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KASAN: slab-use-after-free Read in netfs_unbuffered_write

Status: upstream: reported C repro on 2026/05/29 20:01
Subsystems: netfs
Labels: prio:high
[Documentation on labels]
Reported-by: syzbot+3c74b1f0c372e98efc32@syzkaller.appspotmail.com
First crash: 1d13h, last: 1d11h
▶ ▼ ✨ AI Jobs (1)
ID Workflow Result Correct Bug Created Started Finished Revision Error
b2fe841c-aa24-4cbb-9553-2a32bff4b6eb assessment-security DenialOfService: ✅ Exploitable: ✅ FilesystemTrigger: ❌ NetworkTrigger: ❌ PeripheralTrigger: ❌ RemoteTrigger: ❌ Unprivileged: ✅ UserNamespace: ✅ VMGuestTrigger: ❌ VMHostTrigger: ❌ ❓ KASAN: slab-use-after-free Read in netfs_unbuffered_write 2026/05/29 22:56 2026/05/29 22:56 2026/05/29 23:33 6b4a844333e83556da95d61d7f207e7ef5cd4bc6
▶ ▼ Discussions (2)
Title Replies (including bot) Last reply
[syzbot] [netfs?] KASAN: slab-use-after-free Read in netfs_unbuffered_write 0 (2) 2026/05/30 02:13
[PATCH] netfs: Fix UAF in netfs_unbuffered_write() on failed preparation 1 (1) 2026/05/30 01:14
▶ ▼ Similar bugs (1)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
upstream BUG: unable to handle kernel NULL pointer dereference in netfs_unbuffered_write netfs 19 C 494 21d 85d 29/29 fixed on 2026/05/21 07:50
▶ ▼ Last patch testing requests (1)
Created Duration User Patch Repo Result
2026/05/30 02:13 18m hongao@uniontech.com patch upstream report log

Sample crash report:
==================================================================
BUG: KASAN: slab-use-after-free in netfs_unbuffered_write+0x1c21/0x21e0 fs/netfs/direct_write.c:119
Read of size 2 at addr ffff888022e87864 by task syz.0.17/5936

CPU: 1 UID: 0 PID: 5936 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x100/0x190 lib/dump_stack.c:120
 print_address_description mm/kasan/report.c:378 [inline]
 print_report+0x13d/0x4b0 mm/kasan/report.c:482
 kasan_report+0xdf/0x1d0 mm/kasan/report.c:595
 netfs_unbuffered_write+0x1c21/0x21e0 fs/netfs/direct_write.c:119
 netfs_unbuffered_write_iter_locked+0x7e2/0xa90 fs/netfs/direct_write.c:294
 netfs_unbuffered_write_iter+0x442/0x740 fs/netfs/direct_write.c:386
 v9fs_file_write_iter+0xbf/0x100 fs/9p/vfs_file.c:409
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x6ac/0x1070 fs/read_write.c:688
 ksys_write+0x12a/0x250 fs/read_write.c:740
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f2eecd9ce59
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:00007f2eedc49028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f2eed015fa0 RCX: 00007f2eecd9ce59
RDX: 00000000fffffdd8 RSI: 0000200000000540 RDI: 0000000000000003
RBP: 00007f2eece32d6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f2eed016038 R14: 00007f2eed015fa0 R15: 00007fff0ed7a0b8
 </TASK>

Allocated by task 5936:
 kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
 kasan_save_track+0x14/0x30 mm/kasan/common.c:78
 unpoison_slab_object mm/kasan/common.c:340 [inline]
 __kasan_slab_alloc+0x89/0x90 mm/kasan/common.c:366
 kasan_slab_alloc include/linux/kasan.h:253 [inline]
 slab_post_alloc_hook mm/slub.c:4570 [inline]
 slab_alloc_node mm/slub.c:4899 [inline]
 kmem_cache_alloc_noprof+0x241/0x6e0 mm/slub.c:4906
 mempool_alloc_noprof+0x1b7/0x310 mm/mempool.c:566
 netfs_alloc_subrequest+0xc1/0x3c0 fs/netfs/objects.c:204
 netfs_prepare_write+0x76/0x780 fs/netfs/write_issue.c:172
 netfs_unbuffered_write+0x7b8/0x21e0 fs/netfs/direct_write.c:111
 netfs_unbuffered_write_iter_locked+0x7e2/0xa90 fs/netfs/direct_write.c:294
 netfs_unbuffered_write_iter+0x442/0x740 fs/netfs/direct_write.c:386
 v9fs_file_write_iter+0xbf/0x100 fs/9p/vfs_file.c:409
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x6ac/0x1070 fs/read_write.c:688
 ksys_write+0x12a/0x250 fs/read_write.c:740
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 5936:
 kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
 kasan_save_track+0x14/0x30 mm/kasan/common.c:78
 kasan_save_free_info+0x3b/0x70 mm/kasan/generic.c:584
 poison_slab_object mm/kasan/common.c:253 [inline]
 __kasan_slab_free+0x5f/0x80 mm/kasan/common.c:285
 kasan_slab_free include/linux/kasan.h:235 [inline]
 slab_free_hook mm/slub.c:2689 [inline]
 slab_free mm/slub.c:6251 [inline]
 kmem_cache_free+0x127/0x6c0 mm/slub.c:6378
 mempool_free+0xde/0x130 mm/mempool.c:711
 netfs_free_subrequest fs/netfs/objects.c:239 [inline]
 netfs_put_subrequest+0x282/0x4c0 fs/netfs/objects.c:255
 netfs_unbuffered_write+0x13ed/0x21e0 fs/netfs/direct_write.c:118
 netfs_unbuffered_write_iter_locked+0x7e2/0xa90 fs/netfs/direct_write.c:294
 netfs_unbuffered_write_iter+0x442/0x740 fs/netfs/direct_write.c:386
 v9fs_file_write_iter+0xbf/0x100 fs/9p/vfs_file.c:409
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x6ac/0x1070 fs/read_write.c:688
 ksys_write+0x12a/0x250 fs/read_write.c:740
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The buggy address belongs to the object at ffff888022e877c0
 which belongs to the cache netfs_subrequest of size 200
The buggy address is located 164 bytes inside of
 freed 200-byte region [ffff888022e877c0, ffff888022e87888)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x22e86
head: order:1 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
memcg:ffff888026b62001
flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000040 ffff88801fab5680 dead000000000100 dead000000000122
raw: 0000000000000000 0000000800190019 00000000f5000000 ffff888026b62001
head: 00fff00000000040 ffff88801fab5680 dead000000000100 dead000000000122
head: 0000000000000000 0000000800190019 00000000f5000000 ffff888026b62001
head: 00fff00000000001 ffffffffffffff81 00000000ffffffff 00000000ffffffff
head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000002
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 1, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 1, tgid 1 (swapper/0), ts 4032896346, free_ts 0
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0xfd/0x120 mm/page_alloc.c:1853
 prep_new_page mm/page_alloc.c:1861 [inline]
 get_page_from_freelist+0x11a6/0x3410 mm/page_alloc.c:3941
 __alloc_frozen_pages_noprof+0x27c/0x2bc0 mm/page_alloc.c:5221
 alloc_slab_page mm/slub.c:3278 [inline]
 allocate_slab mm/slub.c:3467 [inline]
 new_slab+0xa6/0x6c0 mm/slub.c:3525
 refill_objects+0x277/0x420 mm/slub.c:7272
 refill_sheaf mm/slub.c:2816 [inline]
 __pcs_replace_empty_main+0x375/0x650 mm/slub.c:4652
 alloc_from_pcs mm/slub.c:4750 [inline]
 slab_alloc_node mm/slub.c:4884 [inline]
 kmem_cache_alloc_noprof+0x480/0x6e0 mm/slub.c:4906
 mempool_init_node+0x2f7/0x6e0 mm/mempool.c:259
 mempool_init_noprof+0x3a/0x50 mm/mempool.c:289
 netfs_init+0x16c/0x2b0 fs/netfs/main.c:128
 do_one_initcall+0x121/0x750 init/main.c:1392
 do_initcall_level init/main.c:1454 [inline]
 do_initcalls init/main.c:1470 [inline]
 do_basic_setup init/main.c:1490 [inline]
 kernel_init_freeable+0x6ea/0x7b0 init/main.c:1703
 kernel_init+0x1f/0x1e0 init/main.c:1593
 ret_from_fork+0x72b/0xd50 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
page_owner free stack trace missing

Memory state around the buggy address:
 ffff888022e87700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff888022e87780: fc fc fc fc fc fc fc fc fa fb fb fb fb fb fb fb
>ffff888022e87800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                                       ^
 ffff888022e87880: fb fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff888022e87900: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

Crashes (4):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/05/29 02:16 upstream 8fde5d1d47f6 4624854e .config console log report syz / log C [disk image (non-bootable)] [vmlinux] [kernel image] ci-qemu-upstream KASAN: slab-use-after-free Read in netfs_unbuffered_write
2026/05/28 23:55 upstream 8fde5d1d47f6 4624854e .config console log report syz / log C [disk image (non-bootable)] [vmlinux] [kernel image] ci-qemu-upstream KASAN: slab-use-after-free Read in netfs_unbuffered_write
2026/05/28 23:29 upstream 8fde5d1d47f6 4624854e .config console log report info [disk image (non-bootable)] [vmlinux] [kernel image] ci-qemu-upstream KASAN: slab-use-after-free Read in netfs_unbuffered_write
2026/05/28 23:29 upstream 8fde5d1d47f6 4624854e .config console log report info [disk image (non-bootable)] [vmlinux] [kernel image] ci-qemu-upstream KASAN: slab-use-after-free Read in netfs_unbuffered_write
* Struck through repros no longer work on HEAD.