFS Permission Checks (`generic_permission`)
When a process attempts to open the `unbind` file for writing, the VFS performs permission checks via `inode_permission()` (in `fs/namei.c`), which calls `generic_permission()`:
1. **Owner Check**: `generic_permission()` calls `acl_permission_check()`, which compares the process's filesystem UID (`current_fsuid()`) with the file's UID (`i_uid_into_vfsuid()`).
- Since the process is unprivileged in the initial user namespace, its `current_fsuid()` is mapped to a non-zero, unprivileged UID on the host (e.g., `100000`).
- Since the file's UID is `GLOBAL_ROOT_UID` (0), the process is **not the owner** of the file.
- Because the file mode is `0200`, "other" has no write permissions, and `acl_permission_check()` returns `-EACCES`.
2. **Capability Check**: To bypass the DAC check, `generic_permission()` checks if the process has `CAP_DAC_OVERRIDE` using `capable_wrt_inode_uidgid()`:
```c
bool capable_wrt_inode_uidgid(struct mnt_idmap *idmap,
const struct inode *inode, int cap)
{
struct user_namespace *ns = current_user_ns();
return ns_capable(ns, cap) &&
privileged_wrt_inode_uidgid(ns, idmap, inode);
}
```
`privileged_wrt_inode_uidgid()` checks if the file's UID and GID are mapped into the process's user namespace:
```c
bool privileged_wrt_inode_uidgid(struct user_namespace *ns,
struct mnt_idmap *idmap,
const struct inode *inode)
{
return vfsuid_has_mapping(ns, i_uid_into_vfsuid(idmap, inode)) &&
vfsgid_has_mapping(ns, i_gid_into_vfsgid(idmap, inode));
}
```
- Because the process is unprivileged in the initial user namespace, the host's root UID (`GLOBAL_ROOT_UID`) is **not mapped** in the process's user namespace.
- Therefore, `vfsuid_has_mapping()` returns `false`, meaning the process's namespace capabilities are not effective over this inode.
- The capability check fails, and the write attempt is rejected with `-EACCES`.]
|
| 22/3 |
2026/07/25 00:53 |
agent |
codeexpert |
6m
Model:
Results: map[Answer:No, a process running as root in a user namespace (but unprivileged in the initial user namespace) **cannot** write to `/sys/bus/platform/drivers/*/unbind`.
Even if the sysfs filesystem is mounted as read-write inside the user namespace, the write attempt will be denied with **`Permission denied` (`-EACCES`)** due to standard Linux VFS discretionary access control (DAC) and capability checks.
### Detailed Step-by-Step Analysis
#### 1. File Definition and Permissions
In `drivers/base/bus.c`, the `unbind` attribute is defined with write-only permissions for the owner:
```c
static DRIVER_ATTR_IGNORE_LOCKDEP(unbind, 0200, NULL, unbind_store);
```
The mode `0200` means only the owner of the file has write permissions; group and "other" have no permissions.
#### 2. Inode Ownership in Kernfs/Sysfs
Sysfs is built on top of `kernfs`. In `fs/kernfs/inode.c`, default attributes for kernfs nodes are initialized to the host's root UID and GID:
```c
/* assign default attributes */
ret->ia_uid = GLOBAL_ROOT_UID;
ret->ia_gid = GLOBAL_ROOT_GID;
```
These attributes are applied to the VFS inode's `i_uid` and `i_gid` fields during inode initialization and refresh. Therefore, the `unbind` file is owned by `GLOBAL_ROOT_UID` (UID 0 in the initial user namespace).
#### 3. VFS Permission Checks (`generic_permission`)
When a process attempts to open the `unbind` file for writing, the VFS performs permission checks via `inode_permission()` (in `fs/namei.c`), which calls `generic_permission()`:
1. **Owner Check**: `generic_permission()` calls `acl_permission_check()`, which compares the process's filesystem UID (`current_fsuid()`) with the file's UID (`i_uid_into_vfsuid()`).
- Since the process is unprivileged in the initial user namespace, its `current_fsuid()` is mapped to a non-zero, unprivileged UID on the host (e.g., `100000`).
- Since the file's UID is `GLOBAL_ROOT_UID` (0), the process is **not the owner** of the file.
- Because the file mode is `0200`, "other" has no write permissions, and `acl_permission_check()` returns `-EACCES`.
2. **Capability Check**: To bypass the DAC check, `generic_permission()` checks if the process has `CAP_DAC_OVERRIDE` using `capable_wrt_inode_uidgid()`:
```c
bool capable_wrt_inode_uidgid(struct mnt_idmap *idmap,
const struct inode *inode, int cap)
{
struct user_namespace *ns = current_user_ns();
return ns_capable(ns, cap) &&
privileged_wrt_inode_uidgid(ns, idmap, inode);
}
```
`privileged_wrt_inode_uidgid()` checks if the file's UID and GID are mapped into the process's user namespace:
```c
bool privileged_wrt_inode_uidgid(struct user_namespace *ns,
struct mnt_idmap *idmap,
const struct inode *inode)
{
return vfsuid_has_mapping(ns, i_uid_into_vfsuid(idmap, inode)) &&
vfsgid_has_mapping(ns, i_gid_into_vfsgid(idmap, inode));
}
```
- Because the process is unprivileged in the initial user namespace, the host's root UID (`GLOBAL_ROOT_UID`) is **not mapped** in the process's user namespace.
- Therefore, `vfsuid_has_mapping()` returns `false`, meaning the process's namespace capabilities are not effective over this inode.
- The capability check fails, and the write attempt is rejected with `-EACCES`.]
Instruction:
You are a capable Linux kernel developer tasked with researching complex questions
about kernel source code. You will be given a concrete question, and need to provide
a concrete answer.
Use tools extensively while researching the question. Don't make assumptions,
or rely on your previous knowledge about the kernel source code, use available tools
to access the actual source code.
Use all available sources of information:
- kernel source code
- documentation in the Documentation dir in the source tree
- git commits descriptions, git blame
Do not guess file names or file paths and attempt to read them without
verifying their existence first using content search or directory listing tools.
If a file, symbol, or directory is not found via content search ('grepper') or
directory listing ('codesearch-dir-index'), treat it as completely absent.
Do not attempt to guess alternative names, extensions, or directories.
Tool Selection Guidelines:
1. For C functions, structs, and variables: Prefer 'codesearch-definition-source'
or 'codesearch-struct-layout' FIRST to retrieve clean, exact definitions
without line-number guessing.
2. If symbol lookup fails (e.g., preprocessor macros, macro-generated code, or
disabled #ifdef branches), fall back to 'read-file' or 'grepper'.
3. For file headers, #include directives, preprocessor macro definitions, and
non-C files (Kconfig, Makefiles, docs): Use 'read-file' or 'grepper' directly.
Do NOT use 'git-log' to search for the presence or existence of files in the
repository. 'git-log' is only for tracing commit history of files that are
already present in the current checkout. If a file does not exist in the
current checkout, it cannot be used for reproduction.
Avoid running broad 'git-log' queries (such as searches on the entire repo)
without a specific 'PathPrefix' to restrict the scope, as these are very
expensive and will time out.
If a 'git-log' tool call times out, do not retry the query with the same broad
scope. You must specify a tighter, narrower 'PathPrefix' for subsequent queries.
Prefer calling several tools at the same time to save round-trips.
Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.
Prompt: Can a process running as root in a user namespace (but unprivileged in the initial user namespace) write to `/sys/bus/platform/drivers/*/unbind`?
|
| 23/4 |
2026/07/25 00:53 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 3924
output: 17
thoughts: 147
Thoughts: **Investigating `/sys/bus/platform/drivers/*/unbind` Write Permissions**
Okay, so the question is, can a process running as root within a user namespace, but lacking those initial privileged credentials, actually write to `/sys/bus/platform/drivers/*/unbind`? That's a good one. It gets to the heart of how namespaces and privilege separation work, and the specific rules around sysfs operations.
Alright, let's dive into the code. The logical first step is to trace the execution path for writes to these unbind files. I need to understand the underlying mechanics. I'll need to figure out which part of the kernel handles these sysfs operations related to `unbind` or `bind` β specifically the interaction with drivers and buses.
My initial approach is to search for keywords related to the process. "Unbind" and "bind" are obvious starting points when looking at the sysfs side. I should look specifically at the driver core code, because sysfs attributes of devices often work that way. I'll probably start with something like `grepper` to search through the `drivers/bus/sysfs` tree.
Then again, the implementation might be named `unbind_store` or `drv_attr_unbind` or something similar, and it would be smart to widen the search a bit. I need to be methodical here, and think like a kernel developer. So, expand the search to `unbind_store`, `drv_attr_unbind`, or anything related to the core driver attributes.
|
| 24/4 |
2026/07/25 00:53 |
tool |
grepper |
0m
Args: map[Expression:unbind_store]
Results: map[Output:drivers/base/bus.c=230=static const struct kset_uevent_ops bus_uevent_ops = {
--
drivers/base/bus.c-234-/* Manually detach a device from its associated driver. */
drivers/base/bus.c:235:static ssize_t unbind_store(struct device_driver *drv, const char *buf,
drivers/base/bus.c-236- size_t count)
--
drivers/base/bus.c-250-}
drivers/base/bus.c:251:static DRIVER_ATTR_IGNORE_LOCKDEP(unbind, 0200, NULL, unbind_store);
drivers/base/bus.c-252-
--
drivers/dma/idxd/compat.c=10=extern void device_driver_detach(struct device *dev);
--
drivers/dma/idxd/compat.c-15-
drivers/dma/idxd/compat.c:16:static ssize_t unbind_store(struct device_driver *drv, const char *buf, size_t count)
drivers/dma/idxd/compat.c-17-{
--
drivers/dma/idxd/compat.c-34-}
drivers/dma/idxd/compat.c:35:static DRIVER_ATTR_IGNORE_LOCKDEP(unbind, 0200, NULL, unbind_store);
drivers/dma/idxd/compat.c-36-
]
|
| 25/4 |
2026/07/25 00:53 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 4782
output: 35
thoughts: 65
Thoughts: Here's my analysis of the provided code snippet, thinking out loud as I process it:
**Decoding the `unbind_store` Function**
Okay, so we're looking at `drivers/base/bus.c` and specifically, that `unbind_store` function. The code mentions `DRIVER_ATTR_IGNORE_LOCKDEP(unbind, 0200, NULL, unbind_store);`. Right away, this screams attribute setup for a driver. The `IGNORE_LOCKDEP` suggests we are bypassing lockdep in this particular attribute which can be useful when you *really* know what you are doing with concurrency. Let me jump into `drivers/base/bus.c` around line 235 as the prompt suggested, and see how this `unbind_store` is utilized within the bus framework. I expect to see its context within the sysfs attribute system, likely linked to a bus's ability to "unbind" a driver from a device. Curious to see what actions this `unbind_store` triggers... is it a simple flag, a more complex unbinding process? I bet this is where the core logic of driver unbinding, or at least a portion of it, lives. Time to delve in.
|
| 26/4 |
2026/07/25 00:53 |
tool |
read-file |
0m
Args: map[File:drivers/base/bus.c FirstLine:220 LineCount:50]
Results: map[Contents: 220:
221: static int bus_uevent_filter(const struct kobject *kobj)
222: {
223: const struct kobj_type *ktype = get_ktype(kobj);
224:
225: if (ktype == &bus_ktype)
226: return 1;
227: return 0;
228: }
229:
230: static const struct kset_uevent_ops bus_uevent_ops = {
231: .filter = bus_uevent_filter,
232: };
233:
234: /* Manually detach a device from its associated driver. */
235: static ssize_t unbind_store(struct device_driver *drv, const char *buf,
236: size_t count)
237: {
238: const struct bus_type *bus = bus_get(drv->bus);
239: struct device *dev;
240: int err = -ENODEV;
241:
242: dev = bus_find_device_by_name(bus, NULL, buf);
243: if (dev && dev->driver == drv) {
244: device_driver_detach(dev);
245: err = count;
246: }
247: put_device(dev);
248: bus_put(bus);
249: return err;
250: }
251: static DRIVER_ATTR_IGNORE_LOCKDEP(unbind, 0200, NULL, unbind_store);
252:
253: /*
254: * Manually attach a device to a driver.
255: * Note: the driver must want to bind to the device,
256: * it is not possible to override the driver's id table.
257: */
258: static ssize_t bind_store(struct device_driver *drv, const char *buf,
259: size_t count)
260: {
261: const struct bus_type *bus = bus_get(drv->bus);
262: struct device *dev;
263: int err = -ENODEV;
264:
265: dev = bus_find_device_by_name(bus, NULL, buf);
266: if (dev && driver_match_device(drv, dev)) {
267: err = device_driver_attach(drv, dev);
268: if (!err) {
269: /* success */
]
|
| 27/4 |
2026/07/25 00:53 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 5892
output: 18
thoughts: 257
Thoughts: **Delving into Sysfs Permissions and User Namespaces**
Okay, so I'm thinking about sysfs and how its permissions are managed. Specifically, I need to understand how write access works. When a process attempts a write operation within sysfs, does it need certain *capabilities* to succeed?
Right, sysfs is a virtual filesystem, so the rules could be a bit different than a standard file system. I should investigate how the files are created and what their default permissions are. For example, I recall the `unbind` attribute has a mode of `0200`, meaning write-only by the owner, root in most cases.
But that brings up a crucial question: who *is* the owner of sysfs files? Usually, it's `root` (UID 0, GID 0) within the initial user namespace. However, the scenario I'm interested in is when a process operates within a *user namespace*. How does sysfs determine file ownership in that case? Does sysfs even support user namespaces? Can files within sysfs be owned by a user ID that's not root in the initial user namespace, but is a valid UID in the context of the user namespace where the process is running?
Time to investigate this in more detail. Let's see if there is any mention of `sysfs` and `ns`, `userns`, or `user_ns` within the kernel source. I could also search for relevant function names like `sysfs_ops`, `sysfs_file_operations`, or `sysfs_permission` using `grepper` to see if I can find an answer.
|
| 28/4 |
2026/07/25 00:53 |
tool |
grepper |
0m
Error:
Args: map[Expression:sysfs_permission]
Results:
|
| 29/4 |
2026/07/25 00:53 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 6525
output: 22
thoughts: 38
Thoughts: Okay, here's my interpretation of those thoughts, framed as my own mental process, given my expertise:
**Diving into the Sysfs Permission Model**
Alright, I need to get a clear picture of how permissions are actually enforced within the sysfs filesystem. My immediate instinct is to jump right into the source code, specifically the `fs/sysfs/` directory. I'm going to start with a targeted search. I'll search for `sysfs` itself within that directory, using it as a starting point, to see how the permissions are being handled. I want to see how the kernel code actually checks and enforces who can do what within that virtual file system.
To get oriented, I'll then list the files within `fs/sysfs/`. This will give me a lay of the land, a quick overview of the key components and hopefully lead me to the right section for understanding how they handle read, write and execute permissions. I'm hoping to quickly identify the relevant functions or data structures responsible for authorization.
|
| 30/4 |
2026/07/25 00:53 |
tool |
codesearch-dir-index |
0m
Args:
Results: map[Files:[dir.c file.c group.c mount.c symlink.c sysfs.h] Subdirs:<nil>]
|
| 31/4 |
2026/07/25 00:53 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 6852
output: 22
thoughts: 73
Thoughts: **Navigating the Kernel Filesystem Permissions Landscape**
Okay, so I need to dig into how permissions are handled in these kernel filesystems. The goal is to understand the authorization process. My initial thought is to start with a search for "permission" directly in `fs/sysfs/` and `kernfs`. But wait, I recall that `sysfs` is built on top of `kernfs`, so exploring the underlying `kernfs` implementation will likely be more fruitful.
Therefore, I'll refine my search strategy. I'll pivot and look for instances of `kernfs_ops` or directly for `kernfs`-related permission checks. This feels like the right path to uncover the core logic. Ultimately, this will involve looking for `permission` in `fs/kernfs/` directly, given the relationship I've just clarified.
|
| 32/4 |
2026/07/25 00:53 |
tool |
codesearch-dir-index |
0m
Args:
Results: map[Files:[dir.c file.c inode.c kernfs-internal.h mount.c symlink.c] Subdirs:<nil>]
|
| 33/4 |
2026/07/25 00:53 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 7178
output: 26
thoughts: 31
Thoughts: Here's my take on the thought process, as requested:
**Diving into Kernfs Permissions**
Okay, so I need to understand how permissions are handled within kernfs, specifically in the context of its inodes. The most direct approach is to jump right into the code. Let's see... I'll start with the likely candidate file, `fs/kernfs/inode.c`. That's where inode-related logic usually lives.
Alternatively, since I'm trying to get at *permission* handling specifically, I could just search within that same file for the string "permission" itself. That's a good way to identify direct code references to permission checks and modifications. Either reading the whole file carefully, or directly searching the code for "permission," should be a quick path to the relevant code. Both approaches are useful to use.
|
| 34/4 |
2026/07/25 00:53 |
tool |
grepper |
0m
Args: map[Expression:permission PathPrefix:fs/kernfs/]
Results: map[Output:fs/kernfs/dir.c=1360=const struct inode_operations kernfs_dir_iops = {
fs/kernfs/dir.c-1361- .lookup = kernfs_iop_lookup,
fs/kernfs/dir.c:1362: .permission = kernfs_iop_permission,
fs/kernfs/dir.c-1363- .setattr = kernfs_iop_setattr,
--
fs/kernfs/inode.c=20=static const struct inode_operations kernfs_iops = {
fs/kernfs/inode.c:21: .permission = kernfs_iop_permission,
fs/kernfs/inode.c-22- .setattr = kernfs_iop_setattr,
--
fs/kernfs/inode.c=266=void kernfs_evict_inode(struct inode *inode)
--
fs/kernfs/inode.c-274-
fs/kernfs/inode.c:275:int kernfs_iop_permission(struct mnt_idmap *idmap,
fs/kernfs/inode.c-276- struct inode *inode, int mask)
--
fs/kernfs/inode.c-289- kernfs_refresh_inode(kn, inode);
fs/kernfs/inode.c:290: ret = generic_permission(&nop_mnt_idmap, inode, mask);
fs/kernfs/inode.c-291- up_read(&root->kernfs_iattr_rwsem);
--
fs/kernfs/kernfs-internal.h=173=void kernfs_evict_inode(struct inode *inode);
fs/kernfs/kernfs-internal.h:174:int kernfs_iop_permission(struct mnt_idmap *idmap,
fs/kernfs/kernfs-internal.h-175- struct inode *inode, int mask);
--
fs/kernfs/symlink.c=149=const struct inode_operations kernfs_symlink_iops = {
--
fs/kernfs/symlink.c-153- .getattr = kernfs_iop_getattr,
fs/kernfs/symlink.c:154: .permission = kernfs_iop_permission,
fs/kernfs/symlink.c-155-};
]
|
| 35/4 |
2026/07/25 00:53 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 8008
output: 38
thoughts: 23
Thoughts: Here's my summary, as though I'm thinking through this myself, given my expertise:
**Diving into kernfs_iop_permission**
Okay, time to dissect `kernfs_iop_permission`. I know this function lives in `fs/kernfs/inode.c`, so I should be able to get a solid grasp of it quickly. I'm expecting this to be the core access control mechanism for kernfs inodes, the gateway controlling read, write, execute permissions β the whole shebang. Knowing kernfs, it likely acts as the permissions layer for the interface that users see in user space and that kernfs manages in the kernel. Let's see how it's implemented. This is probably going to be the lynchpin in how security is controlled on those pseudo filesystems. Gotta pay attention to those details, because a tiny flaw here could be an enormous problem later.
|
| 36/4 |
2026/07/25 00:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/kernfs/inode.c Name:kernfs_iop_permission]
Results: map[SourceCode: 275: int kernfs_iop_permission(struct mnt_idmap *idmap,
276: struct inode *inode, int mask)
277: {
278: struct kernfs_node *kn;
279: struct kernfs_root *root;
280: int ret;
281:
282: if (mask & MAY_NOT_BLOCK)
283: return -ECHILD;
284:
285: kn = inode->i_private;
286: root = kernfs_root(kn);
287:
288: down_read(&root->kernfs_iattr_rwsem);
289: kernfs_refresh_inode(kn, inode);
290: ret = generic_permission(&nop_mnt_idmap, inode, mask);
291: up_read(&root->kernfs_iattr_rwsem);
292:
293: return ret;
294: }
SourceFile:fs/kernfs/inode.c]
|
| 37/4 |
2026/07/25 00:53 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 8569
output: 38
thoughts: 104
Thoughts: **Delving into Kernfs Permissions**
Okay, so `kernfs_iop_permission` calls `generic_permission` with a `nop_mnt_idmap` and the inode mask. I'm immediately wondering about the inode owner here. Where is the inode's owner actually set? This permission check seems a little too generic, and I need to understand how the user and group IDs are being initialized and maintained for these kernfs inodes. I'll need to trace back from here.
My next step is to examine how the `i_uid` and `i_gid` fields of the inode are handled. I need to find the point where these are refreshed or initially set. Let's look for `kernfs_refresh_inode` and see what that does. A quick search should turn up the source code file, probably in `fs/kernfs/inode.c`, where this is defined. That should give me the answers I'm looking for.
|
| 38/4 |
2026/07/25 00:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/kernfs/inode.c Name:kernfs_refresh_inode]
Results: map[SourceCode: 167: static void kernfs_refresh_inode(struct kernfs_node *kn, struct inode *inode)
168: {
169: struct kernfs_iattrs *attrs;
170:
171: inode->i_mode = kn->mode;
172: attrs = kernfs_iattrs_noalloc(kn);
173: if (attrs)
174: /*
175: * kernfs_node has non-default attributes get them from
176: * persistent copy in kernfs_node.
177: */
178: set_inode_attr(inode, attrs);
179:
180: if (kernfs_type(kn) == KERNFS_DIR && !(kn->flags & KERNFS_REMOVING))
181: set_nlink(inode, kn->dir.subdirs + 2);
182: }
SourceFile:fs/kernfs/inode.c]
|
| 39/4 |
2026/07/25 00:53 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 9198
output: 28
thoughts: 53
Thoughts: Here's my thought process, as if I were considering this issue:
**Analyzing Potential Null `attrs` in Kernfs Inode Initialization**
Okay, hold on a second. I need to make sure I'm not missing something fundamental here. What happens if the `attrs` parameter passed to whatever function is initializing these inodes is actually `NULL`? Where are the default user and group IDs going to be set if not there? This could lead to some unexpected security vulnerabilities or just generally incorrect behavior.
Alright, time to dive into the kernel code and figure this out. I think the key lies within the inode initialization functions. My gut tells me to check either `kernfs_init_inode` or perhaps `kernfs_get_inode` within the `fs/kernfs/inode.c` file. Those seem like the likely places where these attributes are handled. Let's see if either of them accounts for `NULL` attributes and how they handle the uid/gid assignments in that scenario. Hopefully, there's a robust mechanism in place to provide safe defaults. I need to understand this completely.
|
| 40/4 |
2026/07/25 00:53 |
tool |
grepper |
0m
Args: map[Expression:i_uid PathPrefix:fs/kernfs/]
Results: map[Output:fs/kernfs/inode.c=157=static inline void set_inode_attr(struct inode *inode,
--
fs/kernfs/inode.c-159-{
fs/kernfs/inode.c:160: inode->i_uid = attrs->ia_uid;
fs/kernfs/inode.c-161- inode->i_gid = attrs->ia_gid;
]
|
| 41/4 |
2026/07/25 00:53 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 9620
output: 36
thoughts: 28
Thoughts: Here's my analysis of the provided section of `fs/kernfs/inode.c`, as I understand it:
**Analyzing kernfs Inode Initialization**
Okay, let's dive into `fs/kernfs/inode.c`, specifically lines 100 to 166. I want to see how they're handling inode creation and initialization within the kernfs context. Right off the bat, I'm looking for the core data structures involved β the `kernfs_node`, the `inode` itself, and how they relate. I'm assuming this section deals with the foundational setup, before we get into the more specific node types (files, directories, etc.).
I'm zeroing in on the initialization flow. Are they allocating the `inode` first, then the `kernfs_node`? Or is it the other way around? What about memory allocation? Are they using `kmalloc`, `kmem_cache_alloc`, or something else entirely? I need to understand the memory management strategy because that's always crucial for performance and preventing leaks. I'm also watching out for error handling. When does a failure occur? How is it propagated? Are they returning error codes directly, or are they using a more complex error-handling mechanism? The interactions between the `inode` and the `kernfs_node` are also something I'm keeping an eye on; are they using pointers or references.
The allocation of the `inode` itself β is this done via `alloc_inode` with a specific `inode_operations` structure assigned? If so, what ops are assigned at this initial stage? Are they generic, or specific to kernfs? Do they immediately set any attributes of the `inode`? And what's being passed as arguments to the allocation function? Is there any specific sizing, and is the allocation taking into account any private data associated with the kernfs inode?
I'm specifically looking for any indication of how they handle different node types within kernfs. Do they use a single allocation path for everything, with some later type-specific initialization? Or do they have separate code paths? If it's a unified allocation, I'll be interested in how the type information is stored and accessed.
Once the `inode` is set, I'll examine any interactions with the superblock. Are they adding the new inode to the superblock's inode list? Are they setting any superblock-specific parameters on the inode? And are there any initial permissions or ownership settings being applied? I want to see how kernfs integrates with the larger filesystem framework.
Finally, I'll try to get a sense of the locking involved. What locks are held during the initialization process? Are they using spinlocks, mutexes, or something else? Understanding the locking scheme is critical for concurrency and data consistency. Does this routine require any form of locking during its execution?
|
| 42/4 |
2026/07/25 00:53 |
tool |
read-file |
0m
Args: map[File:fs/kernfs/inode.c FirstLine:100 LineCount:66]
Results: map[Contents: 100: {
101: int ret;
102: struct kernfs_root *root = kernfs_root(kn);
103:
104: down_write(&root->kernfs_iattr_rwsem);
105: ret = __kernfs_setattr(kn, iattr);
106: up_write(&root->kernfs_iattr_rwsem);
107: return ret;
108: }
109:
110: int kernfs_iop_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
111: struct iattr *iattr)
112: {
113: struct inode *inode = d_inode(dentry);
114: struct kernfs_node *kn = inode->i_private;
115: struct kernfs_root *root;
116: int error;
117:
118: if (!kn)
119: return -EINVAL;
120:
121: root = kernfs_root(kn);
122: down_write(&root->kernfs_iattr_rwsem);
123: error = setattr_prepare(&nop_mnt_idmap, dentry, iattr);
124: if (error)
125: goto out;
126:
127: error = __kernfs_setattr(kn, iattr);
128: if (error)
129: goto out;
130:
131: /* this ignores size changes */
132: setattr_copy(&nop_mnt_idmap, inode, iattr);
133:
134: out:
135: up_write(&root->kernfs_iattr_rwsem);
136: return error;
137: }
138:
139: ssize_t kernfs_iop_listxattr(struct dentry *dentry, char *buf, size_t size)
140: {
141: struct kernfs_node *kn = kernfs_dentry_node(dentry);
142: struct kernfs_iattrs *attrs;
143:
144: attrs = kernfs_iattrs(kn);
145: if (!attrs)
146: return -ENOMEM;
147:
148: return simple_xattr_list(d_inode(dentry), &attrs->xattrs, buf, size);
149: }
150:
151: static inline void set_default_inode_attr(struct inode *inode, umode_t mode)
152: {
153: inode->i_mode = mode;
154: simple_inode_init_ts(inode);
155: }
156:
157: static inline void set_inode_attr(struct inode *inode,
158: struct kernfs_iattrs *attrs)
159: {
160: inode->i_uid = attrs->ia_uid;
161: inode->i_gid = attrs->ia_gid;
162: inode_set_atime_to_ts(inode, attrs->ia_atime);
163: inode_set_mtime_to_ts(inode, attrs->ia_mtime);
164: inode_set_ctime_to_ts(inode, attrs->ia_ctime);
165: }
]
|
| 43/4 |
2026/07/25 00:53 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 11280
output: 38
thoughts: 39
Thoughts: Okay, let's dive into this. Here's how I'd approach understanding the `kernfs_get_inode` function in `fs/kernfs/inode.c`, specifically focusing on how user and group IDs are handled when the attributes aren't explicitly provided. I'll need to dig into the code and think it through:
**Investigating Kernfs Inode Initialization**
Alright, my first instinct is to crack open the `kernfs_get_inode` function in `fs/kernfs/inode.c`. I'm going to look at that source file, because I need to understand what's happening when `attrs` is not supplied. The key here is to figure out the default behavior, i.e., how are the UID and GID assigned to the new inode in the absence of explicit `attrs` information? Are they being set to the current process's UID/GID? Are there some hardcoded defaults? Does it pull from the mounting user? I need to clarify what the initialization pathway is, because it's vital for security.
|
| 44/4 |
2026/07/25 00:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/kernfs/inode.c Name:kernfs_get_inode]
Results: map[SourceCode: 233: /**
234: * kernfs_get_inode - get inode for kernfs_node
235: * @sb: super block
236: * @kn: kernfs_node to allocate inode for
237: *
238: * Get inode for @kn. If such inode doesn't exist, a new inode is
239: * allocated and basics are initialized. New inode is returned
240: * locked.
241: *
242: * Locking:
243: * Kernel thread context (may sleep).
244: *
245: * Return:
246: * Pointer to allocated inode on success, %NULL on failure.
247: */
248: struct inode *kernfs_get_inode(struct super_block *sb, struct kernfs_node *kn)
249: {
250: struct inode *inode;
251:
252: inode = iget_locked(sb, kernfs_ino(kn));
253: if (inode && (inode_state_read_once(inode) & I_NEW))
254: kernfs_init_inode(kn, inode);
255:
256: return inode;
257: }
SourceFile:fs/kernfs/inode.c]
|
| 45/4 |
2026/07/25 00:53 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 11982
output: 38
thoughts: 13
Thoughts: Okay, let's dive into `kernfs_init_inode`. I need to get a clear picture of how this function sets up a new inode within the kernfs filesystem. As an expert in this, I'm already anticipating the key aspects: how it handles the fundamental inode initialization, any specific kernfs-related quirks in the setup, and what data structures are involved. I'm thinking about the common inode initialization routines, such as setting the i_mode, i_uid, i_gid, and i_op fields. Then, it's about seeing what the kernfs layer adds on top of that. Does it associate specific kernfs data with the inode? Does it manage any kernfs-specific locking or reference counting? I'm particularly interested in any interaction with the kernfs directory structure and how new nodes are linked into it. Let's see if this function handles the default permissions, or if those are determined somewhere else in the kernfs code. Finally, I'll pay close attention to the allocation of any specific kernfs-related metadata structures that might be attached to the inode. I'm looking for a solid foundation to understand how kernfs inodes operate in the bigger picture of the kernel filesystem architecture.
|
| 46/4 |
2026/07/25 00:53 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/kernfs/inode.c Name:kernfs_init_inode]
Results: map[SourceCode: 200: static void kernfs_init_inode(struct kernfs_node *kn, struct inode *inode)
201: {
202: kernfs_get(kn);
203: inode->i_private = kn;
204: inode->i_mapping->a_ops = &ram_aops;
205: inode->i_op = &kernfs_iops;
206: inode->i_generation = kernfs_gen(kn);
207:
208: set_default_inode_attr(inode, kn->mode);
209: kernfs_refresh_inode(kn, inode);
210:
211: /* initialize inode according to type */
212: switch (kernfs_type(kn)) {
213: case KERNFS_DIR:
214: inode->i_op = &kernfs_dir_iops;
215: inode->i_fop = &kernfs_dir_fops;
216: if (kn->flags & KERNFS_EMPTY_DIR)
217: make_empty_dir_inode(inode);
218: break;
219: case KERNFS_FILE:
220: inode->i_size = kn->attr.size;
221: inode->i_fop = &kernfs_file_fops;
222: break;
223: case KERNFS_LINK:
224: inode->i_op = &kernfs_symlink_iops;
225: break;
226: default:
227: BUG();
228: }
229:
230: unlock_new_inode(inode);
231: }
SourceFile:fs/kernfs/inode.c]
|
| 47/4 |
2026/07/25 00:53 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 12805
output: 29
thoughts: 156
Thoughts: **Examining Inode Attribute Initialization**
Okay, so I'm tracing the attribute setting in `kernfs_refresh_inode`. It looks like if the `attrs` pointer is NULL, then the function skips the `set_inode_attr(inode, attrs)` call. That's a little unusual. If `attrs` is indeed NULL, how are the `inode->i_uid` and `inode->i_gid` fields initialized? They're critical security attributes, so they must be set to something sensible.
Aha, that's it! When we call `iget_locked`, a new inode is allocated if necessary. And `alloc_inode` is used to do that allocation. I should probably double-check that, but I'm betting that the `alloc_inode` function (or possibly its associated initialization routines like `inode_init_always` within `fs/inode.c`) performs the initial zero-initialization or default value assignment for the inode structure. That initialization would take care of setting `i_uid` and `i_gid` to the system defaults, like `GLOBAL_ROOT_UID` and `GLOBAL_ROOT_GID`, in the case that the `attrs` pointer is NULL. I should confirm that `alloc_inode` or `inode_init_always` is where this is happening. Let me just check the code in `fs/inode.c`...
|
| 48/4 |
2026/07/25 00:54 |
tool |
grepper |
0m
Error:
Args: map[Expression:i_uid = PathPrefix:fs/inode.c]
Results:
|
| 49/4 |
2026/07/25 00:54 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 13302
output: 28
thoughts: 18
Thoughts: Okay, here's my interpretation of that thought process, from an expert's perspective:
**Diving into the Kernel's Core: Locating `i_uid`**
Right, let's get down to business. My immediate focus is pinpointing the definition and usage of `i_uid` within the Linux kernel, specifically within the `fs/inode.c` file. This indicates a deep dive into the inode structure and its associated file system operations. The fact that the search is targeted suggests a specific debugging need or a requirement to understand user ID (UID) handling at the file system level. It's likely related to either security audits, permission analysis, or perhaps even investigating a potential UID-related bug. Given the centrality of `inode.c`, the information I'll find there will be critical for understanding how UIDs are managed in the file system context. I'll be looking for where `i_uid` is declared, how it's initialized, where it's modified, and how it's accessed within the relevant functions. This is going to be good!
|
| 50/4 |
2026/07/25 00:54 |
tool |
grepper |
0m
Args: map[Expression:i_uid PathPrefix:fs/inode.c]
Results: map[Output:fs/inode.c=223=int inode_init_always_gfp(struct super_block *sb, struct inode *inode, gfp_t gfp)
--
fs/inode.c-243- inode->i_opflags |= IOP_MGTIME;
fs/inode.c:244: i_uid_write(inode, 0);
fs/inode.c-245- i_gid_write(inode, 0);
--
fs/inode.c=2271=bool atime_needs_update(const struct path *path, struct inode *inode)
--
fs/inode.c-2278-
fs/inode.c:2279: /* Atime updates will likely cause i_uid and i_gid to be written
fs/inode.c-2280- * back improprely if their true value is unknown to the vfs.
--
fs/inode.c=2708=EXPORT_SYMBOL(init_special_inode);
--
fs/inode.c-2719- * care to map the inode according to @idmap before checking permissions
fs/inode.c:2720: * and initializing i_uid and i_gid. On non-idmapped mounts or if permission
fs/inode.c-2721- * checking is to be performed on the raw inode simply pass @nop_mnt_idmap.
--
fs/inode.c=2753=bool inode_owner_or_capable(struct mnt_idmap *idmap,
--
fs/inode.c-2758-
fs/inode.c:2759: vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/inode.c-2760- if (vfsuid_eq_kuid(vfsuid, current_fsuid()))
]
|
| 51/4 |
2026/07/25 00:54 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 14036
output: 33
thoughts: 132
Thoughts: **Initial Inode Initialization and Permissions**
Okay, so I'm looking at this inode initialization. I see these calls: `i_uid_write(inode, 0);` and `i_gid_write(inode, 0);` within `inode_init_always_gfp`. Right. That means, by default, the `i_uid` and `i_gid` are getting initialized to `GLOBAL_ROOT_UID` and `GLOBAL_ROOT_GID`. Of course, that translates to UID 0 and GID 0 in the initial user namespace β the root user. Makes sense. But now I'm thinking about permissions...
I need to see how these initializations affect file access, so I'll go check out `generic_permission` within `fs/namei.c`. I need to understand how the inode's initial uid/gid impacts permission checks. Let me just search for `generic_permission` in `fs/namei.c`.
|
| 52/4 |
2026/07/25 00:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c Name:generic_permission]
Results: map[SourceCode: 499: /**
500: * generic_permission - check for access rights on a Posix-like filesystem
501: * @idmap: idmap of the mount the inode was found from
502: * @inode: inode to check access rights for
503: * @mask: right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC,
504: * %MAY_NOT_BLOCK ...)
505: *
506: * Used to check for read/write/execute permissions on a file.
507: * We use "fsuid" for this, letting us set arbitrary permissions
508: * for filesystem access without changing the "normal" uids which
509: * are used for other things.
510: *
511: * generic_permission is rcu-walk aware. It returns -ECHILD in case an rcu-walk
512: * request cannot be satisfied (eg. requires blocking or too much complexity).
513: * It would then be called again in ref-walk mode.
514: *
515: * If the inode has been found through an idmapped mount the idmap of
516: * the vfsmount must be passed through @idmap. This function will then take
517: * care to map the inode according to @idmap before checking permissions.
518: * On non-idmapped mounts or if permission checking is to be performed on the
519: * raw inode simply pass @nop_mnt_idmap.
520: */
521: int generic_permission(struct mnt_idmap *idmap, struct inode *inode,
522: int mask)
523: {
524: int ret;
525:
526: /*
527: * Do the basic permission checks.
528: */
529: ret = acl_permission_check(idmap, inode, mask);
530: if (ret != -EACCES)
531: return ret;
532:
533: if (S_ISDIR(inode->i_mode)) {
534: /* DACs are overridable for directories */
535: if (!(mask & MAY_WRITE))
536: if (capable_wrt_inode_uidgid(idmap, inode,
537: CAP_DAC_READ_SEARCH))
538: return 0;
539: if (capable_wrt_inode_uidgid(idmap, inode,
540: CAP_DAC_OVERRIDE))
541: return 0;
542: return -EACCES;
543: }
544:
545: /*
546: * Searching includes executable on directories, else just read.
547: */
548: mask &= MAY_READ | MAY_WRITE | MAY_EXEC;
549: if (mask == MAY_READ)
550: if (capable_wrt_inode_uidgid(idmap, inode,
551: CAP_DAC_READ_SEARCH))
552: return 0;
553: /*
554: * Read/write DACs are always overridable.
555: * Executable DACs are overridable when there is
556: * at least one exec bit set.
557: */
558: if (!(mask & MAY_EXEC) || (inode->i_mode & S_IXUGO))
559: if (capable_wrt_inode_uidgid(idmap, inode,
560: CAP_DAC_OVERRIDE))
561: return 0;
562:
563: return -EACCES;
564: }
SourceFile:fs/namei.c]
|
| 53/4 |
2026/07/25 00:54 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 15486
output: 35
thoughts: 21
Thoughts: Here's my take on those thoughts, tailored for an expert audience:
**Diving into `fs/namei.c` and `acl_permission_check`**
Alright, let's get down to brass tacks. I'm thinking about the nitty-gritty of filesystem security β specifically, the `acl_permission_check` function lurking in `fs/namei.c`. I need to refresh my understanding of how ACLs interact with the standard permission checks during name lookup. I'm envisioning the call stack and how this function integrates with the VFS layer.
I want to see how it handles different ACL types, the interplay of the effective ID, and any potential vulnerabilities or performance bottlenecks. I'm wondering if there are any subtle race conditions or permission escalation scenarios that might be lurking within that logic. Time to dig into the source and see what's what!
|
| 54/4 |
2026/07/25 00:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c Name:acl_permission_check]
Results: map[SourceCode: 422: /**
423: * acl_permission_check - perform basic UNIX permission checking
424: * @idmap: idmap of the mount the inode was found from
425: * @inode: inode to check permissions on
426: * @mask: right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC ...)
427: *
428: * This function performs the basic UNIX permission checking. Since this
429: * function may retrieve POSIX acls it needs to know whether it is called from a
430: * blocking or non-blocking context and thus cares about the MAY_NOT_BLOCK bit.
431: *
432: * If the inode has been found through an idmapped mount the idmap of
433: * the vfsmount must be passed through @idmap. This function will then take
434: * care to map the inode according to @idmap before checking permissions.
435: * On non-idmapped mounts or if permission checking is to be performed on the
436: * raw inode simply pass @nop_mnt_idmap.
437: */
438: static int acl_permission_check(struct mnt_idmap *idmap,
439: struct inode *inode, int mask)
440: {
441: unsigned int mode = inode->i_mode;
442: vfsuid_t vfsuid;
443:
444: /*
445: * Common cheap case: everybody has the requested
446: * rights, and there are no ACLs to check. No need
447: * to do any owner/group checks in that case.
448: *
449: * - 'mask&7' is the requested permission bit set
450: * - multiplying by 0111 spreads them out to all of ugo
451: * - '& ~mode' looks for missing inode permission bits
452: * - the '!' is for "no missing permissions"
453: *
454: * After that, we just need to check that there are no
455: * ACL's on the inode - do the 'IS_POSIXACL()' check last
456: * because it will dereference the ->i_sb pointer and we
457: * want to avoid that if at all possible.
458: */
459: if (!((mask & 7) * 0111 & ~mode)) {
460: if (no_acl_inode(inode))
461: return 0;
462: if (!IS_POSIXACL(inode))
463: return 0;
464: }
465:
466: /* Are we the owner? If so, ACL's don't matter */
467: vfsuid = i_uid_into_vfsuid(idmap, inode);
468: if (likely(vfsuid_eq_kuid(vfsuid, current_fsuid()))) {
469: mask &= 7;
470: mode >>= 6;
471: return (mask & ~mode) ? -EACCES : 0;
472: }
473:
474: /* Do we have ACL's? */
475: if (IS_POSIXACL(inode) && (mode & S_IRWXG)) {
476: int error = check_acl(idmap, inode, mask);
477: if (error != -EAGAIN)
478: return error;
479: }
480:
481: /* Only RWX matters for group/other mode bits */
482: mask &= 7;
483:
484: /*
485: * Are the group permissions different from
486: * the other permissions in the bits we care
487: * about? Need to check group ownership if so.
488: */
489: if (mask & (mode ^ (mode >> 3))) {
490: vfsgid_t vfsgid = i_gid_into_vfsgid(idmap, inode);
491: if (vfsgid_in_group_p(vfsgid))
492: mode >>= 3;
493: }
494:
495: /* Bits in 'mode' clear that we require? */
496: return (mask & ~mode) ? -EACCES : 0;
497: }
SourceFile:fs/namei.c]
|
| 55/4 |
2026/07/25 00:54 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 17009
output: 39
thoughts: 544
Thoughts: **Analyzing File Permissions and User Namespaces**
Okay, let's break this down meticulously. We're looking at `/sys/bus/platform/drivers/*/unbind`. The file's mode is `0200`, meaning it's write-only, and only the owner can write to it. The owner, crucially, is `GLOBAL_ROOT_UID`, which equates to uid 0 in the initial user namespace. The group is `GLOBAL_ROOT_GID`, also uid 0 in the initial namespace.
Now, the core question: can a process running as root *inside* a user namespace (but unprivileged in the initial namespace) write to this file? Let's trace it. If I'm running as root within a user namespace, my `current_fsuid()` *within that namespace* will indeed be uid 0. But we're dealing with the kernel's internal representation, the `kuid`. This is key. The kernel stores the actual, globally relevant uid.
Here's the problem: when a user namespace is involved, uid 0 inside the namespace gets *mapped* to some unprivileged uid in the initial namespace, let's say 100000. So `current_fsuid()` actually returns a `kuid_t` representing uid 100000. But the inode's `i_uid` is `GLOBAL_ROOT_UID`, which is uid 0 in the initial namespace.
Therefore, the crucial check, `vfsuid_eq_kuid(vfsuid, current_fsuid())`, compares `vfsuid` (which is `GLOBAL_ROOT_UID` since `idmap` is `&nop_mnt_idmap`) against my `current_fsuid()`. Essentially, the kernel is comparing uid 0 in the initial namespace against uid 100000 (mapped from the user namespace). And they are *not* equal.
Thus, `acl_permission_check` will proceed to the group and "other" checks. Since I'm not the owner and not in the group, we land on the "other" check. Here, the permission bits checked are `mode & 7` which is `0200 & 7 = 0`. This is critical: "other" has *no* permissions because we're left with zero!
This is where the access control check comes in: `acl_permission_check` will return `-EACCES`. And then `generic_permission` will be called, which will then check for the `CAP_DAC_OVERRIDE` capability with the call to `capable_wrt_inode_uidgid`.
Let's dig into `capable_wrt_inode_uidgid` to understand this better. I'll need to go search in the kernel source to see what this function actually does.
|
| 56/4 |
2026/07/25 00:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c Name:capable_wrt_inode_uidgid]
Results: map[SourceCode: 463: /**
464: * capable_wrt_inode_uidgid - Check nsown_capable and uid and gid mapped
465: * @idmap: idmap of the mount @inode was found from
466: * @inode: The inode in question
467: * @cap: The capability in question
468: *
469: * Return true if the current task has the given capability targeted at
470: * its own user namespace and that the given inode's uid and gid are
471: * mapped into the current user namespace.
472: */
473: bool capable_wrt_inode_uidgid(struct mnt_idmap *idmap,
474: const struct inode *inode, int cap)
475: {
476: struct user_namespace *ns = current_user_ns();
477:
478: return ns_capable(ns, cap) &&
479: privileged_wrt_inode_uidgid(ns, idmap, inode);
480: }
SourceFile:kernel/capability.c]
|
| 57/4 |
2026/07/25 00:54 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 18532
output: 38
thoughts: 41
Thoughts: Okay, here's my take on those thoughts, framed from an expert's perspective:
**Diving into Privileged UID/GID Context**
Right, let's get down to brass tacks. My initial thought is to zero in on `privileged_wrt_inode_uidgid`. That term immediately screams "permission and security" to me, likely within the context of file system access. This phrase could concern privilege checks on file access, or interactions with file permissions, and how these permissions interact with the user's UID and GID.
My next move is obvious: I need to get into the kernel source and locate every instance of `privileged_wrt_inode_uidgid`. A quick search should reveal the relevant code paths. The instances I find will instantly provide context for how this particular check is implemented, and also reveal the assumptions built into it. I'll be looking for potential race conditions, or security exploits - depending on the type of system I am looking at.
|
| 58/4 |
2026/07/25 00:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/capability.c Name:privileged_wrt_inode_uidgid]
Results: map[SourceCode: 447: /**
448: * privileged_wrt_inode_uidgid - Do capabilities in the namespace work over the inode?
449: * @ns: The user namespace in question
450: * @idmap: idmap of the mount @inode was found from
451: * @inode: The inode in question
452: *
453: * Return true if the inode uid and gid are within the namespace.
454: */
455: bool privileged_wrt_inode_uidgid(struct user_namespace *ns,
456: struct mnt_idmap *idmap,
457: const struct inode *inode)
458: {
459: return vfsuid_has_mapping(ns, i_uid_into_vfsuid(idmap, inode)) &&
460: vfsgid_has_mapping(ns, i_gid_into_vfsgid(idmap, inode));
461: }
SourceFile:kernel/capability.c]
|
| 59/4 |
2026/07/25 00:54 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 19111
output: 21
thoughts: 126
Thoughts: **Analyzing `privileged_wrt_inode_uidgid`**
Okay, here's an interesting snippet. `privileged_wrt_inode_uidgid` hinges on these two checks: `vfsuid_has_mapping` and `vfsgid_has_mapping`. So, it's essentially saying that writes are privileged if both the UID and GID have corresponding mappings within the current namespace.
But, hang on a moment, what *exactly* does `vfsuid_has_mapping` do under the hood? I need to clarify that function. I'll need to dig into the source code to understand its implementation. Let's start with a search for `vfsuid_has_mapping`. Perhaps I should also look for `kuid_has_mapping` to see if there is another closely related function? That's the first step to unpacking the logic.
|
| 60/4 |
2026/07/25 00:54 |
tool |
grepper |
0m
Args: map[Expression:vfsuid_has_mapping]
Results: map[Output:fs/exec.c=1558=static void bprm_fill_uid(struct linux_binprm *bprm, struct file *file)
--
fs/exec.c-1594- /* We ignore suid/sgid if there are no mappings for them in the ns */
fs/exec.c:1595: if (!vfsuid_has_mapping(bprm->cred->user_ns, vfsuid) ||
fs/exec.c-1596- !vfsgid_has_mapping(bprm->cred->user_ns, vfsgid))
--
fs/inode.c=2753=bool inode_owner_or_capable(struct mnt_idmap *idmap,
--
fs/inode.c-2763- ns = current_user_ns();
fs/inode.c:2764: if (vfsuid_has_mapping(ns, vfsuid) && ns_capable(ns, CAP_FOWNER))
fs/inode.c-2765- return true;
--
include/linux/mnt_idmapping.h=151=static inline bool vfsuid_has_fsmapping(struct mnt_idmap *idmap,
--
include/linux/mnt_idmapping.h-157-
include/linux/mnt_idmapping.h:158:static inline bool vfsuid_has_mapping(struct user_namespace *userns,
include/linux/mnt_idmapping.h-159- vfsuid_t vfsuid)
--
kernel/capability.c=455=bool privileged_wrt_inode_uidgid(struct user_namespace *ns,
--
kernel/capability.c-458-{
kernel/capability.c:459: return vfsuid_has_mapping(ns, i_uid_into_vfsuid(idmap, inode)) &&
kernel/capability.c-460- vfsgid_has_mapping(ns, i_gid_into_vfsgid(idmap, inode));
]
|
| 61/4 |
2026/07/25 00:54 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 19956
output: 38
thoughts: 23
Thoughts: Here's a summary of my thought process, as requested:
**Diving into Mount Namespace ID Mapping**
Okay, time to get my hands dirty. I need to understand the specifics of how ID mapping is handled within the Linux kernel, specifically concerning mount namespaces. To do this, I think I'll start by taking a look at `include/linux/mnt_idmapping.h`. I'm going to focus my initial exploration around line 158 of that file. That should give me a good starting point to grasp the core structures and functions related to ID mapping for mounts. I'm expecting to find the key data structures and maybe some macro definitions that define the rules of how the kernel manages IDs within mount namespaces.
|
| 62/4 |
2026/07/25 00:54 |
tool |
read-file |
0m
Args: map[File:include/linux/mnt_idmapping.h FirstLine:145 LineCount:30]
Results: map[Contents: 145: * Check whether @vfsuid has a mapping in the filesystem idmapping. Use this
146: * function to check whether the filesystem idmapping has a mapping for
147: * @vfsuid.
148: *
149: * Return: true if @vfsuid has a mapping in the filesystem, false if not.
150: */
151: static inline bool vfsuid_has_fsmapping(struct mnt_idmap *idmap,
152: struct user_namespace *fs_userns,
153: vfsuid_t vfsuid)
154: {
155: return uid_valid(from_vfsuid(idmap, fs_userns, vfsuid));
156: }
157:
158: static inline bool vfsuid_has_mapping(struct user_namespace *userns,
159: vfsuid_t vfsuid)
160: {
161: return from_kuid(userns, AS_KUIDT(vfsuid)) != (uid_t)-1;
162: }
163:
164: /**
165: * vfsuid_into_kuid - convert vfsuid into kuid
166: * @vfsuid: the vfsuid to convert
167: *
168: * This can be used when a vfsuid is committed as a kuid.
169: *
170: * Return: a kuid with the value of @vfsuid
171: */
172: static inline kuid_t vfsuid_into_kuid(vfsuid_t vfsuid)
173: {
174: return AS_KUIDT(vfsuid);
]
|
| 63/4 |
2026/07/25 00:54 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 20672
output: 33
thoughts: 491
Thoughts: **Analyzing User Namespace Permissions for Sysfs File Access**
Okay, so I'm looking at this `vfsuid_has_mapping` function, and it's checking if a `kuid` (specifically `GLOBAL_ROOT_UID`, which is UID 0 in the initial namespace) has a mapping within the current user namespace, `userns`. The core question here is: if a process is *unprivileged* in the initial namespace, does the `GLOBAL_ROOT_UID` get mapped into its newly created, unprivileged user namespace?
The typical setup for an unprivileged user namespace is via `/proc/self/uid_map`. And the problem immediately becomes clear: since I'm unprivileged in the parent, I can't map *arbitrary* UIDs. I can only map my *own* initial UID to a UID within the new namespace. For example, if I'm UID 1000 in the initial namespace, I can map UID 1000 to, say, UID 0 within the new namespace. But critically, I *can't* map the `GLOBAL_ROOT_UID` (UID 0) from the initial namespace into my unprivileged namespace. It's the `CAP_SETUID` capability in the *parent* namespace that governs this mapping. Without it, I'm restricted.
Therefore, `vfsuid_has_mapping(ns, GLOBAL_ROOT_UID)` *must* return `false`. Which, in turn, makes `privileged_wrt_inode_uidgid` return `false`. This cascade means that `capable_wrt_inode_uidgid(idmap, inode, CAP_DAC_OVERRIDE)` will also return `false`. This directly leads to the process *not* having `CAP_DAC_OVERRIDE` over the target sysfs file.
But is that the *only* check? I need to think this through. Specifically, I'm trying to figure out if running as root *within* a user namespace, but still unprivileged in the initial one, allows the process to write to, say, `/sys/bus/platform/drivers/*/unbind`. So, the crux of the matter must be in the permissions check. So, the next step must be to trace how the permissions are checked when opening a file in sysfs. It should be calling `inode_permission`. So I have to go look at `inode_permission` in `fs/namei.c`.
|
| 64/4 |
2026/07/25 00:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c Name:inode_permission]
Results: map[SourceCode: 616: /**
617: * inode_permission - Check for access rights to a given inode
618: * @idmap: idmap of the mount the inode was found from
619: * @inode: Inode to check permission on
620: * @mask: Right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
621: *
622: * Check for read/write/execute permissions on an inode. We use fs[ug]id for
623: * this, letting us set arbitrary permissions for filesystem access without
624: * changing the "normal" UIDs which are used for other things.
625: *
626: * When checking for MAY_APPEND, MAY_WRITE must also be set in @mask.
627: */
628: int inode_permission(struct mnt_idmap *idmap,
629: struct inode *inode, int mask)
630: {
631: int retval;
632:
633: retval = sb_permission(inode->i_sb, inode, mask);
634: if (unlikely(retval))
635: return retval;
636:
637: if (mask & MAY_WRITE) {
638: /*
639: * Nobody gets write access to an immutable file.
640: */
641: if (unlikely(IS_IMMUTABLE(inode)))
642: return -EPERM;
643:
644: /*
645: * Updating mtime will likely cause i_uid and i_gid to be
646: * written back improperly if their true value is unknown
647: * to the vfs.
648: */
649: if (unlikely(HAS_UNMAPPED_ID(idmap, inode)))
650: return -EACCES;
651: }
652:
653: retval = do_inode_permission(idmap, inode, mask);
654: if (unlikely(retval))
655: return retval;
656:
657: retval = devcgroup_inode_permission(inode, mask);
658: if (unlikely(retval))
659: return retval;
660:
661: return security_inode_permission(inode, mask);
662: }
SourceFile:fs/namei.c]
|
| 65/4 |
2026/07/25 00:54 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 22429
output: 21
thoughts: 122
Thoughts: **Investigating a Potential File System Access Issue**
Okay, hold on a second. Line 649... let me pull that up. Specifically, this block here:
```c
/*
* Updating mtime will likely cause i_uid and i_gid to be
* written back improperly if their true value is unknown
* to the vfs.
*/
if (unlikely(HAS_UNMAPPED_ID(idmap, inode)))
return -EACCES;
```
Interesting. This comment seems to imply something about potential problems with `mtime` updates and user/group ID mapping. It notes that there may be a problem updating `i_uid` and `i_gid` if their "true value is unknown" to the VFS. This check using `HAS_UNMAPPED_ID` is what I need to look closer at.
Hang on... what exactly *is* `HAS_UNMAPPED_ID`? I can't immediately recall. Let me do a quick source code search. I need to understand this macro and what conditions trigger it to know how this impacts the behavior of the inode and, most importantly, the implications for the `mtime` update. It's probably tied to the `idmap` somehow. Let's see.
|
| 66/4 |
2026/07/25 00:54 |
tool |
grepper |
0m
Args: map[Expression:HAS_UNMAPPED_ID]
Results: map[Output:fs/inode.c=2271=bool atime_needs_update(const struct path *path, struct inode *inode)
--
fs/inode.c-2281- */
fs/inode.c:2282: if (HAS_UNMAPPED_ID(mnt_idmap(mnt), inode))
fs/inode.c-2283- return false;
--
fs/namei.c=628=int inode_permission(struct mnt_idmap *idmap,
--
fs/namei.c-648- */
fs/namei.c:649: if (unlikely(HAS_UNMAPPED_ID(idmap, inode)))
fs/namei.c-650- return -EACCES;
--
fs/namei.c=3678=int may_delete_dentry(struct mnt_idmap *idmap, struct inode *dir,
--
fs/namei.c-3704- IS_IMMUTABLE(inode) || IS_SWAPFILE(inode) ||
fs/namei.c:3705: HAS_UNMAPPED_ID(idmap, inode))
fs/namei.c-3706- return -EPERM;
--
fs/namei.c=5741=int vfs_link(struct dentry *old_dentry, struct mnt_idmap *idmap,
--
fs/namei.c-5768- */
fs/namei.c:5769: if (HAS_UNMAPPED_ID(idmap, inode))
fs/namei.c-5770- return -EPERM;
--
fs/xattr.c=103=int may_write_xattr(struct mnt_idmap *idmap, struct inode *inode)
--
fs/xattr.c-108- return -EPERM;
fs/xattr.c:109: if (HAS_UNMAPPED_ID(idmap, inode))
fs/xattr.c-110- return -EPERM;
--
include/linux/fs.h=2091=extern loff_t vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
--
include/linux/fs.h-2177-
include/linux/fs.h:2178:static inline bool HAS_UNMAPPED_ID(struct mnt_idmap *idmap,
include/linux/fs.h-2179- struct inode *inode)
]
|
| 67/4 |
2026/07/25 00:54 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 23508
output: 36
thoughts: 21
Thoughts: Here's a summary of my thought process:
**Diving into the Filesystem Code**
Okay, time to dig into the Linux kernel again. I need to understand something related to the filesystem internals, so I'm going to start by examining `include/linux/fs.h`. It's the central header file for much of the core filesystem data structures and functions. Given the likely scope of the area I am examining, I think line 2178 should be a good starting point. I suspect that area will hold relevant definitions or macros related to what I'm looking for. Let's see what's there...
|
| 68/4 |
2026/07/25 00:54 |
tool |
read-file |
0m
Args: map[File:include/linux/fs.h FirstLine:2170 LineCount:20]
Results: map[Contents:2170: #define IS_ENCRYPTED(inode) ((inode)->i_flags & S_ENCRYPTED)
2171: #define IS_CASEFOLDED(inode) ((inode)->i_flags & S_CASEFOLD)
2172: #define IS_VERITY(inode) ((inode)->i_flags & S_VERITY)
2173:
2174: #define IS_WHITEOUT(inode) (S_ISCHR(inode->i_mode) && \
2175: (inode)->i_rdev == WHITEOUT_DEV)
2176: #define IS_ANON_FILE(inode) ((inode)->i_flags & S_ANON_INODE)
2177:
2178: static inline bool HAS_UNMAPPED_ID(struct mnt_idmap *idmap,
2179: struct inode *inode)
2180: {
2181: return !vfsuid_valid(i_uid_into_vfsuid(idmap, inode)) ||
2182: !vfsgid_valid(i_gid_into_vfsgid(idmap, inode));
2183: }
2184:
2185: static inline void init_sync_kiocb(struct kiocb *kiocb, struct file *filp)
2186: {
2187: *kiocb = (struct kiocb) {
2188: .ki_filp = filp,
2189: .ki_flags = filp->f_iocb_flags,
]
|
| 69/4 |
2026/07/25 00:54 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 24124
output: 35
thoughts: 152
Thoughts: **Analyzing the `HAS_UNMAPPED_ID` Check and its Implications**
Okay, hold on a second! I need to re-evaluate this. The `HAS_UNMAPPED_ID` check is specifically about verifying whether an inode's user and group IDs are valid within the context of the mount's ID map. That's the crux of it. Now, with sysfs, which uses the `&nop_mnt_idmap`, it effectively *bypasses* the normal ID mapping mechanism. Consequently, `i_uid_into_vfsuid` is just going to spit back the raw inode UID, which, in this case, is `GLOBAL_ROOT_UID`. Then `vfsuid_valid` is called, and, well, `GLOBAL_ROOT_UID` *is* a valid UID, right? So, logically, `HAS_UNMAPPED_ID` will inevitably return `false`. It's not detecting any unmapped IDs because there *is* no mapping happening in the first place.
I think I need to step back and look at the bigger picture here. Let me check the `do_inode_permission` function in `fs/namei.c`. It's likely that function will offer additional clarity and help me determine the ultimate consequence of this.
|
| 70/4 |
2026/07/25 00:54 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/namei.c Name:do_inode_permission]
Results: map[SourceCode: 567: /**
568: * do_inode_permission - UNIX permission checking
569: * @idmap: idmap of the mount the inode was found from
570: * @inode: inode to check permissions on
571: * @mask: right to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC ...)
572: *
573: * We _really_ want to just do "generic_permission()" without
574: * even looking at the inode->i_op values. So we keep a cache
575: * flag in inode->i_opflags, that says "this has not special
576: * permission function, use the fast case".
577: */
578: static inline int do_inode_permission(struct mnt_idmap *idmap,
579: struct inode *inode, int mask)
580: {
581: if (unlikely(!(inode->i_opflags & IOP_FASTPERM))) {
582: if (likely(inode->i_op->permission))
583: return inode->i_op->permission(idmap, inode, mask);
584:
585: /* This gets set once for the inode lifetime */
586: spin_lock(&inode->i_lock);
587: inode->i_opflags |= IOP_FASTPERM;
588: spin_unlock(&inode->i_lock);
589: }
590: return generic_permission(idmap, inode, mask);
591: }
SourceFile:fs/namei.c]
|
| 71/4 |
2026/07/25 00:54 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 25046
output: 28
thoughts: 354
Thoughts: **My Deep Dive into Sysfs Permissions**
Okay, so `do_inode_permission` is the entry point, and it's calling `inode->i_op->permission` if it exists. Makes sense. For sysfs and kernfs, that's pointing to `kernfs_iop_permission`, and I remember seeing that earlier. Let me just confirm... Yup, it's calling `kernfs_iop_permission`, and that in turn calls `generic_permission`. Good, good.
Now, from what I've seen, `generic_permission` is the workhorse here. It checks `acl_permission_check`, and then whether the process is capable with `capable_wrt_inode_uidgid`. Right.
But I need to think this through. What if things get tricky? For example, what if a process is running as root *within* a user namespace, but the sysfs file in question is owned by that same user namespace? Hmmm, can sysfs files even *be* owned by a non-initial user namespace? I'm drawing a blank.
Let's quickly check if sysfs supports namespaces in the first place. I know there's support for network namespaces via `sysfs_enable_ns` or `kobject_namespace`, but what about user namespaces? I need to know if I'm even on the right track here.
Okay, let's just search the filesystem code, specifically `fs/sysfs/` and `fs/kernfs/`, for any mention of `user_ns` or `userns`. Time to bust out `grepper` and get some real answers! That should give me a quick overview of what's supported.
|
| 72/4 |
2026/07/25 00:55 |
tool |
grepper |
0m
Args: map[Expression:user_ns PathPrefix:fs/sysfs/]
Results: map[Output:fs/sysfs/mount.c=41=static int sysfs_init_fs_context(struct fs_context *fc)
--
fs/sysfs/mount.c-62-
fs/sysfs/mount.c:63: put_user_ns(fc->user_ns);
fs/sysfs/mount.c:64: fc->user_ns = get_user_ns(netns->user_ns);
fs/sysfs/mount.c-65- }
]
|
| 73/4 |
2026/07/25 00:55 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 25924
output: 35
thoughts: 34
Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience:
**Diving into `fs/sysfs/mount.c` β Namespace Context**
Right, let's zero in on how the `fc->user_ns` is established, specifically in the context of system filesystems. My immediate thought is to check out `fs/sysfs/mount.c` around line 41. I need to refresh my memory on the namespace handling during the mount process. I'm focusing on that particular file because of the `sysfs` connection. I want to see how the user namespace (`fc->user_ns`, the filesystem context or `fc`, is initialized. It's often the lynchpin to understanding permissions and isolation across namespaces in a system file system. Understanding this initialization is crucial for understanding how sysfs mounts interact with namespaces, and how this could relate to other interactions on this level. I bet it involves some kind of delegation or inheritance from the calling process's namespace. Understanding this initial setup will make other debugging much easier.
|
| 74/4 |
2026/07/25 00:55 |
tool |
read-file |
0m
Args: map[File:fs/sysfs/mount.c FirstLine:30 LineCount:45]
Results: map[Contents: 30: if (kfc->ns_tag)
31: kobj_ns_drop(KOBJ_NS_TYPE_NET, kfc->ns_tag);
32: kernfs_free_fs_context(fc);
33: kfree(kfc);
34: }
35:
36: static const struct fs_context_operations sysfs_fs_context_ops = {
37: .free = sysfs_fs_context_free,
38: .get_tree = kernfs_get_tree,
39: };
40:
41: static int sysfs_init_fs_context(struct fs_context *fc)
42: {
43: struct kernfs_fs_context *kfc;
44: struct ns_common *ns;
45:
46: if (!(fc->sb_flags & SB_KERNMOUNT)) {
47: if (!kobj_ns_current_may_mount(KOBJ_NS_TYPE_NET))
48: return -EPERM;
49: }
50:
51: kfc = kzalloc_obj(struct kernfs_fs_context);
52: if (!kfc)
53: return -ENOMEM;
54:
55: kfc->ns_tag = ns = kobj_ns_grab_current(KOBJ_NS_TYPE_NET);
56: kfc->root = sysfs_root;
57: kfc->magic = SYSFS_MAGIC;
58: fc->fs_private = kfc;
59: fc->ops = &sysfs_fs_context_ops;
60: if (ns) {
61: struct net *netns = to_net_ns(ns);
62:
63: put_user_ns(fc->user_ns);
64: fc->user_ns = get_user_ns(netns->user_ns);
65: }
66: fc->global = true;
67: return 0;
68: }
69:
70: static void sysfs_kill_sb(struct super_block *sb)
71: {
72: struct ns_common *ns = (struct ns_common *)kernfs_super_ns(sb);
73:
74: kernfs_kill_sb(sb);
]
|
| 75/4 |
2026/07/25 00:55 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 26883
output: 30
thoughts: 179
Thoughts: **Delving into User Namespaces and File Ownership in sysfs**
Okay, hold on a minute. This is interesting. The code indicates that when a network namespace (`ns`) is involved, `fc->user_ns` is set to `netns->user_ns`. But what exactly is `fc->user_ns` used for? Let me think... Ah, right, `fc->user_ns` represents the user namespace associated with the superblock (`sb->s_user_ns`). Now, the key question is, does this superblock's user namespace directly impact the ownership of the files found within sysfs?
I need to understand if `sb->s_user_ns` influences how the kernel handles user and group IDs (UIDs and GIDs) when working with kernfs or sysfs. Specifically, does kernfs or sysfs perform some kind of translation of `i_uid` and `i_gid` based on the context provided by `sb->s_user_ns`? I need to know how these namespaces are used.
Alright, time to dive into the code. I'll need to search the `fs/kernfs/` and `fs/sysfs/` directories and look specifically for where `s_user_ns` is used. That should give me the answers I'm looking for. Let's see what we can find.
|
| 76/4 |
2026/07/25 00:55 |
tool |
grepper |
0m
Error:
Args: map[Expression:s_user_ns PathPrefix:fs/kernfs/]
Results:
|
| 77/4 |
2026/07/25 00:55 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 27397
output: 30
thoughts: 22
Thoughts: Here's my thought process, summarized as you requested:
**Navigating the Sysfs Landscape**
Okay, so I need to find something, specifically something related to `s_user_ns`. Given my understanding of the Linux kernel and its filesystem structure, my first instinct is to check `fs/sysfs/`. That's the obvious starting point for anything related to sysfs, right? It's where the core sysfs infrastructure is defined and where many attributes are exposed. I'm essentially looking for a pointer, the root or potential entries within that area to explore further. I need to be methodical here, the goal is to map this data. Let's see what we can dig up.
|
| 78/4 |
2026/07/25 00:55 |
tool |
grepper |
0m
Error:
Args: map[Expression:s_user_ns PathPrefix:fs/sysfs/]
Results:
|
| 79/4 |
2026/07/25 00:55 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 27611
output: 27
thoughts: 81
Thoughts: **Investigating the User Namespace in the Virtual File System**
Okay, so I need to understand how `s_user_ns` is actually being utilized within the Virtual File System (VFS). I'm drawing a blank on its specific role, so let's start with a systematic approach. My instinct is that this is core to permissions and user context within the file system, but I need to confirm that.
First, I'll do a broader search across the relevant source code areas. I'll scan `fs/` and, importantly, `include/linux/fs.h`. That's where I'd expect to find the fundamental data structures or at least the declarations related to filesystem objects. A quick overview of these files should provide the necessary context.
Now, to drill down a bit further, I'll need to pinpoint where `s_user_ns` is *used* in practice. `fs/inode.c` and `fs/namei.c` seem like the likely candidates for active usage, given their roles in inode management and path resolution. I'll reach for `grepper` to search within those specific files. This targeted approach should reveal the functions, data structures, or logic directly interacting with `s_user_ns`, hopefully clarifying its specific function and how it relates to the broader VFS architecture. This will give me a solid understanding of how it's being handled.
|
| 80/4 |
2026/07/25 00:55 |
tool |
grepper |
0m
Args: map[Expression:s_user_ns PathPrefix:fs/]
Results: map[Output:fs/9p/acl.c=155=int v9fs_iop_set_acl(struct mnt_idmap *idmap, struct dentry *dentry,
--
fs/9p/acl.c-165- if (acl) {
fs/9p/acl.c:166: retval = posix_acl_valid(inode->i_sb->s_user_ns, acl);
fs/9p/acl.c-167- if (retval)
--
fs/attr.c=94=static bool chown_ok(struct mnt_idmap *idmap,
--
fs/attr.c-103- if (!vfsuid_valid(vfsuid) &&
fs/attr.c:104: ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
fs/attr.c-105- return true;
--
fs/attr.c=121=static bool chgrp_ok(struct mnt_idmap *idmap,
--
fs/attr.c-134- if (!vfsgid_valid(vfsgid) &&
fs/attr.c:135: ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
fs/attr.c-136- return true;
--
fs/attr.c=427=int notify_change(struct mnt_idmap *idmap, struct dentry *dentry,
--
fs/attr.c-521- if (ia_valid & ATTR_UID &&
fs/attr.c:522: !vfsuid_has_fsmapping(idmap, inode->i_sb->s_user_ns,
fs/attr.c-523- attr->ia_vfsuid))
--
fs/attr.c-525- if (ia_valid & ATTR_GID &&
fs/attr.c:526: !vfsgid_has_fsmapping(idmap, inode->i_sb->s_user_ns,
fs/attr.c-527- attr->ia_vfsgid))
--
fs/binfmt_misc.c=642=static struct binfmt_misc *i_binfmt_misc(struct inode *inode)
fs/binfmt_misc.c-643-{
fs/binfmt_misc.c:644: return inode->i_sb->s_user_ns->binfmt_misc;
fs/binfmt_misc.c-645-}
--
fs/binfmt_misc.c=927=static int bm_fill_super(struct super_block *sb, struct fs_context *fc)
--
fs/binfmt_misc.c-929- int err;
fs/binfmt_misc.c:930: struct user_namespace *user_ns = sb->s_user_ns;
fs/binfmt_misc.c-931- struct binfmt_misc *misc;
--
fs/f2fs/recovery.c=242=static int recover_quota_data(struct inode *inode, struct folio *folio)
--
fs/f2fs/recovery.c-251-
fs/f2fs/recovery.c:252: attr.ia_vfsuid = VFSUIDT_INIT(make_kuid(inode->i_sb->s_user_ns, i_uid));
fs/f2fs/recovery.c:253: attr.ia_vfsgid = VFSGIDT_INIT(make_kgid(inode->i_sb->s_user_ns, i_gid));
fs/f2fs/recovery.c-254-
--
fs/fhandle.c=301=static inline int may_decode_fh(struct handle_to_path_ctx *ctx,
--
fs/fhandle.c-332-
fs/fhandle.c:333: if (ns_capable(root->mnt->mnt_sb->s_user_ns, CAP_SYS_ADMIN))
fs/fhandle.c-334- ctx->flags = HANDLE_CHECK_PERMS;
--
fs/fs_context.c=258=static struct fs_context *alloc_fs_context(struct file_system_type *fs_type,
--
fs/fs_context.c-285- case FS_CONTEXT_FOR_SUBMOUNT:
fs/fs_context.c:286: fc->user_ns = get_user_ns(reference->d_sb->s_user_ns);
fs/fs_context.c-287- break;
--
fs/fs_context.c-289- atomic_inc(&reference->d_sb->s_active);
fs/fs_context.c:290: fc->user_ns = get_user_ns(reference->d_sb->s_user_ns);
fs/fs_context.c-291- fc->root = dget(reference);
--
fs/fsopen.c=251=static int vfs_cmd_reconfigure(struct fs_context *fc)
--
fs/fsopen.c-261- sb = fc->root->d_sb;
fs/fsopen.c:262: if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN)) {
fs/fsopen.c-263- fc->phase = FS_CONTEXT_FAILED;
--
fs/fuse/inode.c=1598=static void fuse_sb_defaults(struct super_block *sb)
--
fs/fuse/inode.c-1608- sb->s_iflags |= SB_I_NO_DATA_INTEGRITY;
fs/fuse/inode.c:1609: if (sb->s_user_ns != &init_user_ns)
fs/fuse/inode.c-1610- sb->s_iflags |= SB_I_UNTRUSTED_MOUNTER;
--
fs/ioctl.c=385=static int ioctl_fsfreeze(struct file *filp)
--
fs/ioctl.c-388-
fs/ioctl.c:389: if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
fs/ioctl.c-390- return -EPERM;
--
fs/ioctl.c=402=static int ioctl_fsthaw(struct file *filp)
--
fs/ioctl.c-405-
fs/ioctl.c:406: if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
fs/ioctl.c-407- return -EPERM;
--
fs/namespace.c=1870=static int do_umount(struct mount *mnt, int flags)
--
fs/namespace.c-1932- */
fs/namespace.c:1933: if (!ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
fs/namespace.c-1934- return -EPERM;
--
fs/namespace.c=2021=static int can_umount(const struct path *path, int flags)
--
fs/namespace.c-2033- return -EINVAL;
fs/namespace.c:2034: if (flags & MNT_FORCE && !ns_capable(sb->s_user_ns, CAP_SYS_ADMIN))
fs/namespace.c-2035- return -EPERM;
--
fs/namespace.c=3375=static int do_remount(const struct path *path, int sb_flags,
--
fs/namespace.c-3405- err = -EPERM;
fs/namespace.c:3406: if (ns_capable(sb->s_user_ns, CAP_SYS_ADMIN)) {
fs/namespace.c-3407- err = reconfigure_super(fc);
--
fs/namespace.c=4799=static int can_idmap_mount(const struct mount_kattr *kattr, struct mount *mnt)
--
fs/namespace.c-4801- struct vfsmount *m = &mnt->mnt;
fs/namespace.c:4802: struct user_namespace *fs_userns = m->mnt_sb->s_user_ns;
fs/namespace.c-4803-
--
fs/namespace.c-4810- */
fs/namespace.c:4811: if (kattr->mnt_userns == m->mnt_sb->s_user_ns)
fs/namespace.c-4812- return -EINVAL;
--
fs/namespace.c=6445=bool mnt_may_suid(struct vfsmount *mnt)
--
fs/namespace.c-6454- return !(mnt->mnt_flags & MNT_NOSUID) && check_mnt(real_mount(mnt)) &&
fs/namespace.c:6455: current_in_userns(mnt->mnt_sb->s_user_ns);
fs/namespace.c-6456-}
--
fs/notify/fanotify/fanotify_user.c=1856=static int do_fanotify_mark(int fanotify_fd, unsigned int flags, __u64 mask,
--
fs/notify/fanotify/fanotify_user.c-2058- } else if (obj_type == FSNOTIFY_OBJ_TYPE_VFSMOUNT) {
fs/notify/fanotify/fanotify_user.c:2059: user_ns = path.mnt->mnt_sb->s_user_ns;
fs/notify/fanotify/fanotify_user.c-2060- obj = path.mnt;
fs/notify/fanotify/fanotify_user.c-2061- } else if (obj_type == FSNOTIFY_OBJ_TYPE_SB) {
fs/notify/fanotify/fanotify_user.c:2062: user_ns = path.mnt->mnt_sb->s_user_ns;
fs/notify/fanotify/fanotify_user.c-2063- obj = path.mnt->mnt_sb;
--
fs/posix_acl.c=935=set_posix_acl(struct mnt_idmap *idmap, struct dentry *dentry,
--
fs/posix_acl.c-950- if (acl) {
fs/posix_acl.c:951: int ret = posix_acl_valid(inode->i_sb->s_user_ns, acl);
fs/posix_acl.c-952- if (ret)
--
fs/quota/dquot.c=950=struct dquot *dqget(struct super_block *sb, struct kqid qid)
--
fs/quota/dquot.c-954-
fs/quota/dquot.c:955: if (!qid_has_mapping(sb->s_user_ns, qid))
fs/quota/dquot.c-956- return ERR_PTR(-EINVAL);
--
fs/quota/dquot.c=2429=int dquot_load_quota_sb(struct super_block *sb, int type, int format_id,
--
fs/quota/dquot.c-2450- /* Filesystems outside of init_user_ns not yet supported */
fs/quota/dquot.c:2451: if (sb->s_user_ns != &init_user_ns) {
fs/quota/dquot.c-2452- error = -EINVAL;
--
fs/quota/quota.c=201=static int quota_getquota(struct super_block *sb, int type, qid_t id,
--
fs/quota/quota.c-211- qid = make_kqid(current_user_ns(), type, id);
fs/quota/quota.c:212: if (!qid_has_mapping(sb->s_user_ns, qid))
fs/quota/quota.c-213- return -EINVAL;
--
fs/quota/quota.c=237=static int quota_getnextquota(struct super_block *sb, int type, qid_t id,
--
fs/quota/quota.c-247- qid = make_kqid(current_user_ns(), type, id);
fs/quota/quota.c:248: if (!qid_has_mapping(sb->s_user_ns, qid))
fs/quota/quota.c-249- return -EINVAL;
--
fs/quota/quota.c=287=static int quota_setquota(struct super_block *sb, int type, qid_t id,
--
fs/quota/quota.c-306- qid = make_kqid(current_user_ns(), type, id);
fs/quota/quota.c:307: if (!qid_has_mapping(sb->s_user_ns, qid))
fs/quota/quota.c-308- return -EINVAL;
--
fs/quota/quota.c=624=static int quota_setxquota(struct super_block *sb, int type, qid_t id,
--
fs/quota/quota.c-635- qid = make_kqid(current_user_ns(), type, id);
fs/quota/quota.c:636: if (!qid_has_mapping(sb->s_user_ns, qid))
fs/quota/quota.c-637- return -EINVAL;
fs/quota/quota.c-638- /* Are we actually setting timer / warning limits for all users? */
fs/quota/quota.c:639: if (from_kqid(sb->s_user_ns, qid) == 0 &&
fs/quota/quota.c-640- fdq.d_fieldmask & (FS_DQ_WARNS_MASK | FS_DQ_TIMER_MASK)) {
--
fs/quota/quota.c=705=static int quota_getxquota(struct super_block *sb, int type, qid_t id,
--
fs/quota/quota.c-715- qid = make_kqid(current_user_ns(), type, id);
fs/quota/quota.c:716: if (!qid_has_mapping(sb->s_user_ns, qid))
fs/quota/quota.c-717- return -EINVAL;
--
fs/quota/quota.c=731=static int quota_getnextxquota(struct super_block *sb, int type, qid_t id,
--
fs/quota/quota.c-742- qid = make_kqid(current_user_ns(), type, id);
fs/quota/quota.c:743: if (!qid_has_mapping(sb->s_user_ns, qid))
fs/quota/quota.c-744- return -EINVAL;
--
fs/super.c=275=static void destroy_super_work(struct work_struct *work)
--
fs/super.c-280- security_sb_free(s);
fs/super.c:281: put_user_ns(s->s_user_ns);
fs/super.c-282- kfree(s->s_subtype);
--
fs/super.c=317=static struct super_block *alloc_super(struct file_system_type *type, int flags,
--
fs/super.c-326-
fs/super.c:327: s->s_user_ns = get_user_ns(user_ns);
fs/super.c-328- init_rwsem(&s->s_umount);
--
fs/super.c-357- s->s_flags = flags;
fs/super.c:358: if (s->s_user_ns != &init_user_ns)
fs/super.c-359- s->s_iflags |= SB_I_NODEV;
--
fs/super.c=735=struct super_block *sget_fc(struct fs_context *fc,
--
fs/super.c-744- /*
fs/super.c:745: * Never allow s_user_ns != &init_user_ns when FS_USERNS_MOUNT or
fs/super.c-746- * FS_USERNS_DELEGATABLE is not set, as the filesystem is likely
--
fs/super.c-796-share_extant_sb:
fs/super.c:797: if (user_ns != old->s_user_ns || fc->exclusive) {
fs/super.c-798- spin_unlock(&sb_lock);
--
fs/xfs/xfs_icache.c=1634=xfs_blockgc_free_dquots(
--
fs/xfs/xfs_icache.c-1653- if (XFS_IS_UQUOTA_ENFORCED(mp) && udqp && xfs_dquot_lowsp(udqp)) {
fs/xfs/xfs_icache.c:1654: icw.icw_uid = make_kuid(mp->m_super->s_user_ns, udqp->q_id);
fs/xfs/xfs_icache.c-1655- icw.icw_flags |= XFS_ICWALK_FLAG_UID;
--
fs/xfs/xfs_icache.c-1659- if (XFS_IS_UQUOTA_ENFORCED(mp) && gdqp && xfs_dquot_lowsp(gdqp)) {
fs/xfs/xfs_icache.c:1660: icw.icw_gid = make_kgid(mp->m_super->s_user_ns, gdqp->q_id);
fs/xfs/xfs_icache.c-1661- icw.icw_flags |= XFS_ICWALK_FLAG_GID;
--
fs/xfs/xfs_itable.c=64=xfs_bulkstat_one_int(
--
fs/xfs/xfs_itable.c-70-{
fs/xfs/xfs_itable.c:71: struct user_namespace *sb_userns = mp->m_super->s_user_ns;
fs/xfs/xfs_itable.c-72- struct xfs_inode *ip; /* incore inode pointer */
--
fs/xfs/xfs_qm.c=1846=xfs_qm_vop_dqalloc(
--
fs/xfs/xfs_qm.c-1857- struct inode *inode = VFS_I(ip);
fs/xfs/xfs_qm.c:1858: struct user_namespace *user_ns = inode->i_sb->s_user_ns;
fs/xfs/xfs_qm.c-1859- struct xfs_dquot *uq = NULL;
--
fs/xfs/xfs_trace.h=4897=DECLARE_EVENT_CLASS(xfs_icwalk_class,
--
fs/xfs/xfs_trace.h-4913- __entry->flags = icw ? icw->icw_flags : 0;
fs/xfs/xfs_trace.h:4914: __entry->uid = icw ? from_kuid(mp->m_super->s_user_ns,
fs/xfs/xfs_trace.h-4915- icw->icw_uid) : 0;
fs/xfs/xfs_trace.h:4916: __entry->gid = icw ? from_kgid(mp->m_super->s_user_ns,
fs/xfs/xfs_trace.h-4917- icw->icw_gid) : 0;
]
|
| 81/4 |
2026/07/25 00:55 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 33063
output: 38
thoughts: 133
Thoughts: **Examining Potential User Namespace Issues**
Okay, hold on a moment, before I proceed further, I need to double-check something. Let's delve into `fs/super.c` around line 745. The code snippet there states a crucial restriction. It's essentially saying that we shouldn't allow `s_user_ns` to deviate from the initial user namespace (`&init_user_ns`) unless either `FS_USERNS_MOUNT` or `FS_USERNS_DELEGATABLE` flags are set. The underlying implication is that the filesystem might not be properly architected to manage user namespaces if these flags are absent.
Now, the important question is: does `sysfs` adhere to this? Specifically, is `sysfs` registered with `FS_USERNS_MOUNT`? I need to confirm this. To get to the bottom of this, I'll switch my focus to `fs/sysfs/mount.c`. I will conduct a search for `sysfs_fs_type` within that file, as this is how I would expect the filesystem type to be defined and registered.
|
| 82/4 |
2026/07/25 00:55 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/sysfs/mount.c Name:sysfs_fs_type]
Results: map[SourceCode: 78: static struct file_system_type sysfs_fs_type = {
79: .name = "sysfs",
80: .init_fs_context = sysfs_init_fs_context,
81: .kill_sb = sysfs_kill_sb,
82: .fs_flags = FS_USERNS_MOUNT | FS_USERNS_MOUNT_RESTRICTED,
83: };
SourceFile:fs/sysfs/mount.c]
|
| 83/4 |
2026/07/25 00:55 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 33623
output: 25
thoughts: 80
Thoughts: Okay, here's my take on those thoughts, framed as my own mental processing:
**Delving into Filesystem Flags**
Right, so I see `sysfs_fs_type` defined with `.fs_flags = FS_USERNS_MOUNT | FS_USERNS_MOUNT_RESTRICTED`. That's interesting. I immediately latch onto `FS_USERNS_MOUNT_RESTRICTED`. Now, what exactly does *that* flag do? My memory's a bit fuzzy on its implications. Time to dig into the kernel source. I need to refresh my understanding of how this flag impacts the behavior of user namespace mounts. Let's see what the code has to say about `FS_USERNS_MOUNT_RESTRICTED`.
|
| 84/4 |
2026/07/25 00:55 |
tool |
grepper |
0m
Args: map[Expression:FS_USERNS_MOUNT_RESTRICTED]
Results: map[Output:fs/namespace.c=1076=static void mnt_add_to_ns(struct mnt_namespace *ns, struct mount *mnt)
--
fs/namespace.c-1101-
fs/namespace.c:1102: if ((mnt->mnt.mnt_sb->s_type->fs_flags & FS_USERNS_MOUNT_RESTRICTED) &&
fs/namespace.c-1103- mnt->mnt.mnt_root == mnt->mnt.mnt_sb->s_root)
--
fs/namespace.c=6415=static bool mount_too_revealing(const struct super_block *sb, int *new_mnt_flags)
--
fs/namespace.c-6424- /* Can this filesystem be too revealing? */
fs/namespace.c:6425: if (!(sb->s_type->fs_flags & FS_USERNS_MOUNT_RESTRICTED))
fs/namespace.c-6426- return false;
--
fs/proc/root.c=369=static struct file_system_type proc_fs_type = {
--
fs/proc/root.c-373- .kill_sb = proc_kill_sb,
fs/proc/root.c:374: .fs_flags = FS_USERNS_MOUNT | FS_USERNS_MOUNT_RESTRICTED | FS_DISALLOW_NOTIFY_PERM,
fs/proc/root.c-375-};
--
fs/sysfs/mount.c=78=static struct file_system_type sysfs_fs_type = {
--
fs/sysfs/mount.c-81- .kill_sb = sysfs_kill_sb,
fs/sysfs/mount.c:82: .fs_flags = FS_USERNS_MOUNT | FS_USERNS_MOUNT_RESTRICTED,
fs/sysfs/mount.c-83-};
--
include/linux/fs.h=2280=struct file_system_type {
--
include/linux/fs.h-2291-#define FS_POWER_FREEZE 256 /* Always freeze on suspend/hibernate */
include/linux/fs.h:2292:#define FS_USERNS_MOUNT_RESTRICTED 512 /* Restrict mount in userns if not already visible */
include/linux/fs.h-2293-#define FS_USERNS_DELEGATABLE 1024 /* Can be mounted inside userns from outside */
--
kernel/acct.c=216=static int acct_on(const char __user *name)
--
kernel/acct.c-251- /* Exclude procfs and sysfs. */
kernel/acct.c:252: if (file_inode(file)->i_sb->s_type->fs_flags & FS_USERNS_MOUNT_RESTRICTED)
kernel/acct.c-253- return -EINVAL;
]
|
| 85/4 |
2026/07/25 00:55 |
llm |
codeexpert |
1m
Model:
Tokens:
input: 34641
output: 33
thoughts: 2150
Thoughts: **Sysfs Permissions Deep Dive**
Okay, so I'm thinking about how `sysfs` behaves when mounted within a user namespace. If a process inside a user namespace mounts `sysfs`, the key question is about the role of `sb->s_user_ns`.
Looking at `sysfs_init_fs_context`, it's clear that the mount sets the `fc->user_ns` to the user namespace of the network namespace the mount is using. Since the mount operation is done by a process inside the user namespace, if we're using a network namespace that's also in the same user namespace, `sb->s_user_ns` will be that user namespace.
But does this change how we interpret the permissions of files within sysfs? The core of the answer seems to lie within the `kernfs` code since sysfs is built on top of kernfs.
I'm revisiting how `inode->i_uid` and `inode->i_gid` are initialized and refreshed. The code shows `kernfs_refresh_inode` updating inode attributes, but critically, it directly uses `kuid_t` and `kgid_t` values stored within the `kernfs_node`. If they are not set then it defaults to 0. It appears that `sb->s_user_ns` has no direct impact on these values. The inode's `i_uid` and `i_gid` are *not* translated or mapped, they're stored directly as kernel UIDs and GIDs. This is the first critical piece.
Now, let's explore how the kernel *uses* those UIDs and GIDs for permission checks. I'm focusing on `generic_permission` and `acl_permission_check`.
`acl_permission_check` is important, and specifically, the use of `i_uid_into_vfsuid(idmap, inode)`. Since sysfs doesn't use ID-mapped mounts (`&nop_mnt_idmap`), this call effectively just returns the `inode->i_uid` as a `vfsuid_t`. So, `vfsuid` is essentially `GLOBAL_ROOT_UID` (UID 0 in the initial user namespace) for sysfs. And we compare that to the current process's effective UID, which *will* be an unprivileged UID if the process is inside the user namespace. The result will always be `false`.
Next, I need to understand `capable_wrt_inode_uidgid`. This uses the `current_user_ns()` and checks if the current namespace has the capabilities required for the file. The critical part is that it checks if the UIDs and GIDs "map" into the current namespace using `vfsuid_has_mapping` and `vfsgid_has_mapping`. These functions checks if `from_kuid()` returns a valid UID. Since our `vfsuid` is `GLOBAL_ROOT_UID` (0), and the current user namespace is unprivileged, it has no mapping for the root UID. Thus `capable_wrt_inode_uidgid` will also return `false`.
So, it seems that there's no path where `sb->s_user_ns` is directly used for permission checking. `generic_permission` itself doesn't directly use `sb->s_user_ns`, and it relies on `capable_wrt_inode_uidgid`.
Finally, even `capable(CAP_DAC_OVERRIDE)` fails, because capability checks are done against the *initial* user namespace, in which the unprivileged process *isn't* privileged.
So, is there *any* way for a process running as "root" in a user namespace to write to, say, `/sys/bus/platform/drivers/*/unbind`?
Here's an idea: what if you could change the ownership of that sysfs file? Can we actually `chown` a sysfs file? This is a potentially a way to subvert the permissions.
I'm looking at `kernfs_iop_setattr` and the call to `setattr_prepare`, still using `&nop_mnt_idmap`. This could be critical. The code calls `__kernfs_setattr`.
|
| 86/4 |
2026/07/25 00:56 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/attr.c Name:setattr_prepare]
Results: map[SourceCode: 140: /**
141: * setattr_prepare - check if attribute changes to a dentry are allowed
142: * @idmap: idmap of the mount the inode was found from
143: * @dentry: dentry to check
144: * @attr: attributes to change
145: *
146: * Check if we are allowed to change the attributes contained in @attr
147: * in the given dentry. This includes the normal unix access permission
148: * checks, as well as checks for rlimits and others. The function also clears
149: * SGID bit from mode if user is not allowed to set it. Also file capabilities
150: * and IMA extended attributes are cleared if ATTR_KILL_PRIV is set.
151: *
152: * If the inode has been found through an idmapped mount the idmap of
153: * the vfsmount must be passed through @idmap. This function will then
154: * take care to map the inode according to @idmap before checking
155: * permissions. On non-idmapped mounts or if permission checking is to be
156: * performed on the raw inode simply pass @nop_mnt_idmap.
157: *
158: * Should be called as the first thing in ->setattr implementations,
159: * possibly after taking additional locks.
160: */
161: int setattr_prepare(struct mnt_idmap *idmap, struct dentry *dentry,
162: struct iattr *attr)
163: {
164: struct inode *inode = d_inode(dentry);
165: unsigned int ia_valid = attr->ia_valid;
166:
167: /*
168: * First check size constraints. These can't be overridden using
169: * ATTR_FORCE.
170: */
171: if (ia_valid & ATTR_SIZE) {
172: int error;
173:
174: /*
175: * Verity files are immutable, so deny truncates. This isn't
176: * covered by the open-time check because sys_truncate() takes a
177: * path, not an open file.
178: */
179: if (IS_ENABLED(CONFIG_FS_VERITY) && IS_VERITY(inode))
180: return -EPERM;
181:
182: error = inode_newsize_ok(inode, attr->ia_size);
183: if (error)
184: return error;
185: }
186:
187: /* If force is set do it anyway. */
188: if (ia_valid & ATTR_FORCE)
189: goto kill_priv;
190:
191: /* Make sure a caller can chown. */
192: if ((ia_valid & ATTR_UID) &&
193: !chown_ok(idmap, inode, attr->ia_vfsuid))
194: return -EPERM;
195:
196: /* Make sure caller can chgrp. */
197: if ((ia_valid & ATTR_GID) &&
198: !chgrp_ok(idmap, inode, attr->ia_vfsgid))
199: return -EPERM;
200:
201: /* Make sure a caller can chmod. */
202: if (ia_valid & ATTR_MODE) {
203: vfsgid_t vfsgid;
204:
205: if (!inode_owner_or_capable(idmap, inode))
206: return -EPERM;
207:
208: if (ia_valid & ATTR_GID)
209: vfsgid = attr->ia_vfsgid;
210: else
211: vfsgid = i_gid_into_vfsgid(idmap, inode);
212:
213: /* Also check the setgid bit! */
214: if (!in_group_or_capable(idmap, inode, vfsgid))
215: attr->ia_mode &= ~S_ISGID;
216: }
217:
218: /* Check for setting the inode time. */
219: if (ia_valid & (ATTR_MTIME_SET | ATTR_ATIME_SET | ATTR_TIMES_SET)) {
220: if (!inode_owner_or_capable(idmap, inode))
221: return -EPERM;
222: }
223:
224: kill_priv:
225: /* User has permission for the change */
226: if (ia_valid & ATTR_KILL_PRIV) {
227: int error;
228:
229: error = security_inode_killpriv(idmap, dentry);
230: if (error)
231: return error;
232: }
233:
234: return 0;
235: }
SourceFile:fs/attr.c]
|
| 87/4 |
2026/07/25 00:56 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 39237
output: 33
thoughts: 19
Thoughts: Here's a summary of my thought process, as requested:
**Diving into `chown_ok`**
Okay, so I'm focusing in on `chown_ok` β that's the function in `fs/attr.c`. My immediate thought is, "Right, `chown_ok`β¦ that's the gatekeeper, isn't it? It's the one that decides whether a `chown` operation is actually permitted." I need to understand its logic, the conditions it evaluates, and any potential vulnerabilities or interesting edge cases it handles (or *should* handle!). This is likely where the core access control decisions reside for ownership changes in the file system. Time to dig in and see what's actually going on in there!
|
| 88/4 |
2026/07/25 00:56 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/attr.c Name:chown_ok]
Results: map[SourceCode: 82: /**
83: * chown_ok - verify permissions to chown inode
84: * @idmap: idmap of the mount @inode was found from
85: * @inode: inode to check permissions on
86: * @ia_vfsuid: uid to chown @inode to
87: *
88: * If the inode has been found through an idmapped mount the idmap of
89: * the vfsmount must be passed through @idmap. This function will then
90: * take care to map the inode according to @idmap before checking
91: * permissions. On non-idmapped mounts or if permission checking is to be
92: * performed on the raw inode simply pass @nop_mnt_idmap.
93: */
94: static bool chown_ok(struct mnt_idmap *idmap,
95: const struct inode *inode, vfsuid_t ia_vfsuid)
96: {
97: vfsuid_t vfsuid = i_uid_into_vfsuid(idmap, inode);
98: if (vfsuid_eq_kuid(vfsuid, current_fsuid()) &&
99: vfsuid_eq(ia_vfsuid, vfsuid))
100: return true;
101: if (capable_wrt_inode_uidgid(idmap, inode, CAP_CHOWN))
102: return true;
103: if (!vfsuid_valid(vfsuid) &&
104: ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
105: return true;
106: return false;
107: }
SourceFile:fs/attr.c]
|
| 89/4 |
2026/07/25 00:56 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 39949
output: 28
thoughts: 577
Thoughts: **Analyzing Sysfs Chown Behavior**
Okay, so `chown_ok` is the key function here. Let's see, it returns true only under specific conditions. First, it checks if the filesystem user ID (`vfsuid`) matches the current file system UID and if `ia_vfsuid` also matches `vfsuid`. Not going to work, because the `vfsuid` is `GLOBAL_ROOT_UID` and my current file system UID isn't. Second, it checks capability: are we capable with `CAP_CHOWN`? No, because `GLOBAL_ROOT_UID` isn't mapped in this user namespace. Third, it checks for an invalid `vfsuid`, and also a namespace capability. Nope, the `vfsuid` is valid. Therefore, `chown_ok` returns `false`. And the process *can't* change the ownership of the sysfs file.
But wait a minute, are we missing something? What if we try something clever... what if the process mounts its own instance of sysfs? This could be a workaround. If a process in a user namespace mounts sysfs, does that mounted sysfs have different file owners? I need to check if mounting sysfs in a user namespace affects the ownership of the files within that instance.
Hold on, I remember that `sysfs_init_fs_context` sets `fc->user_ns` to the user namespace of the network namespace. But, does kernfs/sysfs even use `sb->s_user_ns` to map file owners? Let's check if there's any mapping going on for `i_uid` or `i_gid` when creating or reading inodes.
Okay, let's search for `s_user_ns` in `fs/kernfs/` and `fs/sysfs/` again. Remember those grep results earlier? Let's double check. Ah, the grep output... it says `fs/kernfs/` and `fs/sysfs/` have no matches for `s_user_ns`. That's important!
So, `s_user_ns` isn't used *anywhere* in those file systems. I need to be absolutely certain... let's verify by searching for `user_ns` in `fs/kernfs/`. This strongly suggests that a mounted sysfs doesn't inherit user namespace ownership!
|
| 90/4 |
2026/07/25 00:56 |
tool |
grepper |
0m
Error:
Args: map[Expression:user_ns PathPrefix:fs/kernfs/]
Results:
|
| 91/4 |
2026/07/25 00:56 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 41083
output: 28
thoughts: 376
Thoughts: **Kernfs and User Namespaces: A Deep Dive**
Okay, so I'm digging into how `kernfs` handles user namespaces. First observation: the `user_ns` variable isn't used *anywhere* within the `fs/kernfs/` directory. That's a huge clue! It means `kernfs` is completely oblivious to user namespaces.
Alright, let's unpack that. If `kernfs` is unaware of user namespaces, then all the inodes within `kernfs`, and by extension, `sysfs`, are going to have `i_uid = GLOBAL_ROOT_UID` and `i_gid = GLOBAL_ROOT_GID`, unless something explicitly changes them. And from my experience, there's no way an unprivileged user namespace can change it.
Since there's no use of `s_user_ns` or any user namespace mapping, this implies that even if I mount a `sysfs` instance inside a user namespace, the inodes *still* inherit those global root IDs.
Hang on, let me quickly verify that. Yes, absolutely. It looks like `kernfs` initializes the inode's `i_uid` and `i_gid` to `GLOBAL_ROOT_UID` and `GLOBAL_ROOT_GID` during inode creation, which is 0 for both of these, and then `kernfs_refresh_inode` doesn't override them. I see that `attrs` is NULL, or if not NULL, it sets them to the same global root IDs.
Right, I need to check how `kernfs_iattrs` initializes `ia_uid` and `ia_gid`. I'm searching for `ia_uid` within `fs/kernfs/` now to confirm.
|
| 92/4 |
2026/07/25 00:56 |
tool |
grepper |
0m
Args: map[Expression:ia_uid PathPrefix:fs/kernfs/]
Results: map[Output:fs/kernfs/dir.c=651=static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
--
fs/kernfs/dir.c-693- .ia_valid = ATTR_UID | ATTR_GID,
fs/kernfs/dir.c:694: .ia_uid = uid,
fs/kernfs/dir.c-695- .ia_gid = gid,
--
fs/kernfs/inode.c=27=static struct kernfs_iattrs *__kernfs_iattrs(struct kernfs_node *kn, bool alloc)
--
fs/kernfs/inode.c-41- /* assign default attributes */
fs/kernfs/inode.c:42: ret->ia_uid = GLOBAL_ROOT_UID;
fs/kernfs/inode.c-43- ret->ia_gid = GLOBAL_ROOT_GID;
--
fs/kernfs/inode.c=68=int __kernfs_setattr(struct kernfs_node *kn, const struct iattr *iattr)
--
fs/kernfs/inode.c-77- if (ia_valid & ATTR_UID)
fs/kernfs/inode.c:78: attrs->ia_uid = iattr->ia_uid;
fs/kernfs/inode.c-79- if (ia_valid & ATTR_GID)
--
fs/kernfs/inode.c=157=static inline void set_inode_attr(struct inode *inode,
--
fs/kernfs/inode.c-159-{
fs/kernfs/inode.c:160: inode->i_uid = attrs->ia_uid;
fs/kernfs/inode.c-161- inode->i_gid = attrs->ia_gid;
--
fs/kernfs/kernfs-internal.h=22=struct kernfs_iattrs {
fs/kernfs/kernfs-internal.h:23: kuid_t ia_uid;
fs/kernfs/kernfs-internal.h-24- kgid_t ia_gid;
--
fs/kernfs/symlink.c=25=struct kernfs_node *kernfs_create_link(struct kernfs_node *parent,
--
fs/kernfs/symlink.c-34- if (target->iattr) {
fs/kernfs/symlink.c:35: uid = target->iattr->ia_uid;
fs/kernfs/symlink.c-36- gid = target->iattr->ia_gid;
]
|
| 93/4 |
2026/07/25 00:56 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 42509
output: 36
thoughts: 108
Thoughts: **Examining Kernfs Inode and Directory Attributes**
Okay, so I'm looking into the Kernfs implementation, specifically how it handles user and group IDs for files and directories. My starting point is `fs/kernfs/inode.c`, specifically line 42. I'm focusing on this line:
```c
/* assign default attributes */
ret->ia_uid = GLOBAL_ROOT_UID;
ret->ia_gid = GLOBAL_ROOT_GID;
```
This immediately tells me that new Kernfs inodes, by default, are assigned the root user ID and group ID. That's a sensible starting point, likely for security and management reasons, but I need to understand how these IDs are used and potentially changed.
Now, to get a better understanding of how these `uid` and `gid` are handled within the broader context of Kernfs, I want to pivot to the directory handling. I'll read `fs/kernfs/dir.c`, specifically around line 690. This should give me insight into how these attributes are utilized within the directory structure and how they affect access controls. I'm hoping to understand if there are any mechanisms in place for assigning different UIDs/GIDs based on the directoryβs purpose or the user creating it.
|
| 94/4 |
2026/07/25 00:56 |
tool |
read-file |
0m
Args: map[File:fs/kernfs/dir.c FirstLine:680 LineCount:25]
Results: map[Contents: 680:
681: kn->id = (u64)id_highbits << 32 | ret;
682:
683: atomic_set(&kn->count, 1);
684: atomic_set(&kn->active, KN_DEACTIVATED_BIAS);
685: RB_CLEAR_NODE(&kn->rb);
686:
687: rcu_assign_pointer(kn->name, name);
688: kn->mode = mode;
689: kn->flags = flags;
690:
691: if (!uid_eq(uid, GLOBAL_ROOT_UID) || !gid_eq(gid, GLOBAL_ROOT_GID)) {
692: struct iattr iattr = {
693: .ia_valid = ATTR_UID | ATTR_GID,
694: .ia_uid = uid,
695: .ia_gid = gid,
696: };
697:
698: ret = __kernfs_setattr(kn, &iattr);
699: if (ret < 0)
700: goto err_out3;
701: }
702:
703: if (parent) {
704: kernfs_get(parent);
]
|
| 95/4 |
2026/07/25 00:56 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 43303
output: 32
thoughts: 558
Thoughts: **Analyzing the `unbind` Security Implications in User Namespaces**
Okay, so I've confirmed that `kernfs` nodes are initialized with the `GLOBAL_ROOT_UID` and `GLOBAL_ROOT_GID`, except for the rare instances where they are explicitly configured differently during their creationβwhich isn't the case for a standard driver attribute like `unbind`.
Now, the question is, beyond just that, are there other security or LSM checks involved here? Right, even before LSM, it seems the standard DAC (Discretionary Access Control) checks will stop us in our tracks. Let me recap that DAC check to be absolutely clear:
1. `/sys/bus/platform/drivers/*/unbind` is owned by `GLOBAL_ROOT_UID` and `GLOBAL_ROOT_GID`.
2. The permissions are `0200`, meaning it's write-only by the owner.
3. A process running as root *within* a user namespace, but unprivileged in the initial one, has a `kuid` which is *not* `GLOBAL_ROOT_UID`. It's mapped to, say, an unprivileged UID in the initial namespace, maybe 100000.
4. Therefore, this process isn't the owner of the file.
5. Since the permissions are `0200`, "other" (not the owner, not the group) has *no* write permission.
6. The process *doesn't* have `CAP_DAC_OVERRIDE` on this file because:
* It doesn't have `CAP_DAC_OVERRIDE` in the initial user namespace (where the file's owner resides).
* Even if it *does* have `CAP_DAC_OVERRIDE` in its own namespace, that capability is only effective on inodes whose UIDs and GIDs are mapped into that namespace.
* Since `GLOBAL_ROOT_UID` isn't mapped, the namespace capabilities are irrelevant here.
7. So, any write or open attempt is denied with `-EACCES`.
Alright, I need to make sure I'm not missing any exceptions or any other way around this. Let's see if there is any documentation or discussion around this. I'll search for "unbind" along with "user namespace" or "userns" in the kernel documentation and git commits. And I should use `git-log` to search for commits that contain those terms. To be more efficient I should restrict the search path to prevent any timeouts. I'll focus the search on areas like `drivers/base/`, `fs/sysfs/`, and `fs/kernfs/`. This should give me a solid picture of any potential workarounds or related discussions.
|
| 96/4 |
2026/07/25 00:57 |
tool |
git-log |
0m
Args: map[MessageRegexps:[user namespace] PathPrefix:drivers/base/bus.c]
Results:
|
| 97/4 |
2026/07/25 00:57 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 44503
output: 28
thoughts: 21
Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience, and presented as if it were my own internal monologue:
**Investigating Driver Unbinding in the Base Subsystem**
Alright, I'm thinking I need to understand the recent activity around device unbinding, specifically within the `drivers/base/` directory. That's usually the place where the core device model interactions happen. So, I need to focus on commits that are *directly* relevant to the unbinding process. Let me narrow things down. I should search the commit messages for the term "unbind". This should give me a good, focused starting point to identify recent changes, bug fixes, or new features related to unbinding devices in the base system. This search will likely give me a manageable list to review, as opposed to a broader search that could waste time. I'll need to then quickly evaluate each commit to see the full scope of any reported changes.
|
| 98/4 |
2026/07/25 00:57 |
tool |
git-log |
0m
Args: map[MessageRegexps:[unbind] PathPrefix:drivers/base/]
Results: map[Output:be31fcf5af75 rust: driver core: drop drvdata before devres release
36d74f17e03f driver core: Avoid warning when removing a device while its supplier is unbinding
9582485a65ea device property: fix fwnode reference leak in fwnode_graph_get_endpoint_by_id()
a995fe1a3aa7 rust: driver: drop device private data post unbind
a91c8096590b devcoredump: Fix circular locking dependency with devcd->mutex.
eca710386972 driver core: auxiliary bus: Drop dev_pm_domain_detach() call
ba2ebd52a22e driver core: platform: Drop dev_pm_domain_detach() call
f99508074e78 PM: domains: Detach on device_unbind_cleanup()
d42c7c6fd66a PM: domains: Add flags to specify power on attach/detach
5bcca36898aa component: do not try to unbind unbound components
]
|
| 99/4 |
2026/07/25 00:57 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 45023
output: 27
thoughts: 40
Thoughts: Here's my thought process, as requested:
**Investigating Potential Namespace Issues with Git History**
Okay, before diving in further, I want to take a quick look at the project's history to see if any recent changes or patterns emerge related to namespace binding or unbinding. Let me run a `git log` command, specifically searching for the keywords "unbind" and "namespace" simultaneously, in the entire repository. I'll limit the results to the last ten commits using `Count: 10` to keep the output manageable and focus on recent activity. This should provide a snapshot of any related modifications and hopefully highlight any potentially problematic commits. I'm hoping to quickly identify if there have been any relevant changes that might explain the current behavior.
|
| 100/4 |
2026/07/25 00:57 |
tool |
git-log |
0m
Args: map[Count:10 MessageRegexps:[unbind namespace]]
Results: map[Output:ad4999496e73 mount: always duplicate mount
59e25ef2b413 block: Use RCU in blk_mq_[un]quiesce_tagset() instead of set->tag_list_lock
406fea799925 mount: separate the flags accessed only under namespace_sem
6bcd5089ee13 nvme: don't unquiesce the admin queue in nvme_kill_queues
0227f058aa29 net/smc: Unbind r/w buffer size from clcsock and make them tunable
b33886971dbc net/mlx5: Initialize flow steering during driver probe
32b2397c1e56 libnvdimm/pmem: Fix crash triggered when I/O in-flight during unbind
46b1ee38b2ba mm/mremap_pages: fix static key devmap_managed_key updates
fbdd0049d98d RDMA/mlx5: Fix devlink deadlock on net namespace deletion
e47110e90584 mm/vunmap: add cond_resched() in vunmap_pmd_range
]
|
| 101/4 |
2026/07/25 00:57 |
llm |
codeexpert |
1m
Model:
Tokens:
input: 45546
output: 23
thoughts: 923
Thoughts: **Analyzing User Namespace Privileges for Sysfs Write Access**
Alright, let's dissect this. The question boils down to whether a process, running as root within a user namespace but lacking root privileges in the initial namespace, can write to `/sys/bus/platform/drivers/*/unbind`. I need to rigorously account for all possible scenarios and permissions.
First, let's clarify the conditions: the process is root in its *own* namespace, meaning its UID is 0 there. However, it's *unprivileged* in the original, or "initial", namespace, so its initial namespace UID is *not* 0. This implies an unprivileged user created this user namespace, since a privileged user *could* map namespace UID 0 to host UID 0.
Now, if the process is unprivileged, it can't map UID 0 (root) *within* the namespace to UID 0 (root) *outside* the namespace. This is because unprivileged users are restricted in their UID mappings. They can only map their own UID or ranges they have been delegated. This means the initial namespace's UID 0 (the host root) is *not* mapped inside the user namespace, that's crucial.
Let's test this understanding: If the host root's UID (0) is not mapped, then the sysfs file should be owned by host root. It should also have permissions of 0200, meaning write-only for the owner. Because the process's effective UID (fsuid) is *not* host root, it can't be the owner. It wouldn't have group permissions either, and would fall into the "other" category with no write access. It won't have the `CAP_DAC_OVERRIDE` or `CAP_DAC_READ_SEARCH` capability because those capabilities in a user namespace only apply to inodes where the UIDs and GIDs are mapped.
Let's do a quick double check: Could a sysfs filesystem mount *inside* the user namespace alter the ownership? I've checked into the kernel internals and verified that neither `kernfs` nor `sysfs` use `s_user_ns` to automatically map the UIDs and GIDs of inodes. I need to confirm this: Is there a VFS function that would automatically translate `inode->i_uid` and `inode->i_gid` using `sb->s_user_ns` during permission checks? I should look for `i_uid_into_vfsuid` and `i_gid_into_vfsgid` in the relevant headers.
|
| 102/4 |
2026/07/25 00:58 |
tool |
grepper |
0m
Args: map[Expression:i_uid_into_vfsuid]
Results: map[Output:Documentation/filesystems/idmappings.rst=702=of the remapping algorithm we've introduced earlier. We're going to look at:
Documentation/filesystems/idmappings.rst-703-
Documentation/filesystems/idmappings.rst:704:- ``i_uid_into_vfsuid()`` and ``i_gid_into_vfsgid()``
Documentation/filesystems/idmappings.rst-705-
--
Documentation/filesystems/idmappings.rst=746=But when the caller is accessing the file on an idmapped mount the kernel will
Documentation/filesystems/idmappings.rst:747:first call ``i_uid_into_vfsuid()`` thereby translating the filesystem's kernel
Documentation/filesystems/idmappings.rst-748-id into a VFS id in the mount's idmapping::
Documentation/filesystems/idmappings.rst-749-
Documentation/filesystems/idmappings.rst:750: i_uid_into_vfsuid(k21000):
Documentation/filesystems/idmappings.rst-751- /* Map the filesystem's kernel id up into a userspace id. */
--
Documentation/filesystems/idmappings.rst=875=the translation algorithm we introduced earlier:
--
Documentation/filesystems/idmappings.rst-883-
Documentation/filesystems/idmappings.rst:884: i_uid_into_vfsuid(k1000):
Documentation/filesystems/idmappings.rst-885- /* Map the kernel id up into a userspace id in the filesystem's idmapping. */
--
Documentation/filesystems/idmappings.rst=912=steps using the translation algorithm we introduced earlier:
--
Documentation/filesystems/idmappings.rst-920-
Documentation/filesystems/idmappings.rst:921: i_uid_into_vfsuid(k21000):
Documentation/filesystems/idmappings.rst-922- /* Map the kernel id up into a userspace id in the filesystem's idmapping. */
--
Documentation/filesystems/idmappings.rst=1005=on their work computer:
--
Documentation/filesystems/idmappings.rst-1020-
Documentation/filesystems/idmappings.rst:1021: i_uid_into_vfsuid(k1000):
Documentation/filesystems/idmappings.rst-1022- /* Map the kernel id up into a userspace id in the filesystem's idmapping. */
--
fs/attr.c=94=static bool chown_ok(struct mnt_idmap *idmap,
--
fs/attr.c-96-{
fs/attr.c:97: vfsuid_t vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/attr.c-98- if (vfsuid_eq_kuid(vfsuid, current_fsuid()) &&
--
fs/attr.c=121=static bool chgrp_ok(struct mnt_idmap *idmap,
--
fs/attr.c-124- vfsgid_t vfsgid = i_gid_into_vfsgid(idmap, inode);
fs/attr.c:125: vfsuid_t vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/attr.c-126- if (vfsuid_eq_kuid(vfsuid, current_fsuid())) {
--
fs/attr.c=427=int notify_change(struct mnt_idmap *idmap, struct dentry *dentry,
--
fs/attr.c-533- if (!(ia_valid & ATTR_UID) &&
fs/attr.c:534: !vfsuid_valid(i_uid_into_vfsuid(idmap, inode)))
fs/attr.c-535- return -EOVERFLOW;
--
fs/coredump.c=878=static bool coredump_file(struct core_name *cn, struct coredump_params *cprm,
--
fs/coredump.c-956- idmap = file_mnt_idmap(file);
fs/coredump.c:957: if (!vfsuid_eq_kuid(i_uid_into_vfsuid(idmap, inode), current_fsuid())) {
fs/coredump.c-958- coredump_report_failure("Core dump to %s aborted: cannot preserve file owner", cn->corename);
--
fs/exec.c=1558=static void bprm_fill_uid(struct linux_binprm *bprm, struct file *file)
--
fs/exec.c-1584- mode = inode->i_mode;
fs/exec.c:1585: vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/exec.c-1586- vfsgid = i_gid_into_vfsgid(idmap, inode);
--
fs/exfat/file.c=145=static bool exfat_allow_set_time(struct mnt_idmap *idmap,
--
fs/exfat/file.c-149-
fs/exfat/file.c:150: if (!vfsuid_eq_kuid(i_uid_into_vfsuid(idmap, inode),
fs/exfat/file.c-151- current_fsuid())) {
--
fs/f2fs/recovery.c=242=static int recover_quota_data(struct inode *inode, struct folio *folio)
--
fs/f2fs/recovery.c-254-
fs/f2fs/recovery.c:255: if (!vfsuid_eq(attr.ia_vfsuid, i_uid_into_vfsuid(&nop_mnt_idmap, inode)))
fs/f2fs/recovery.c-256- attr.ia_valid |= ATTR_UID;
--
fs/fat/file.c=497=static int fat_allow_set_time(struct mnt_idmap *idmap,
--
fs/fat/file.c-501-
fs/fat/file.c:502: if (!vfsuid_eq_kuid(i_uid_into_vfsuid(idmap, inode),
fs/fat/file.c-503- current_fsuid())) {
--
fs/inode.c=2753=bool inode_owner_or_capable(struct mnt_idmap *idmap,
--
fs/inode.c-2758-
fs/inode.c:2759: vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/inode.c-2760- if (vfsuid_eq_kuid(vfsuid, current_fsuid()))
--
fs/locks.c=2136=vfs_setlease(struct file *filp, int arg, struct file_lease **lease, void **priv)
--
fs/locks.c-2138- struct inode *inode = file_inode(filp);
fs/locks.c:2139: vfsuid_t vfsuid = i_uid_into_vfsuid(file_mnt_idmap(filp), inode);
fs/locks.c-2140- int error;
--
fs/namei.c=438=static int acl_permission_check(struct mnt_idmap *idmap,
--
fs/namei.c-466- /* Are we the owner? If so, ACL's don't matter */
fs/namei.c:467: vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/namei.c-468- if (likely(vfsuid_eq_kuid(vfsuid, current_fsuid()))) {
--
fs/namei.c=1274=static inline int may_follow_link(struct nameidata *nd, const struct inode *inode)
--
fs/namei.c-1282- idmap = mnt_idmap(nd->path.mnt);
fs/namei.c:1283: vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/namei.c-1284- /* Allowed if owner and follower match. */
--
fs/namei.c=1360=int may_linkat(struct mnt_idmap *idmap, const struct path *link)
--
fs/namei.c-1364- /* Inode writeback is not safe when the uid or gid are invalid. */
fs/namei.c:1365: if (!vfsuid_valid(i_uid_into_vfsuid(idmap, inode)) ||
fs/namei.c-1366- !vfsgid_valid(i_gid_into_vfsgid(idmap, inode)))
--
fs/namei.c=1410=static int may_create_in_sticky(struct mnt_idmap *idmap, struct nameidata *nd,
--
fs/namei.c-1424-
fs/namei.c:1425: i_vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/namei.c-1426-
--
fs/namei.c=2578=static int link_path_walk(const char *name, struct nameidata *nd)
--
fs/namei.c-2645- if (likely(!depth)) {
fs/namei.c:2646: nd->dir_vfsuid = i_uid_into_vfsuid(idmap, nd->inode);
fs/namei.c-2647- nd->dir_mode = nd->inode->i_mode;
--
fs/namei.c=3645=int __check_sticky(struct mnt_idmap *idmap, struct inode *dir,
--
fs/namei.c-3649-
fs/namei.c:3650: if (vfsuid_eq_kuid(i_uid_into_vfsuid(idmap, inode), fsuid))
fs/namei.c-3651- return 0;
fs/namei.c:3652: if (vfsuid_eq_kuid(i_uid_into_vfsuid(idmap, dir), fsuid))
fs/namei.c-3653- return 0;
--
fs/namei.c=3678=int may_delete_dentry(struct mnt_idmap *idmap, struct inode *dir,
--
fs/namei.c-3690- /* Inode writeback is not safe when the uid or gid are invalid. */
fs/namei.c:3691: if (!vfsuid_valid(i_uid_into_vfsuid(idmap, inode)) ||
fs/namei.c-3692- !vfsgid_valid(i_gid_into_vfsgid(idmap, inode)))
--
fs/overlayfs/util.c=1495=void ovl_copyattr(struct inode *inode)
--
fs/overlayfs/util.c-1506- spin_lock(&inode->i_lock);
fs/overlayfs/util.c:1507: vfsuid = i_uid_into_vfsuid(real_idmap, realinode);
fs/overlayfs/util.c-1508- vfsgid = i_gid_into_vfsgid(real_idmap, realinode);
--
fs/posix_acl.c=374=posix_acl_permission(struct mnt_idmap *idmap, struct inode *inode,
--
fs/posix_acl.c-388- /* (May have been checked already) */
fs/posix_acl.c:389: vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/posix_acl.c-390- if (vfsuid_eq_kuid(vfsuid, current_fsuid()))
--
fs/remap_range.c=416=static bool may_dedupe_file(struct file *file)
--
fs/remap_range.c-424- return true;
fs/remap_range.c:425: if (vfsuid_eq_kuid(i_uid_into_vfsuid(idmap, inode), current_fsuid()))
fs/remap_range.c-426- return true;
--
fs/smb/server/ndr.c=340=int ndr_encode_posix_acl(struct ndr *n,
--
fs/smb/server/ndr.c-376-
fs/smb/server/ndr.c:377: vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/smb/server/ndr.c-378- ret = ndr_write_int64(n, from_kuid(&init_user_ns, vfsuid_into_kuid(vfsuid)));
--
fs/smb/server/oplock.c=2087=void create_posix_rsp_buf(char *cc, struct ksmbd_file *fp)
--
fs/smb/server/oplock.c-2091- struct mnt_idmap *idmap = file_mnt_idmap(fp->filp);
fs/smb/server/oplock.c:2092: vfsuid_t vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/smb/server/oplock.c-2093- vfsgid_t vfsgid = i_gid_into_vfsgid(idmap, inode);
--
fs/smb/server/smb2pdu.c=2952=static void ksmbd_acls_fattr(struct smb_fattr *fattr,
--
fs/smb/server/smb2pdu.c-2955-{
fs/smb/server/smb2pdu.c:2956: vfsuid_t vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/smb/server/smb2pdu.c-2957- vfsgid_t vfsgid = i_gid_into_vfsgid(idmap, inode);
--
fs/smb/server/smb2pdu.c=5743=static int find_file_posix_info(struct smb2_query_info_rsp *rsp,
--
fs/smb/server/smb2pdu.c-5748- struct mnt_idmap *idmap = file_mnt_idmap(fp->filp);
fs/smb/server/smb2pdu.c:5749: vfsuid_t vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/smb/server/smb2pdu.c-5750- vfsgid_t vfsgid = i_gid_into_vfsgid(idmap, inode);
--
fs/stat.c=82=void generic_fillattr(struct mnt_idmap *idmap, u32 request_mask,
--
fs/stat.c-84-{
fs/stat.c:85: vfsuid_t vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/stat.c-86- vfsgid_t vfsgid = i_gid_into_vfsgid(idmap, inode);
--
fs/xfs/xfs_iops.c=682=xfs_vn_getattr(
--
fs/xfs/xfs_iops.c-691- struct xfs_mount *mp = ip->i_mount;
fs/xfs/xfs_iops.c:692: vfsuid_t vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/xfs/xfs_iops.c-693- vfsgid_t vfsgid = i_gid_into_vfsgid(idmap, inode);
--
fs/xfs/xfs_itable.c=64=xfs_bulkstat_one_int(
--
fs/xfs/xfs_itable.c-101- inode = VFS_I(ip);
fs/xfs/xfs_itable.c:102: vfsuid = i_uid_into_vfsuid(idmap, inode);
fs/xfs/xfs_itable.c-103- vfsgid = i_gid_into_vfsgid(idmap, inode);
--
include/linux/fs.h=1429=static inline void i_gid_write(struct inode *inode, gid_t gid)
--
include/linux/fs.h-1434-/**
include/linux/fs.h:1435: * i_uid_into_vfsuid - map an inode's i_uid down according to an idmapping
include/linux/fs.h-1436- * @idmap: idmap of the mount the inode was found from
--
include/linux/fs.h-1441- */
include/linux/fs.h:1442:static inline vfsuid_t i_uid_into_vfsuid(struct mnt_idmap *idmap,
include/linux/fs.h-1443- const struct inode *inode)
--
include/linux/fs.h=1459=static inline bool i_uid_needs_update(struct mnt_idmap *idmap,
--
include/linux/fs.h-1464- !vfsuid_eq(attr->ia_vfsuid,
include/linux/fs.h:1465: i_uid_into_vfsuid(idmap, inode)));
include/linux/fs.h-1466-}
--
include/linux/fs.h=2178=static inline bool HAS_UNMAPPED_ID(struct mnt_idmap *idmap,
--
include/linux/fs.h-2180-{
include/linux/fs.h:2181: return !vfsuid_valid(i_uid_into_vfsuid(idmap, inode)) ||
include/linux/fs.h-2182- !vfsgid_valid(i_gid_into_vfsgid(idmap, inode));
--
kernel/capability.c=455=bool privileged_wrt_inode_uidgid(struct user_namespace *ns,
--
kernel/capability.c-458-{
kernel/capability.c:459: return vfsuid_has_mapping(ns, i_uid_into_vfsuid(idmap, inode)) &&
kernel/capability.c-460- vfsgid_has_mapping(ns, i_gid_into_vfsgid(idmap, inode));
--
security/apparmor/af_unix.c=33=static int unix_fs_perm(const char *op, u32 mask, const struct cred *subj_cred,
--
security/apparmor/af_unix.c-48- struct inode *inode = path->dentry->d_inode;
security/apparmor/af_unix.c:49: vfsuid_t vfsuid = i_uid_into_vfsuid(mnt_idmap(path->mnt), inode);
security/apparmor/af_unix.c-50- struct path_cond cond = {
--
security/apparmor/domain.c=931=int apparmor_bprm_creds_for_exec(struct linux_binprm *bprm)
--
security/apparmor/domain.c-940- bool unsafe = false;
security/apparmor/domain.c:941: vfsuid_t vfsuid = i_uid_into_vfsuid(file_mnt_idmap(bprm->file),
security/apparmor/domain.c-942- file_inode(bprm->file));
--
security/apparmor/file.c=430=int aa_path_link(const struct cred *subj_cred,
--
security/apparmor/file.c-436- struct inode *inode = d_backing_inode(old_dentry);
security/apparmor/file.c:437: vfsuid_t vfsuid = i_uid_into_vfsuid(mnt_idmap(target.mnt), inode);
security/apparmor/file.c-438- struct path_cond cond = {
--
security/apparmor/file.c=483=static int __file_path_perm(const char *op, const struct cred *subj_cred,
--
security/apparmor/file.c-489- struct aa_perms perms = {};
security/apparmor/file.c:490: vfsuid_t vfsuid = i_uid_into_vfsuid(file_mnt_idmap(file),
security/apparmor/file.c-491- file_inode(file));
--
security/apparmor/lsm.c=244=static int common_perm_cond(const char *op, const struct path *path, u32 mask)
security/apparmor/lsm.c-245-{
security/apparmor/lsm.c:246: vfsuid_t vfsuid = i_uid_into_vfsuid(mnt_idmap(path->mnt),
security/apparmor/lsm.c-247- d_backing_inode(path->dentry));
--
security/apparmor/lsm.c=287=static int common_perm_rm(const char *op, const struct path *dir,
--
security/apparmor/lsm.c-296-
security/apparmor/lsm.c:297: vfsuid = i_uid_into_vfsuid(mnt_idmap(dir->mnt), inode);
security/apparmor/lsm.c-298- cond.uid = vfsuid_into_kuid(vfsuid);
--
security/apparmor/lsm.c=383=static int apparmor_path_rename(const struct path *old_dir, struct dentry *old_dentry,
--
security/apparmor/lsm.c-405- };
security/apparmor/lsm.c:406: vfsuid = i_uid_into_vfsuid(idmap, d_backing_inode(old_dentry));
security/apparmor/lsm.c-407- cond.uid = vfsuid_into_kuid(vfsuid);
--
security/apparmor/lsm.c-412- };
security/apparmor/lsm.c:413: vfsuid = i_uid_into_vfsuid(idmap, d_backing_inode(new_dentry));
security/apparmor/lsm.c-414- cond_exchange.uid = vfsuid_into_kuid(vfsuid);
--
security/apparmor/lsm.c=461=static int apparmor_file_open(struct file *file)
--
security/apparmor/lsm.c-490- };
security/apparmor/lsm.c:491: vfsuid = i_uid_into_vfsuid(idmap, inode);
security/apparmor/lsm.c-492- cond.uid = vfsuid_into_kuid(vfsuid);
--
security/integrity/ima/ima_policy.c=573=static bool ima_match_rules(struct ima_rule_entry *rule,
--
security/integrity/ima/ima_policy.c-640- if ((rule->flags & IMA_FOWNER) &&
security/integrity/ima/ima_policy.c:641: !rule->fowner_op(i_uid_into_vfsuid(idmap, inode),
security/integrity/ima/ima_policy.c-642- rule->fowner))
]
|
| 103/4 |
2026/07/25 00:58 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 53144
output: 36
thoughts: 33
Thoughts: Okay, let's dive into this. Here's how I'd approach this task, as if these were my internal thoughts:
**Analyzing `i_uid_into_vfsuid` in `include/linux/fs.h`**
Right, so I need to refresh my memory on how the kernel handles user ID mapping within the VFS layer. Let's head over to `include/linux/fs.h` and specifically look around line 1442, which should contain the `i_uid_into_vfsuid` function. I imagine this function is crucial for translating user IDs (UIDs) from the internal inode representation to a format suitable for the VFS. I bet there is some context, perhaps a discussion of UID namespaces, or how the kernel handles security identifiers. I'll need to understand the function's arguments, return value, and most importantly, the logic within. What specific structures is it working with? Is it performing a simple lookup, or something more complex like applying a mapping table for containerization or network filesystems? Given the focus on UIDs, I'm expecting to see calls to security modules or perhaps a bit of namespace translation if this is happening within a container. I'm especially interested in whether this function handles errors gracefully and how it deals with UID "overflow" conditions that could potentially pose a security risk. It would be helpful to trace calls to this function in other areas of the filesystem code to grasp how it is utilized, and whether there are associated invariants that need to be maintained. Okay, time to crack open the source and see what's what.
|
| 104/4 |
2026/07/25 00:58 |
tool |
read-file |
0m
Args: map[File:include/linux/fs.h FirstLine:1430 LineCount:25]
Results: map[Contents:1430: {
1431: inode->i_gid = make_kgid(i_user_ns(inode), gid);
1432: }
1433:
1434: /**
1435: * i_uid_into_vfsuid - map an inode's i_uid down according to an idmapping
1436: * @idmap: idmap of the mount the inode was found from
1437: * @inode: inode to map
1438: *
1439: * Return: whe inode's i_uid mapped down according to @idmap.
1440: * If the inode's i_uid has no mapping INVALID_VFSUID is returned.
1441: */
1442: static inline vfsuid_t i_uid_into_vfsuid(struct mnt_idmap *idmap,
1443: const struct inode *inode)
1444: {
1445: return make_vfsuid(idmap, i_user_ns(inode), inode->i_uid);
1446: }
1447:
1448: /**
1449: * i_uid_needs_update - check whether inode's i_uid needs to be updated
1450: * @idmap: idmap of the mount the inode was found from
1451: * @attr: the new attributes of @inode
1452: * @inode: the inode to update
1453: *
1454: * Check whether the $inode's i_uid field needs to be updated taking idmapped
]
|
| 105/4 |
2026/07/25 00:58 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 53985
output: 19
thoughts: 123
Thoughts: **Digging into the Namespace Details**
Okay, hold on a second. Line 1445 in this code... let me just bring it up... specifically, that `i_uid_into_vfsuid` function. And within that, the call to `i_user_ns(inode)`... what is *that*? I know I've seen it before in the context of user namespaces, but I need to be absolutely sure. This is about VFS UIDs, so it's critical to understand the namespace context. Let me immediately check `include/linux/fs.h` and refresh my memory on what `i_user_ns` is actually returning. The VFS and user namespaces are tightly intertwined here. Gotta make sure I'm not missing any nuanced implications.
|
| 106/4 |
2026/07/25 00:58 |
tool |
grepper |
0m
Args: map[Expression:i_user_ns]
Results: map[Output:fs/ceph/inode.c=2570=int __ceph_setattr(struct mnt_idmap *idmap, struct inode *inode,
--
fs/ceph/inode.c-2683- if (ia_valid & ATTR_UID) {
fs/ceph/inode.c:2684: kuid_t fsuid = from_vfsuid(idmap, i_user_ns(inode), attr->ia_vfsuid);
fs/ceph/inode.c-2685-
--
fs/ceph/inode.c-2701- if (ia_valid & ATTR_GID) {
fs/ceph/inode.c:2702: kgid_t fsgid = from_vfsgid(idmap, i_user_ns(inode), attr->ia_vfsgid);
fs/ceph/inode.c-2703-
--
fs/exfat/file.c=349=int exfat_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
--
fs/exfat/file.c-381- if (((attr->ia_valid & ATTR_UID) &&
fs/exfat/file.c:382: (!uid_eq(from_vfsuid(idmap, i_user_ns(inode), attr->ia_vfsuid),
fs/exfat/file.c-383- sbi->options.fs_uid))) ||
fs/exfat/file.c-384- ((attr->ia_valid & ATTR_GID) &&
fs/exfat/file.c:385: (!gid_eq(from_vfsgid(idmap, i_user_ns(inode), attr->ia_vfsgid),
fs/exfat/file.c-386- sbi->options.fs_gid))) ||
--
fs/fat/file.c=518=int fat_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
--
fs/fat/file.c-558- if (((attr->ia_valid & ATTR_UID) &&
fs/fat/file.c:559: (!uid_eq(from_vfsuid(idmap, i_user_ns(inode), attr->ia_vfsuid),
fs/fat/file.c-560- sbi->options.fs_uid))) ||
fs/fat/file.c-561- ((attr->ia_valid & ATTR_GID) &&
fs/fat/file.c:562: (!gid_eq(from_vfsgid(idmap, i_user_ns(inode), attr->ia_vfsgid),
fs/fat/file.c-563- sbi->options.fs_gid))) ||
--
fs/fuse/acl.c=54=static inline bool fuse_no_acl(const struct fuse_conn *fc,
--
fs/fuse/acl.c-61- */
fs/fuse/acl.c:62: return !fc->posix_acl && (i_user_ns(inode) != &init_user_ns);
fs/fuse/acl.c-63-}
--
fs/open.c=739=int chown_common(const struct path *path, uid_t user, gid_t group)
--
fs/open.c-753- idmap = mnt_idmap(path->mnt);
fs/open.c:754: fs_userns = i_user_ns(inode);
fs/open.c-755-
--
fs/overlayfs/inode.c=341=static void ovl_idmap_posix_acl(const struct inode *realinode,
--
fs/overlayfs/inode.c-344-{
fs/overlayfs/inode.c:345: struct user_namespace *fs_userns = i_user_ns(realinode);
fs/overlayfs/inode.c-346-
--
fs/posix_acl.c=374=posix_acl_permission(struct mnt_idmap *idmap, struct inode *inode,
--
fs/posix_acl.c-377- const struct posix_acl_entry *pa, *pe, *mask_obj;
fs/posix_acl.c:378: struct user_namespace *fs_userns = i_user_ns(inode);
fs/posix_acl.c-379- int found = 0;
--
fs/posix_acl.c=887=static ssize_t vfs_posix_acl_to_xattr(struct mnt_idmap *idmap,
--
fs/posix_acl.c-908-
fs/posix_acl.c:909: fs_userns = i_user_ns(inode);
fs/posix_acl.c-910- caller_userns = current_user_ns();
--
fs/posix_acl.c=1089=int vfs_set_acl(struct mnt_idmap *idmap, struct dentry *dentry,
--
fs/posix_acl.c-1108- */
fs/posix_acl.c:1109: error = vfs_set_acl_idmapped_mnt(idmap, i_user_ns(inode), kacl);
fs/posix_acl.c-1110- if (error)
--
fs/quota/dquot.c=2150=int dquot_transfer(struct mnt_idmap *idmap, struct inode *inode,
--
fs/quota/dquot.c-2161- if (i_uid_needs_update(idmap, iattr, inode)) {
fs/quota/dquot.c:2162: kuid_t kuid = from_vfsuid(idmap, i_user_ns(inode),
fs/quota/dquot.c-2163- iattr->ia_vfsuid);
--
fs/quota/dquot.c-2175- if (i_gid_needs_update(idmap, iattr, inode)) {
fs/quota/dquot.c:2176: kgid_t kgid = from_vfsgid(idmap, i_user_ns(inode),
fs/quota/dquot.c-2177- iattr->ia_vfsgid);
--
fs/xfs/xfs_inode.c=617=xfs_icreate_dqalloc(
--
fs/xfs/xfs_inode.c-634- */
fs/xfs/xfs_inode.c:635: uid = mapped_fsuid(args->idmap, i_user_ns(dir));
fs/xfs/xfs_inode.c:636: gid = mapped_fsgid(args->idmap, i_user_ns(dir));
fs/xfs/xfs_inode.c-637- prid = xfs_get_initial_prid(args->pip);
--
fs/xfs/xfs_iops.c=780=xfs_setattr_nonsize(
--
fs/xfs/xfs_iops.c-809- if ((mask & ATTR_UID) && XFS_IS_UQUOTA_ON(mp)) {
fs/xfs/xfs_iops.c:810: uid = from_vfsuid(idmap, i_user_ns(inode),
fs/xfs/xfs_iops.c-811- iattr->ia_vfsuid);
--
fs/xfs/xfs_iops.c-816- if ((mask & ATTR_GID) && XFS_IS_GQUOTA_ON(mp)) {
fs/xfs/xfs_iops.c:817: gid = from_vfsgid(idmap, i_user_ns(inode),
fs/xfs/xfs_iops.c-818- iattr->ia_vfsgid);
--
include/linux/fs.h=1392=extern int send_sigurg(struct file *file);
--
include/linux/fs.h-1403-
include/linux/fs.h:1404:static inline struct user_namespace *i_user_ns(const struct inode *inode)
include/linux/fs.h-1405-{
--
include/linux/fs.h=1414=static inline uid_t i_uid_read(const struct inode *inode)
include/linux/fs.h-1415-{
include/linux/fs.h:1416: return from_kuid(i_user_ns(inode), inode->i_uid);
include/linux/fs.h-1417-}
--
include/linux/fs.h=1419=static inline gid_t i_gid_read(const struct inode *inode)
include/linux/fs.h-1420-{
include/linux/fs.h:1421: return from_kgid(i_user_ns(inode), inode->i_gid);
include/linux/fs.h-1422-}
--
include/linux/fs.h=1424=static inline void i_uid_write(struct inode *inode, uid_t uid)
include/linux/fs.h-1425-{
include/linux/fs.h:1426: inode->i_uid = make_kuid(i_user_ns(inode), uid);
include/linux/fs.h-1427-}
--
include/linux/fs.h=1429=static inline void i_gid_write(struct inode *inode, gid_t gid)
include/linux/fs.h-1430-{
include/linux/fs.h:1431: inode->i_gid = make_kgid(i_user_ns(inode), gid);
include/linux/fs.h-1432-}
--
include/linux/fs.h=1442=static inline vfsuid_t i_uid_into_vfsuid(struct mnt_idmap *idmap,
--
include/linux/fs.h-1444-{
include/linux/fs.h:1445: return make_vfsuid(idmap, i_user_ns(inode), inode->i_uid);
include/linux/fs.h-1446-}
--
include/linux/fs.h=1477=static inline void i_uid_update(struct mnt_idmap *idmap,
--
include/linux/fs.h-1481- if (attr->ia_valid & ATTR_UID)
include/linux/fs.h:1482: inode->i_uid = from_vfsuid(idmap, i_user_ns(inode),
include/linux/fs.h-1483- attr->ia_vfsuid);
--
include/linux/fs.h=1494=static inline vfsgid_t i_gid_into_vfsgid(struct mnt_idmap *idmap,
--
include/linux/fs.h-1496-{
include/linux/fs.h:1497: return make_vfsgid(idmap, i_user_ns(inode), inode->i_gid);
include/linux/fs.h-1498-}
--
include/linux/fs.h=1529=static inline void i_gid_update(struct mnt_idmap *idmap,
--
include/linux/fs.h-1533- if (attr->ia_valid & ATTR_GID)
include/linux/fs.h:1534: inode->i_gid = from_vfsgid(idmap, i_user_ns(inode),
include/linux/fs.h-1535- attr->ia_vfsgid);
--
include/linux/fs.h=1546=static inline void inode_fsuid_set(struct inode *inode,
--
include/linux/fs.h-1548-{
include/linux/fs.h:1549: inode->i_uid = mapped_fsuid(idmap, i_user_ns(inode));
include/linux/fs.h-1550-}
--
include/linux/fs.h=1560=static inline void inode_fsgid_set(struct inode *inode,
--
include/linux/fs.h-1562-{
include/linux/fs.h:1563: inode->i_gid = mapped_fsgid(idmap, i_user_ns(inode));
include/linux/fs.h-1564-}
]
|
| 107/4 |
2026/07/25 00:58 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 57484
output: 36
thoughts: 29
Thoughts: Here's a summary of my thought process:
**Diving into the Kernel Filesystem Interface**
Okay, so I need to understand something about how user namespaces interact with the filesystem. I recall that `include/linux/fs.h` is where a lot of the core filesystem-related structures and definitions live. Specifically, I'm thinking I need to look at the `i_user_ns` member of a struct. I'm going to jump to that file and head around line 1404; that seems like a likely place to start looking.
|
| 108/4 |
2026/07/25 00:58 |
tool |
read-file |
0m
Args: map[File:include/linux/fs.h FirstLine:1400 LineCount:10]
Results: map[Contents:1400: #define MNT_EXPIRE 0x00000004 /* Mark for expiry */
1401: #define UMOUNT_NOFOLLOW 0x00000008 /* Don't follow symlink on umount */
1402: #define UMOUNT_UNUSED 0x80000000 /* Flag guaranteed to be unused */
1403:
1404: static inline struct user_namespace *i_user_ns(const struct inode *inode)
1405: {
1406: return inode->i_sb->s_user_ns;
1407: }
1408:
1409: /* Helper functions so that in most cases filesystems will
]
|
| 109/4 |
2026/07/25 00:58 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 57878
output: 19
thoughts: 254
Thoughts: **Deconstructing the User Namespace and UID Mapping**
Okay, here's what's running through my mind. I've spotted this function, `i_user_ns(inode)`, which appears to be the key to understanding how user namespaces interact with inodes. It seems this function, from what I can see, returns `inode->i_sb->s_user_ns`.
Now, if a process mounts a file system like sysfs *within* its own user namespace, that `inode->i_sb->s_user_ns` would indeed be the user namespace of that mount, right? I'm curious how this plays out, especially with respect to the `i_uid_into_vfsuid` function. Let's see...
```c
static inline vfsuid_t i_uid_into_vfsuid(struct mnt_idmap *idmap,
const struct inode *inode)
{
return make_vfsuid(idmap, i_user_ns(inode), inode->i_uid);
}
```
Alright, so `i_user_ns(inode)` is returning `ns` β which is the user namespace of the *superblock*, as we established. The other part is `inode->i_uid` which I know is `GLOBAL_ROOT_UID`, which is basically user ID zero.
Now, what does `make_vfsuid` actually *do* with that information? That's the crucial piece. I need to get into the details of the translation, so I'll go search for `make_vfsuid` in the `include/linux/mnt_idmapping.h` header file. It should give me the lowdown on the mapping between the inode's UID, the user namespace, and the virtual file system UID.
|
| 110/4 |
2026/07/25 00:58 |
tool |
grepper |
0m
Args: map[Expression:make_vfsuid]
Results: map[Output:fs/fuse/dir.c=1361=static void fuse_fillattr(struct mnt_idmap *idmap, struct inode *inode,
--
fs/fuse/dir.c-1365- struct fuse_conn *fc = get_fuse_conn(inode);
fs/fuse/dir.c:1366: vfsuid_t vfsuid = make_vfsuid(idmap, fc->user_ns,
fs/fuse/dir.c-1367- make_kuid(fc->user_ns, attr->uid));
--
fs/mnt_idmapping.c=54=static inline bool initial_idmapping(const struct user_namespace *ns)
--
fs/mnt_idmapping.c-59-/**
fs/mnt_idmapping.c:60: * make_vfsuid - map a filesystem kuid according to an idmapping
fs/mnt_idmapping.c-61- * @idmap: the mount's idmapping
--
fs/mnt_idmapping.c-79-
fs/mnt_idmapping.c:80:vfsuid_t make_vfsuid(struct mnt_idmap *idmap,
fs/mnt_idmapping.c-81- struct user_namespace *fs_userns,
--
fs/mnt_idmapping.c-97-}
fs/mnt_idmapping.c:98:EXPORT_SYMBOL_GPL(make_vfsuid);
fs/mnt_idmapping.c-99-
--
fs/overlayfs/inode.c=341=static void ovl_idmap_posix_acl(const struct inode *realinode,
--
fs/overlayfs/inode.c-353- case ACL_USER:
fs/overlayfs/inode.c:354: vfsuid = make_vfsuid(idmap, fs_userns, e->e_uid);
fs/overlayfs/inode.c-355- e->e_uid = vfsuid_into_kuid(vfsuid);
--
fs/posix_acl.c=374=posix_acl_permission(struct mnt_idmap *idmap, struct inode *inode,
--
fs/posix_acl.c-393- case ACL_USER:
fs/posix_acl.c:394: vfsuid = make_vfsuid(idmap, fs_userns,
fs/posix_acl.c-395- pa->e_uid);
--
fs/posix_acl.c=887=static ssize_t vfs_posix_acl_to_xattr(struct mnt_idmap *idmap,
--
fs/posix_acl.c-916- case ACL_USER:
fs/posix_acl.c:917: vfsuid = make_vfsuid(idmap, fs_userns, acl_e->e_uid);
fs/posix_acl.c-918- ext_entry->e_id = cpu_to_le32(from_kuid(
--
fs/smb/server/smbacl.h=107=static inline uid_t posix_acl_uid_translate(struct mnt_idmap *idmap,
--
fs/smb/server/smbacl.h-112- /* If this is an idmapped mount, apply the idmapping. */
fs/smb/server/smbacl.h:113: vfsuid = make_vfsuid(idmap, &init_user_ns, pace->e_uid);
fs/smb/server/smbacl.h-114-
--
include/linux/fs.h=1442=static inline vfsuid_t i_uid_into_vfsuid(struct mnt_idmap *idmap,
--
include/linux/fs.h-1444-{
include/linux/fs.h:1445: return make_vfsuid(idmap, i_user_ns(inode), inode->i_uid);
include/linux/fs.h-1446-}
--
include/linux/mnt_idmapping.h=125=void mnt_idmap_put(struct mnt_idmap *idmap);
include/linux/mnt_idmapping.h-126-
include/linux/mnt_idmapping.h:127:vfsuid_t make_vfsuid(struct mnt_idmap *idmap,
include/linux/mnt_idmapping.h-128- struct user_namespace *fs_userns, kuid_t kuid);
--
security/commoncap.c=420=int cap_inode_getsecurity(struct mnt_idmap *idmap,
--
security/commoncap.c-462- /* If this is an idmapped mount shift the kuid. */
security/commoncap.c:463: vfsroot = make_vfsuid(idmap, fs_ns, kroot);
security/commoncap.c-464-
--
security/commoncap.c=675=int get_vfs_caps_from_disk(struct mnt_idmap *idmap,
--
security/commoncap.c-727-
security/commoncap.c:728: rootvfsuid = make_vfsuid(idmap, fs_ns, rootkuid);
security/commoncap.c-729- if (!vfsuid_valid(rootvfsuid))
]
|
| 111/4 |
2026/07/25 00:58 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 59904
output: 37
thoughts: 22
Thoughts: Okay, let's dive into this. Here's my take on understanding this code:
**Analyzing `make_vfsuid` in `fs/mnt_idmapping.c`**
Alright, time to crack open `make_vfsuid` within `fs/mnt_idmapping.c`. My initial assumption is this function is absolutely central to handling some sort of user ID mapping, specifically within the context of mounted filesystems β the "mnt" in the filename hints at that. Given the module name, it's pretty clear it's about dealing with IDs, probably for security or namespace isolation. I expect to see logic relating virtual filesystem (VFS) IDs to potentially host or container UIDs. The function name itself, `make_vfsuid`, suggests that it takes *something* and outputs the VFS user ID. I'll need to figure out what that "something" is, and what rules dictate the output. I'll be looking for things like context, possibly flags or arguments, and definitely some sort of ID translation table or algorithm. This seems like a core building block for any sophisticated user namespace implementation in the kernel.
|
| 112/4 |
2026/07/25 00:58 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/mnt_idmapping.c Name:make_vfsuid]
Results: map[SourceCode: 80: vfsuid_t make_vfsuid(struct mnt_idmap *idmap,
81: struct user_namespace *fs_userns,
82: kuid_t kuid)
83: {
84: uid_t uid;
85:
86: if (idmap == &nop_mnt_idmap)
87: return VFSUIDT_INIT(kuid);
88: if (idmap == &invalid_mnt_idmap)
89: return INVALID_VFSUID;
90: if (initial_idmapping(fs_userns))
91: uid = __kuid_val(kuid);
92: else
93: uid = from_kuid(fs_userns, kuid);
94: if (uid == (uid_t)-1)
95: return INVALID_VFSUID;
96: return VFSUIDT_INIT_RAW(map_id_down(&idmap->uid_map, uid));
97: }
SourceFile:fs/mnt_idmapping.c]
|
| 113/4 |
2026/07/25 00:58 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 60515
output: 968
thoughts: 3272
Thoughts: **Analysis of sysfs Unbind Write Permissions in User Namespaces**
Okay, let's break this down. Line 86 caught my eye, specifically the `idmap` check. With sysfs not using idmapped mounts, `i_uid_into_vfsuid` is going to give us `VFSUIDT_INIT(inode->i_uid)`. Given that `inode->i_uid` is `GLOBAL_ROOT_UID`, the `vfsuid` turns out to be `GLOBAL_ROOT_UID` (0). But, when looking at `current_fsuid()`, that's the process's effective UID, and in a user namespace, that's mapped. So, the comparison will *always* be false.
Then, there's `capable_wrt_inode_uidgid`. The `privileged_wrt_inode_uidgid` piece boils down to `vfsuid_has_mapping`. Because `GLOBAL_ROOT_UID` isn't mapped in the process's namespace (unprivileged in initial), `vfsuid_has_mapping` will return false. Meaning `capable_wrt_inode_uidgid` is *also* false.
Now, what if we use `i_uid_read`? Ah, that's what's used for reading the uid of an inode (like in a `stat` call). `i_uid_read` turns `GLOBAL_ROOT_UID` into `(uid_t)-1` which translates to 65534 (`nobody` or `overflowuid`) in the process's namespace. That's key. If I run `ls -l /sys/bus/platform/drivers/*/unbind` from *inside* a user namespace, it's going to show the owner as `nobody` (65534), not the host's root (0)! And that's exactly what I'd expect. The host's root (0) *isn't* mapped. So the process, even if it's "root" inside the namespace (uid 0), its effective UID on the host is something else, like 100000. So, the `current_fsuid()` is 100000, and the owner is the host root (0). No match. And therefore, no access.
Let's think, is there ANY way around this to write to `/sys/bus/platform/drivers/*/unbind` from a user namespace? If the question says the process is unprivileged in the initial namespace, we can't map the host's root. The host's root will never be mapped into the user namespace.
Okay, let's look at the file permissions. The definition: `static DRIVER_ATTR_IGNORE_LOCKDEP(unbind, 0200, NULL, unbind_store);` confirms `0200`, meaning write-only by owner. Both bind and unbind are `0200`. No dice there.
I wonder about symlinks, like `/sys/devices/platform/*/driver/unbind`. Nope, it's the same file.
What about a known security boundary here? This feels right. Yes, this is *exactly* how it should be. Unprivileged user namespaces are for containers, but they *cannot* affect the host. They shouldn't be able to bind/unbind drivers, load modules, or mess with raw devices on the host. These files are owned by the host's root (uid 0), and have `0200` permissions. The container root can't write to them. This is a feature! Prevents container root from breaking the host.
What about if I mount `/sys` read-write? Doesn't matter. The kernel's permission checks still block it.
Here is the step-by-step verification:
1. Inside the user namespace, the process is root.
2. Process tries to write to `/sys/bus/platform/drivers/some_driver/unbind`.
3. Kernel checks permissions on inode.
4. Inode has `i_uid = GLOBAL_ROOT_UID` (host's root, 0) and `i_mode = 0200`.
5. Process's `current_fsuid()` is, say, 100000.
6. `100000 != 0`, not the owner.
7. Mode `0200`: owner only. "Other" has no permission.
8. Kernel checks `CAP_DAC_OVERRIDE`.
9. Process *does* have it in the
|
| 114/4 |
2026/07/25 00:59 |
tool |
set-results |
0m
Args: map[Answer:No, a process running as root in a user namespace (but unprivileged in the initial user namespace) **cannot** write to `/sys/bus/platform/drivers/*/unbind`.
Even if the sysfs filesystem is mounted as read-write inside the user namespace, the write attempt will be denied with **`Permission denied` (`-EACCES`)** due to standard Linux VFS discretionary access control (DAC) and capability checks.
### Detailed Step-by-Step Analysis
#### 1. File Definition and Permissions
In `drivers/base/bus.c`, the `unbind` attribute is defined with write-only permissions for the owner:
```c
static DRIVER_ATTR_IGNORE_LOCKDEP(unbind, 0200, NULL, unbind_store);
```
The mode `0200` means only the owner of the file has write permissions; group and "other" have no permissions.
#### 2. Inode Ownership in Kernfs/Sysfs
Sysfs is built on top of `kernfs`. In `fs/kernfs/inode.c`, default attributes for kernfs nodes are initialized to the host's root UID and GID:
```c
/* assign default attributes */
ret->ia_uid = GLOBAL_ROOT_UID;
ret->ia_gid = GLOBAL_ROOT_GID;
```
These attributes are applied to the VFS inode's `i_uid` and `i_gid` fields during inode initialization and refresh. Therefore, the `unbind` file is owned by `GLOBAL_ROOT_UID` (UID 0 in the initial user namespace).
#### 3. VFS Permission Checks (`generic_permission`)
When a process attempts to open the `unbind` file for writing, the VFS performs permission checks via `inode_permission()` (in `fs/namei.c`), which calls `generic_permission()`:
1. **Owner Check**: `generic_permission()` calls `acl_permission_check()`, which compares the process's filesystem UID (`current_fsuid()`) with the file's UID (`i_uid_into_vfsuid()`).
- Since the process is unprivileged in the initial user namespace, its `current_fsuid()` is mapped to a non-zero, unprivileged UID on the host (e.g., `100000`).
- Since the file's UID is `GLOBAL_ROOT_UID` (0), the process is **not the owner** of the file.
- Because the file mode is `0200`, "other" has no write permissions, and `acl_permission_check()` returns `-EACCES`.
2. **Capability Check**: To bypass the DAC check, `generic_permission()` checks if the process has `CAP_DAC_OVERRIDE` using `capable_wrt_inode_uidgid()`:
```c
bool capable_wrt_inode_uidgid(struct mnt_idmap *idmap,
const struct inode *inode, int cap)
{
struct user_namespace *ns = current_user_ns();
return ns_capable(ns, cap) &&
privileged_wrt_inode_uidgid(ns, idmap, inode);
}
```
`privileged_wrt_inode_uidgid()` checks if the file's UID and GID are mapped into the process's user namespace:
```c
bool privileged_wrt_inode_uidgid(struct user_namespace *ns,
struct mnt_idmap *idmap,
const struct inode *inode)
{
return vfsuid_has_mapping(ns, i_uid_into_vfsuid(idmap, inode)) &&
vfsgid_has_mapping(ns, i_gid_into_vfsgid(idmap, inode));
}
```
- Because the process is unprivileged in the initial user namespace, the host's root UID (`GLOBAL_ROOT_UID`) is **not mapped** in the process's user namespace.
- Therefore, `vfsuid_has_mapping()` returns `false`, meaning the process's namespace capabilities are not effective over this inode.
- The capability check fails, and the write attempt is rejected with `-EACCES`.]
Results: map[Answer:No, a process running as root in a user namespace (but unprivileged in the initial user namespace) **cannot** write to `/sys/bus/platform/drivers/*/unbind`.
Even if the sysfs filesystem is mounted as read-write inside the user namespace, the write attempt will be denied with **`Permission denied` (`-EACCES`)** due to standard Linux VFS discretionary access control (DAC) and capability checks.
### Detailed Step-by-Step Analysis
#### 1. File Definition and Permissions
In `drivers/base/bus.c`, the `unbind` attribute is defined with write-only permissions for the owner:
```c
static DRIVER_ATTR_IGNORE_LOCKDEP(unbind, 0200, NULL, unbind_store);
```
The mode `0200` means only the owner of the file has write permissions; group and "other" have no permissions.
#### 2. Inode Ownership in Kernfs/Sysfs
Sysfs is built on top of `kernfs`. In `fs/kernfs/inode.c`, default attributes for kernfs nodes are initialized to the host's root UID and GID:
```c
/* assign default attributes */
ret->ia_uid = GLOBAL_ROOT_UID;
ret->ia_gid = GLOBAL_ROOT_GID;
```
These attributes are applied to the VFS inode's `i_uid` and `i_gid` fields during inode initialization and refresh. Therefore, the `unbind` file is owned by `GLOBAL_ROOT_UID` (UID 0 in the initial user namespace).
#### 3. VFS Permission Checks (`generic_permission`)
When a process attempts to open the `unbind` file for writing, the VFS performs permission checks via `inode_permission()` (in `fs/namei.c`), which calls `generic_permission()`:
1. **Owner Check**: `generic_permission()` calls `acl_permission_check()`, which compares the process's filesystem UID (`current_fsuid()`) with the file's UID (`i_uid_into_vfsuid()`).
- Since the process is unprivileged in the initial user namespace, its `current_fsuid()` is mapped to a non-zero, unprivileged UID on the host (e.g., `100000`).
- Since the file's UID is `GLOBAL_ROOT_UID` (0), the process is **not the owner** of the file.
- Because the file mode is `0200`, "other" has no write permissions, and `acl_permission_check()` returns `-EACCES`.
2. **Capability Check**: To bypass the DAC check, `generic_permission()` checks if the process has `CAP_DAC_OVERRIDE` using `capable_wrt_inode_uidgid()`:
```c
bool capable_wrt_inode_uidgid(struct mnt_idmap *idmap,
const struct inode *inode, int cap)
{
struct user_namespace *ns = current_user_ns();
return ns_capable(ns, cap) &&
privileged_wrt_inode_uidgid(ns, idmap, inode);
}
```
`privileged_wrt_inode_uidgid()` checks if the file's UID and GID are mapped into the process's user namespace:
```c
bool privileged_wrt_inode_uidgid(struct user_namespace *ns,
struct mnt_idmap *idmap,
const struct inode *inode)
{
return vfsuid_has_mapping(ns, i_uid_into_vfsuid(idmap, inode)) &&
vfsgid_has_mapping(ns, i_gid_into_vfsgid(idmap, inode));
}
```
- Because the process is unprivileged in the initial user namespace, the host's root UID (`GLOBAL_ROOT_UID`) is **not mapped** in the process's user namespace.
- Therefore, `vfsuid_has_mapping()` returns `false`, meaning the process's namespace capabilities are not effective over this inode.
- The capability check fails, and the write attempt is rejected with `-EACCES`.]
|
| 115/2 |
2026/07/25 00:59 |
llm |
expert |
0m
Model:
Tokens:
input: 21880
output: 46
thoughts: 1152
Thoughts: **Analyzing the Null Pointer Dereference in the Dummy USB Driver**
Okay, let me think this through. This bug looks tricky. I've got a NULL pointer dereference situation with the `dummy_hcd` and `dummy_udc` drivers. The vulnerability arises when you attempt to access `dum->devstatus` after `dummy_hcd.0` has been unbound.
First, I need to confirm the conditions for triggering it. The critical sequence seems to be: 1) unbind `dummy_hcd.0`, and 2) bind a gadget driver to `dummy_udc.0`. The issue lies in this code:
```c
dum = gadget_to_dummy_hcd(_gadget)->dum;
if (value)
dum->devstatus |= (1 << USB_DEVICE_SELF_POWERED);
else
dum->devstatus &= ~(1 << USB_DEVICE_SELF_POWERED);
```
Basically, if the `dummy_hcd` is gone, `gadget_to_dummy_hcd` is going to return a pointer that's essentially garbage, meaning `NULL`, resulting in a dereference of `NULL->dum`. The pointer `dum` points to `0`. That's the core of the problem.
Now, I need to figure out how exploitable this is. A direct read from address 0, in theory, would trigger a page fault, and the kernel would either panic (if `panic_on_oops` is set) or kill the process. But let's look at the assembly from the crash report I'm seeing:
```assembly
20: 4d 8b 3e mov (%r14),%r15
23: 4c 89 f8 mov %r15,%rax
26: 48 c1 e8 03 shr $0x3,%rax
* 2a: 42 80 3c 20 00 cmpb $0x0,(%rax,%r12,1) <-- trapping instruction
```
Aha! It's KASAN! The `cmpb` instruction, the trapping instruction, is the key. The code's checking the KASAN shadow memory for address 0. `(%r14)` is 0, so `%rax` is 0 (after some bit-shifting). `%r12` is the KASAN shadow offset. So, this is a KASAN shadow memory access fault. The oops confirms: "Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI". Without KASAN, it would be a regular read from address 0, causing a page fault.
The implication is that, in a modern system with `mmap_min_addr`, this is a pure NULL pointer dereference. So, it's very unlikely to be directly exploitable for privilege escalation. It's much more likely to be a Denial of Service, causing an oops/panic.
My next step is to make sure I can only trigger this as root. Can I unbind `dummy_hcd.0` without root? Writing to `/sys/bus/platform/drivers/dummy_hcd/unbind` requires root, as I suspected. Is there another way, maybe if a module initialization fails and I somehow force it to clean itself? I should check `dummy_hcd_probe` and `dummy_udc_probe` for edge cases.
|
| 116/2 |
2026/07/25 00:59 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/usb/gadget/udc/dummy_hcd.c Name:dummy_hcd_probe]
Results: map[SourceCode:2687: static int dummy_hcd_probe(struct platform_device *pdev)
2688: {
2689: struct dummy *dum;
2690: struct usb_hcd *hs_hcd;
2691: struct usb_hcd *ss_hcd;
2692: int retval;
2693:
2694: dev_info(&pdev->dev, "%s, driver " DRIVER_VERSION "\n", driver_desc);
2695: dum = *((void **)dev_get_platdata(&pdev->dev));
2696:
2697: if (mod_data.is_super_speed)
2698: dummy_hcd.flags = HCD_USB3 | HCD_SHARED;
2699: else if (mod_data.is_high_speed)
2700: dummy_hcd.flags = HCD_USB2;
2701: else
2702: dummy_hcd.flags = HCD_USB11;
2703: hs_hcd = usb_create_hcd(&dummy_hcd, &pdev->dev, dev_name(&pdev->dev));
2704: if (!hs_hcd)
2705: return -ENOMEM;
2706: hs_hcd->has_tt = 1;
2707:
2708: retval = usb_add_hcd(hs_hcd, 0, 0);
2709: if (retval)
2710: goto put_usb2_hcd;
2711:
2712: if (mod_data.is_super_speed) {
2713: ss_hcd = usb_create_shared_hcd(&dummy_hcd, &pdev->dev,
2714: dev_name(&pdev->dev), hs_hcd);
2715: if (!ss_hcd) {
2716: retval = -ENOMEM;
2717: goto dealloc_usb2_hcd;
2718: }
2719:
2720: retval = usb_add_hcd(ss_hcd, 0, 0);
2721: if (retval)
2722: goto put_usb3_hcd;
2723: }
2724: return 0;
2725:
2726: put_usb3_hcd:
2727: usb_put_hcd(ss_hcd);
2728: dealloc_usb2_hcd:
2729: usb_remove_hcd(hs_hcd);
2730: put_usb2_hcd:
2731: usb_put_hcd(hs_hcd);
2732: dum->hs_hcd = dum->ss_hcd = NULL;
2733: return retval;
2734: }
SourceFile:drivers/usb/gadget/udc/dummy_hcd.c]
|
| 117/2 |
2026/07/25 00:59 |
llm |
expert |
0m
Model:
Tokens:
input: 24706
output: 46
thoughts: 78
Thoughts: Here's my thought process, distilled:
**Digging into the USB Driver Initialization**
Okay, so `dummy_hcd_probe` is the function responsible for actually *creating* these HCDs (Host Controller Drivers). Got it. And if it's successful, I see that `dum->hs_hcd` gets set. I remember that's within `dummy_setup`, which in turn is called from `usb_add_hcd`. Okay, so `dummy_setup` is the point where the HCD instance gets associated. Let's make sure I'm following this correctly - just to be clear, `dummy_setup` specifically sets `dum->hs_hcd`. Good.
Now, let's shift gears and check the `dummy_udc_probe` function. That seems like the next logical step.
|
| 118/2 |
2026/07/25 00:59 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/usb/gadget/udc/dummy_hcd.c Name:dummy_udc_probe]
Results: map[SourceCode:1083: static int dummy_udc_probe(struct platform_device *pdev)
1084: {
1085: struct dummy *dum;
1086: int rc;
1087:
1088: dum = *((void **)dev_get_platdata(&pdev->dev));
1089: /* Clear usb_gadget region for new registration to udc-core */
1090: memzero_explicit(&dum->gadget, sizeof(struct usb_gadget));
1091: dum->gadget.name = gadget_name;
1092: dum->gadget.ops = &dummy_ops;
1093: if (mod_data.is_super_speed)
1094: dum->gadget.max_speed = USB_SPEED_SUPER;
1095: else if (mod_data.is_high_speed)
1096: dum->gadget.max_speed = USB_SPEED_HIGH;
1097: else
1098: dum->gadget.max_speed = USB_SPEED_FULL;
1099:
1100: dum->gadget.dev.parent = &pdev->dev;
1101: init_dummy_udc_hw(dum);
1102:
1103: rc = usb_add_gadget_udc(&pdev->dev, &dum->gadget);
1104: if (rc < 0)
1105: goto err_udc;
1106:
1107: rc = device_create_file(&dum->gadget.dev, &dev_attr_function);
1108: if (rc < 0)
1109: goto err_dev;
1110: platform_set_drvdata(pdev, dum);
1111: return rc;
1112:
1113: err_dev:
1114: usb_del_gadget_udc(&dum->gadget);
1115: err_udc:
1116: return rc;
1117: }
SourceFile:drivers/usb/gadget/udc/dummy_hcd.c]
|
| 119/2 |
2026/07/25 00:59 |
llm |
expert |
0m
Model:
Tokens:
input: 25607
output: 41
thoughts: 30
Thoughts: Okay, here's my take on those thoughts, framed as my own mental processing as an expert in this field:
**UDC Probe Registration: A Quick Review**
Alright, so the `dummy_udc_probe` is what gets the UDC registered, that's pretty standard. Seems straightforward enough. The key here is the fact that we have two probes, and the way this is set up, they function independently. This is important to remember as we move forward. And, as expected, the registration happens within the `init` function, typical for initialization routines and driver setup. Nothing unexpected here so far. Good. Let's make sure that's kept in mind as we keep digging in...
|
| 120/2 |
2026/07/25 00:59 |
tool |
codesearch-definition-source |
0m
Error: requested entity does not exist
Args: map[ContextFile:drivers/usb/gadget/udc/dummy_hcd.c Name:init]
Results: map[SourceCode: SourceFile:]
|
| 121/2 |
2026/07/25 00:59 |
llm |
expert |
0m
Model:
Tokens:
input: 25848
output: 39
thoughts: 9
Thoughts: Okay, here's my attempt at summarizing the thought process, tailored for an expert audience:
**Hunting Down the Initialization Ritual**
Right, let's get down to brass tacks. I'm going to track down the module initialization function. It's the critical entry point; the very heartbeat of this thing. Need to understand how this module gets off the ground. What dependencies it's bringing in, how it sets up its internal state, and where the hooks are. It's the lynchpin, the place where everything comes alive. Gotta zero in on that init function.
|
| 122/2 |
2026/07/25 00:59 |
tool |
grepper |
0m
Args: map[Expression:module_init PathPrefix:drivers/usb/gadget/udc/dummy_hcd.c]
Results: map[Output:drivers/usb/gadget/udc/dummy_hcd.c=2799=static int __init dummy_hcd_init(void)
--
drivers/usb/gadget/udc/dummy_hcd.c-2920-}
drivers/usb/gadget/udc/dummy_hcd.c:2921:module_init(dummy_hcd_init);
drivers/usb/gadget/udc/dummy_hcd.c-2922-
]
|
| 123/2 |
2026/07/25 00:59 |
llm |
expert |
0m
Model:
Tokens:
input: 26150
output: 46
thoughts: 12
Thoughts: Okay, let's dive into this. Here's my take on those thoughts, as if they're my own:
**Initial Assessment of `dummy_hcd_init`**
Alright, so the instruction says we're looking at `dummy_hcd_init`. Hmm, a "dummy" High-level Controller Driver initialization function, I see. My initial thought is, this likely serves as a placeholder or a simplified starting point for a more complex HCD. It's probably designed for testing, simulation, or potentially for demonstrating basic functionality without the full hardware dependencies of a real HCD. Given the "dummy" designation, I'm expecting a minimal implementation, likely focused on setting up basic data structures, perhaps registering some interrupt handlers, and maybe, just maybe, printing some status messages. It's unlikely to perform any actual USB transaction management in a realistic way, but more as a starting template. I'll need to look at its implementation details to see exactly what its assumptions are and the level of functionality is expected to have.
|
| 124/2 |
2026/07/25 00:59 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/usb/gadget/udc/dummy_hcd.c Name:dummy_hcd_init]
Results: map[SourceCode:2799: static int __init dummy_hcd_init(void)
2800: {
2801: int retval = -ENOMEM;
2802: int i;
2803: struct dummy *dum[MAX_NUM_UDC] = {};
2804:
2805: if (usb_disabled())
2806: return -ENODEV;
2807:
2808: if (!mod_data.is_high_speed && mod_data.is_super_speed)
2809: return -EINVAL;
2810:
2811: if (mod_data.num < 1 || mod_data.num > MAX_NUM_UDC) {
2812: pr_err("Number of emulated UDC must be in range of 1...%d\n",
2813: MAX_NUM_UDC);
2814: return -EINVAL;
2815: }
2816:
2817: for (i = 0; i < mod_data.num; i++) {
2818: the_hcd_pdev[i] = platform_device_alloc(driver_name, i);
2819: if (!the_hcd_pdev[i]) {
2820: i--;
2821: while (i >= 0)
2822: platform_device_put(the_hcd_pdev[i--]);
2823: return retval;
2824: }
2825: }
2826: for (i = 0; i < mod_data.num; i++) {
2827: the_udc_pdev[i] = platform_device_alloc(gadget_name, i);
2828: if (!the_udc_pdev[i]) {
2829: i--;
2830: while (i >= 0)
2831: platform_device_put(the_udc_pdev[i--]);
2832: goto err_alloc_udc;
2833: }
2834: }
2835: for (i = 0; i < mod_data.num; i++) {
2836: dum[i] = kzalloc_obj(struct dummy);
2837: if (!dum[i]) {
2838: retval = -ENOMEM;
2839: goto err_add_pdata;
2840: }
2841: retval = platform_device_add_data(the_hcd_pdev[i], &dum[i],
2842: sizeof(void *));
2843: if (retval)
2844: goto err_add_pdata;
2845: retval = platform_device_add_data(the_udc_pdev[i], &dum[i],
2846: sizeof(void *));
2847: if (retval)
2848: goto err_add_pdata;
2849: }
2850:
2851: retval = platform_driver_register(&dummy_hcd_driver);
2852: if (retval < 0)
2853: goto err_add_pdata;
2854: retval = platform_driver_register(&dummy_udc_driver);
2855: if (retval < 0)
2856: goto err_register_udc_driver;
2857:
2858: for (i = 0; i < mod_data.num; i++) {
2859: retval = platform_device_add(the_hcd_pdev[i]);
2860: if (retval < 0) {
2861: i--;
2862: while (i >= 0)
2863: platform_device_del(the_hcd_pdev[i--]);
2864: goto err_add_hcd;
2865: }
2866: }
2867: for (i = 0; i < mod_data.num; i++) {
2868: if (!dum[i]->hs_hcd ||
2869: (!dum[i]->ss_hcd && mod_data.is_super_speed)) {
2870: /*
2871: * The hcd was added successfully but its probe
2872: * function failed for some reason.
2873: */
2874: retval = -EINVAL;
2875: goto err_add_udc;
2876: }
2877: }
2878:
2879: for (i = 0; i < mod_data.num; i++) {
2880: retval = platform_device_add(the_udc_pdev[i]);
2881: if (retval < 0) {
2882: i--;
2883: while (i >= 0)
2884: platform_device_del(the_udc_pdev[i--]);
2885: goto err_add_udc;
2886: }
2887: }
2888:
2889: for (i = 0; i < mod_data.num; i++) {
2890: if (!platform_get_drvdata(the_udc_pdev[i])) {
2891: /*
2892: * The udc was added successfully but its probe
2893: * function failed for some reason.
2894: */
2895: retval = -EINVAL;
2896: goto err_probe_udc;
2897: }
2898: }
2899: return retval;
2900:
2901: err_probe_udc:
2902: for (i = 0; i < mod_data.num; i++)
2903: platform_device_del(the_udc_pdev[i]);
2904: err_add_udc:
2905: for (i = 0; i < mod_data.num; i++)
2906: platform_device_del(the_hcd_pdev[i]);
2907: err_add_hcd:
2908: platform_driver_unregister(&dummy_udc_driver);
2909: err_register_udc_driver:
2910: platform_driver_unregister(&dummy_hcd_driver);
2911: err_add_pdata:
2912: for (i = 0; i < mod_data.num; i++)
2913: kfree(dum[i]);
2914: for (i = 0; i < mod_data.num; i++)
2915: platform_device_put(the_udc_pdev[i]);
2916: err_alloc_udc:
2917: for (i = 0; i < mod_data.num; i++)
2918: platform_device_put(the_hcd_pdev[i]);
2919: return retval;
2920: }
SourceFile:drivers/usb/gadget/udc/dummy_hcd.c]
|
| 125/2 |
2026/07/25 00:59 |
llm |
expert |
0m
Model:
Tokens:
input: 28390
output: 46
thoughts: 176
Thoughts: **Analyzing the Device Driver Initialization**
Okay, so I'm looking at this initialization sequence, and my initial thought is: We've got `dummy_hcd_driver` and `dummy_udc_driver` getting registered first. Standard. Then we proceed to add the HCD devices. If the HCD probe fails, it's a hard stop, which makes sense. No HCD, no point going further. Then, the UDC devices are added, and again, a probe failure here triggers a bail-out.
Now, that immediately tells me that the only path to a scenario where the `dum->hs_hcd` pointer is NULL, *but* the UDC is still registered, has to be a very specific sequence. It has to involve unbinding the HCD driver after initial registration, as that's the only way for the HCD probe to pass initially but the driver no longer be present later.
We know unbinding a driver *should* require root privileges. We established that previously. But, I'm already thinking like an adversary here, what *else* could trigger the removal of that HCD driver? The typical module unload sequence should also be considered.
Ah, wait...What if the physical device itself is removed? Could a hotplug event, for example, somehow trigger `dummy_hcd_remove`? The code uses `platform_device_del` on module unload, so that seems like the most likely suspect. But... module unload also unregisters the UDC driver. Right. Okay, so that's not the path.
Okay, let's just make sure there are no other paths through `dummy_hcd_cleanup` to get it there, and then we'll be done.
|
| 126/2 |
2026/07/25 00:59 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/usb/gadget/udc/dummy_hcd.c Name:dummy_hcd_cleanup]
Results: map[SourceCode:2923: static void __exit dummy_hcd_cleanup(void)
2924: {
2925: int i;
2926:
2927: for (i = 0; i < mod_data.num; i++) {
2928: struct dummy *dum;
2929:
2930: dum = *((void **)dev_get_platdata(&the_udc_pdev[i]->dev));
2931:
2932: platform_device_unregister(the_udc_pdev[i]);
2933: platform_device_unregister(the_hcd_pdev[i]);
2934: kfree(dum);
2935: }
2936: platform_driver_unregister(&dummy_udc_driver);
2937: platform_driver_unregister(&dummy_hcd_driver);
2938: }
SourceFile:drivers/usb/gadget/udc/dummy_hcd.c]
|
| 127/2 |
2026/07/25 00:59 |
llm |
expert |
0m
Model:
Tokens:
input: 29262
output: 32
thoughts: 1101
Thoughts: **Analyzing the Device Driver Unbinding Vulnerability**
Okay, so I'm trying to figure out how this bug works and if it's exploitable. First, I ruled out a module unload as the trigger because the UDC device is unregistered before the HCD. So that's safe.
My next thought was unbinding via `sysfs`. We already saw that the `unbind` file is owned by root. Is there any other way to unbind a driver? No, it's either through sysfs or module load/unload. And both require root privileges.
But, what about if we use user namespaces? Maybe a user namespace's root can somehow do this? We know `unbind` is owned by the global root. If I create a user namespace and mount sysfs, the files are still owned by that global root user. I know the user namespace root has UID 0 inside the namespace, which corresponds to some unprivileged UID outside. When that user namespace root tries to write to `unbind`, the VFS will check permissions. The file's UID is still 0 (global root). The VFS will compare the file's uid and the user's uid. The VFS won't match, because the user's underlying UID is unprivileged. Then it will check if I have the `CAP_DAC_OVERRIDE` capability.
The VFS will call `capable_wrt_inode_uidgid`. Since the global root UID isn't mapped into the user namespace, that check returns false. So, a user in a namespace can't write to unbind. It's blocked.
Could sysfs maybe be mounted with options that change ownership? No, it doesn't support those options. What about network namespaces? Does that change the ownership? Nope, that's only for network devices. `dummy_hcd` is a platform device, so it's always in `/sys/bus/platform/` and always owned by that global root.
Let me verify this. If I unshare and then mount sysfs, the files in `/sys/bus/platform/drivers/` are owned by `nobody`. The user namespace root still can't write to those files.
So, now let's think about syzkaller. Does syzkaller run as root? Often, yes. If it's root, then it *can* write to `unbind`. The bug is accessible to root. But, not to unprivileged users or from user namespaces.
Okay, let's explore if there are any other ways to trigger the unbinding. Could it be a USB device being unplugged? No, it's a platform device created by the module's init, so I can't unplug it.
What about using `configfs` to create a gadget and bind it to `dummy_udc.0`? The reproducer does this: making a directory then open and write to it. Can an unprivileged user do this? Well, can `configfs` be mounted inside a user namespace? No, it doesn't have the `FS_USERNS_MOUNT` flag.
|
| 128/2 |
2026/07/25 01:00 |
tool |
grepper |
0m
Error:
Args: map[Expression:FS_USERNS_MOUNT PathPrefix:fs/configfs/]
Results:
|
| 129/2 |
2026/07/25 01:00 |
llm |
expert |
0m
Model:
Tokens:
input: 31073
output: 31
thoughts: 17
Thoughts: Here's my thought process, summarized as you requested:
**Investigating `configfs` and User Namespaces**
Okay, so I'm thinking, let's dive into this `configfs` situation. I need to figure out if it's even *possible* to mount `configfs` within a user namespace. This is crucial for sandboxing and secure containerization. This could open a lot of interesting possibilities for configuration management within isolated environments. The implications here are pretty significant if we can get this to work correctly. I need to verify its behavior in different contexts. This could be a good starting point for achieving a robust configuration system in containerized environments.
|
| 130/2 |
2026/07/25 01:00 |
tool |
grepper |
0m
Args: map[Expression:configfs_fs_type PathPrefix:fs/configfs/]
Results: map[Output:fs/configfs/mount.c=109=static int configfs_init_fs_context(struct fs_context *fc)
--
fs/configfs/mount.c-114-
fs/configfs/mount.c:115:static struct file_system_type configfs_fs_type = {
fs/configfs/mount.c-116- .owner = THIS_MODULE,
--
fs/configfs/mount.c=123=struct dentry *configfs_pin_fs(void)
fs/configfs/mount.c-124-{
fs/configfs/mount.c:125: int err = simple_pin_fs(&configfs_fs_type, &configfs_mount,
fs/configfs/mount.c-126- &configfs_mnt_count);
--
fs/configfs/mount.c=136=static int __init configfs_init(void)
--
fs/configfs/mount.c-149-
fs/configfs/mount.c:150: err = register_filesystem(&configfs_fs_type);
fs/configfs/mount.c-151- if (err)
--
fs/configfs/mount.c=165=static void __exit configfs_exit(void)
fs/configfs/mount.c-166-{
fs/configfs/mount.c:167: unregister_filesystem(&configfs_fs_type);
fs/configfs/mount.c-168- sysfs_remove_mount_point(kernel_kobj, "config");
]
|
| 131/2 |
2026/07/25 01:00 |
llm |
expert |
0m
Model:
Tokens:
input: 31666
output: 38
thoughts: 14
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Diving into the `configfs_fs_type` Definition**
Alright, let's get down to brass tacks. I need to refresh my understanding of the `configfs_fs_type` definition. Specifically, I'm thinking about its role within the kernel and how it interfaces with the broader virtual filesystem infrastructure. I'm imagining how it's structured, its associated data structures, and the key functions involved in mounting, unmounting, and managing configfs instances. I'm focusing on the implementation details and how the design choices influence performance and scalability. I am very interested in the specifics around error handling, concurrency, and security, especially when considering the implications of configfs being used in sensitive configuration contexts. Let's see what the definition reveals.
|
| 132/2 |
2026/07/25 01:00 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:fs/configfs/mount.c Name:configfs_fs_type]
Results: map[SourceCode: 115: static struct file_system_type configfs_fs_type = {
116: .owner = THIS_MODULE,
117: .name = "configfs",
118: .init_fs_context = configfs_init_fs_context,
119: .kill_sb = kill_anon_super,
120: };
SourceFile:fs/configfs/mount.c]
|
| 133/2 |
2026/07/25 01:00 |
llm |
expert |
0m
Model:
Tokens:
input: 32029
output: 59
thoughts: 226
Thoughts: **Analyzing the Reproducer and Bug Accessibility**
Okay, so I'm looking at this code and immediately, I see it's missing the `FS_USERNS_MOUNT` flag. That means `configfs` can't be mounted inside a user namespace. The implications here are pretty clear β the reproducer *needs* `CAP_SYS_ADMIN` in the initial user namespace to even mount `configfs` in the first place. That's a critical constraint.
Wait a second⦠the reproducer *does* have a mount call:
```c
syscall(__NR_mount, /*src=*/0ul, /*dst=*/0x200000000080ul, /*type=*/0x2000000000c0ul, /*flags=*/0ul, /*data=*/0ul);
```
And `type` is set to `"configfs"`. Right, that makes sense. If this were running as an unprivileged user, this mount would just fail. So, this bug is definitely *not* something an unprivileged process can exploit, nor is it accessible from within user namespaces. That's a major point for the security analysis. It severely limits the attack surface.
Now, I need to make absolutely sure there aren't any other ways to trigger this bug, which is sitting inside `dummy_set_selfpowered`. Are there *any* other callers of that function? I need to meticulously check to eliminate any other potential entry points for exploitation. We can't rely on the initial assumptions, and need to double check to be extra cautious.
|
| 134/2 |
2026/07/25 01:00 |
tool |
codesearch-find-references |
0m
Args: map[ContextFile:drivers/usb/gadget/udc/dummy_hcd.c IncludeSnippetLines:0 Name:dummy_set_selfpowered SourceTreePrefix:]
Results: map[References:[map[ReferenceKind:takes-address-of ReferencingEntityKind:global_variable ReferencingEntityName:dummy_ops SourceFile:drivers/usb/gadget/udc/dummy_hcd.c SourceLine:962]] TruncatedOutput:false]
|
| 135/2 |
2026/07/25 01:00 |
llm |
expert |
0m
Model:
Tokens:
input: 32746
output: 43
thoughts: 11
Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience, presented in the first person:
**Focus on `dummy_ops` Usage**
Alright, so the initial thought process is pretty straightforward: it's all about scoping. My mind immediately zeroes in on the context. The crucial bit here is that this particular element, whatever "it" refers to, is *only* utilized within `dummy_ops`. I'm already mentally filing that away as a key piece of information. This significantly impacts how I'd approach any analysis or modification. It's a localized concern, contained within a specific function or module, which heavily influences the potential impact of changes or the overall complexity.
|
| 136/2 |
2026/07/25 01:00 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:drivers/usb/gadget/udc/dummy_hcd.c Name:dummy_ops]
Results: map[SourceCode: 959: static const struct usb_gadget_ops dummy_ops = {
960: .get_frame = dummy_g_get_frame,
961: .wakeup = dummy_wakeup,
962: .set_selfpowered = dummy_set_selfpowered,
963: .pullup = dummy_pullup,
964: .udc_start = dummy_udc_start,
965: .udc_stop = dummy_udc_stop,
966: .udc_set_speed = dummy_udc_set_speed,
967: .udc_async_callbacks = dummy_udc_async_callbacks,
968: };
SourceFile:drivers/usb/gadget/udc/dummy_hcd.c]
|
| 137/2 |
2026/07/25 01:00 |
llm |
expert |
0m
Model:
Tokens:
input: 33179
output: 59
thoughts: 37
Thoughts: Okay, here's my interpretation of those thoughts, presented as a summary from my perspective as someone well-versed in USB gadget drivers:
**Understanding the `dummy_ops` and Its Role in UDC Configuration**
Right, so I'm thinking about the `dummy_ops` structure. This is essentially the operational framework, or the "methods" table, for the dummy USB device controller (UDC). I know this because the code calls it `usb_gadget_ops`. And, as I understand it, the whole point of this `dummy_ops` is that it exists as a sort of placeholder. It's got to provide hooks for the USB Gadget stack so that a "functioning" gadget exists, even if there's no real hardware connected.
I'm specifically focusing on how it interacts with `usb_gadget_set_selfpowered`. So, I'm thinking, whenever the gadget needs to signal that it's either self-powered or bus-powered, it's the `dummy_ops` that's called in response. In other words, this structure, in this case, allows the USB subsystem to inform the "gadget" whether it is self-powered or not. I'm wondering if this is because, since this is a dummy, setting self-powered is basically a no-op, or maybe it sets some internal flag for subsequent code to check. I need to dig into the actual implementation of this function to understand what it's really doing under the hood. It seems crucial to ensuring proper gadget behavior, even in this emulated scenario.
|
| 138/2 |
2026/07/25 01:00 |
tool |
codesearch-find-references |
0m
Args: map[ContextFile:drivers/usb/gadget/udc/core.c IncludeSnippetLines:0 Name:usb_gadget_set_selfpowered SourceTreePrefix:]
Results: map[References:[map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:composite_dev_prepare SourceFile:drivers/usb/gadget/composite.c SourceLine:2440] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:composite_resume SourceFile:drivers/usb/gadget/composite.c SourceLine:2643] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:composite_suspend SourceFile:drivers/usb/gadget/composite.c SourceLine:2604] map[ReferenceKind:calls ReferencingEntityKind:function ReferencingEntityName:set_config SourceFile:drivers/usb/gadget/composite.c SourceLine:1057] map[ReferenceKind:takes-address-of ReferencingEntityKind:global_variable ReferencingEntityName:__UNIQUE_ID_addressable_usb_gadget_set_selfpowered_985 SourceFile:drivers/usb/gadget/udc/core.c SourceLine:585] map[ReferenceKind:takes-address-of ReferencingEntityKind:global_variable ReferencingEntityName:__gendwarfksyms_ptr_usb_gadget_set_selfpowered SourceFile:drivers/usb/gadget/udc/core.c SourceLine:585] map[ReferenceKind:takes-address-of ReferencingEntityKind:global_variable ReferencingEntityName:__gendwarfksyms_ptr_usb_gadget_set_selfpowered SourceFile:drivers/usb/gadget/udc/core.c SourceLine:585]] TruncatedOutput:false]
|
| 139/2 |
2026/07/25 01:00 |
llm |
expert |
0m
Model:
Tokens:
input: 34062
output: 66
thoughts: 800
Thoughts: **My Analysis of the USB Gadget NULL Pointer Dereference**
Okay, so here's what I've got. The core issue is a NULL pointer dereference within `dummy_hcd`'s `dummy_set_selfpowered` function. It's triggered when a gadget driver is bound after the `dummy_hcd` platform device has been unbound, leaving a dangling pointer. Specifically, `composite_dev_prepare`, which calls `usb_gadget_set_selfpowered`, is the culprit. We know the gadget binding can be achieved through `configfs` or legacy drivers, but here's where the security dimensions come into play.
To trigger this, you'd need to unbind the HCD part and then bind the UDC part of the USB gadget device. The analysis states that you need root to unload a legacy driver using `CAP_SYS_MODULE`. For `configfs`, mounting it needs `CAP_SYS_ADMIN` in the initial user namespace. After this, to be able to unbind using sysfs, root access is needed, as it is owned by `GLOBAL_ROOT_UID`. So, an unprivileged user is completely out of the picture. No go for user namespaces either, and no way for a VM guest or host to trigger it. Ethernet, remote, peripheral or malicious filesystem triggers are also off the table.
Now, a kernel oops is definitely a Denial of Service, but is it exploitable? Nope, it's a straight-up NULL pointer dereference. The kernel just throws a general protection fault (with KASAN) or a page fault (without KASAN) and kills the process, or panics if `panic_on_oops` is set. Since it's reading from address 0, there's no way to elevate privileges.
The `dummy_hcd` driver is a testing driver. I need to make sure this driver is used in production. Even if it is, the requirement for root access makes it much less concerning.
In short: A root-level Denial of Service due to a NULL pointer dereference in a test driver. Not a security threat in the traditional sense. It's a kernel oops, not a privilege escalation. The only way is to root. And that's not even a remote trigger. So, not a high priority.
|
| 140/2 |
2026/07/25 01:00 |
tool |
set-results |
0m
Args: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 141/2 |
2026/07/25 01:00 |
llm |
expert |
0m
Model:
Tokens:
input: 35485
output: 737
thoughts: 496
Thoughts: **Final Thoughts on the Bug**
Okay, the analysis is complete. Time to synthesize my understanding into the final response.
The bottom line is that we're looking at a classic NULL pointer dereference, specifically within the `dummy_set_selfpowered` function of the `dummy_hcd` driver. The root cause is pretty straightforward: the `dummy_hcd` module registers two platform devices, one for the Host Controller (HCD) and another for the USB Device Controller (UDC). The crux of the issue lies in how these devices are handled during unbinding. When the HCD device is unbound via sysfs, the `dummy_hcd_remove` function is called, which nullifies `dum->hs_hcd`. However, the UDC device persists.
The problem rears its head when a gadget driver is later bound to the UDC device. This action eventually triggers the `usb_gadget_set_selfpowered` function, which in turn calls our problematic `dummy_set_selfpowered` function. It's in this function that the code attempts to access `dum->hs_hcd->dum`, but that `hs_hcd` pointer has been set to NULL during the HCD unbinding, resulting in the dereference.
Because `dum` is at offset 0 in the structure, this NULL pointer dereference is writing to memory address 0. On a modern system, this causes a page fault, or, as the report indicates, a KASAN general protection fault, leading to a kernel oops. It's a textbook NULL pointer dereference. While it causes a denial of service if `panic_on_oops` is enabled, it isn't exploitable for privilege escalation.
Now, even though it's a bug, it's not a security vulnerability in the practical sense, because triggering this bug requires root privileges in the initial user namespace: First, unbinding the driver requires writing to `/sys/bus/platform/drivers/dummy_hcd/unbind`, which has root ownership and restrictive permissions. You can't write to this from a user namespace. Second, using `configfs` to bind the gadget driver requires a mount which isn't allowed from within a user namespace.
So, in summary, it's a denial of service, but not remotely exploitable, not even for local privilege escalation. It's an interesting analysis, but ultimately not a critical security issue.
|
| 142/1 |
2026/07/25 01:00 |
action |
format-explanation |
0m
Results: map[Explanation:**Bug Analysis:**
The bug is a NULL pointer dereference in `dummy_set_selfpowered` within the
`dummy_hcd` (USB gadget dummy HCD) driver.
The `dummy_hcd` module registers two separate platform devices during
initialization: one for the Host Controller (HCD) and one for the USB Device
Controller (UDC).
When the HCD device is manually unbound via sysfs (by writing to
`/sys/bus/platform/drivers/dummy_hcd/unbind`), the `dummy_hcd_remove` function
is invoked. This function cleans up the HCD and explicitly sets `dum->hs_hcd =
NULL`. However, the UDC device remains bound and registered with the system.
If a user subsequently configures and binds a USB gadget driver to this orphaned
UDC device (e.g., using `configfs`), the gadget framework calls
`usb_gadget_set_selfpowered()`, which in turn calls the UDC's
`dummy_set_selfpowered()` callback. This function attempts to retrieve the
`dummy` struct by dereferencing `dum->hs_hcd`, which was previously set to NULL:
```c
dum = gadget_to_dummy_hcd(_gadget)->dum; // gadget_to_dummy_hcd returns NULL
if (value)
dum->devstatus |= (1 << USB_DEVICE_SELF_POWERED);
```
Because the `dum` field is at offset 0 of `struct dummy_hcd`, this results in a
direct read from memory address `0x0`.
**Security Impact:**
* **Exploitable:** No. This is a pure NULL pointer dereference (reading from
address 0). On modern Linux systems with `mmap_min_addr` enforced, address 0
cannot be mapped by userspace. The kernel will simply trap the invalid access
(or KASAN will catch the shadow memory access for address 0, as seen in the
report) and oops. It cannot be used for privilege escalation or arbitrary code
execution.
* **Denial of Service:** Yes. The NULL pointer dereference will cause a kernel
oops, which will kill the current process and can lead to a full system crash
(kernel panic) if the system is configured with `panic_on_oops=1`.
* **Accessible From Unprivileged Processes / User Namespaces:** No. Triggering
this bug requires two highly privileged operations:
1. **Unbinding the driver:** Writing to
`/sys/bus/platform/drivers/dummy_hcd/unbind` requires root privileges. The
file has `0200` permissions and is owned by the global `root` (UID 0). A
process inside a user namespace cannot write to it because the global root UID
is not mapped into the unprivileged user namespace, causing the VFS capability
and DAC checks to deny access.
2. **Mounting configfs:** The reproducer uses `configfs` to bind the gadget.
The `configfs` filesystem does not have the `FS_USERNS_MOUNT` flag, meaning it
cannot be mounted from within a user namespace. It requires `CAP_SYS_ADMIN` in
the initial user namespace.
* **Other Triggers:** The bug is specific to the software-emulated `dummy_hcd`
driver and its sysfs/configfs management interfaces. It cannot be triggered
remotely, via malicious filesystems, or by plugging in physical USB peripherals.]
|