KASAN: use-after-free Write in skb_dequeue

Status: premoderation: reported on 2026/07/07 22:41
Reported-by: syzbot+efa8b5673fa3ecdc4c5b@syzkaller.appspotmail.com
First crash: 42d, last: 42d
▶ ▼ Similar bugs (8)
Kernel Title Rank 🛈 Repro Cause bisect Fix bisect Count Last Reported Patched Status
linux-4.14 KASAN: use-after-free Write in skb_dequeue 22 1 2449d 2449d 0/1 auto-closed as invalid on 2020/04/02 21:00
android-49 KASAN: use-after-free Write in skb_dequeue 22 syz, C 10 3111d 3311d 0/3 closed as invalid on 2019/02/20 20:15
android-54 KASAN: use-after-free Read in skb_dequeue 19 syz 1 1828d 1828d 0/2 auto-obsoleted due to no activity on 2023/04/23 02:47
upstream general protection fault in skb_dequeue (3) wireless 2 syz, C done 6 1285d 1294d 22/29 fixed on 2023/06/08 14:41
upstream KASAN: slab-use-after-free Read in skb_dequeue bluetooth 19 4 341d 438d 0/29 auto-obsoleted due to no activity on 2025/12/10 14:34
upstream general protection fault in skb_dequeue (2) bluetooth 19 syz, C inconclusive done 9 1773d 1858d 0/29 auto-closed as invalid on 2022/10/03 17:36
upstream KASAN: slab-use-after-free Read in skb_dequeue (2) actionable prio:high bluetooth 19 2 38d 91d 0/29 upstream: reported on 2026/05/19 10:47
linux-4.19 KASAN: use-after-free Read in skb_dequeue (2) 19 syz, C done 2 1798d 1828d 1/1 fixed on 2021/10/15 14:38

Sample crash report:
==================================================================
BUG: KASAN: use-after-free in instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
BUG: KASAN: use-after-free in atomic_try_cmpxchg_acquire include/asm-generic/atomic-instrumented.h:705 [inline]
BUG: KASAN: use-after-free in queued_spin_lock include/asm-generic/qspinlock.h:82 [inline]
BUG: KASAN: use-after-free in do_raw_spin_lock_flags include/linux/spinlock.h:196 [inline]
BUG: KASAN: use-after-free in __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:119 [inline]
BUG: KASAN: use-after-free in _raw_spin_lock_irqsave+0xb5/0x130 kernel/locking/spinlock.c:159
Write of size 4 at addr ffff88810d7e0c5c by task kworker/u4:3/356

CPU: 0 PID: 356 Comm: kworker/u4:3 Tainted: G        W         syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
Workqueue: l2tp l2tp_tunnel_del_work
Call Trace:
 __dump_stack+0x21/0x24 lib/dump_stack.c:77
 dump_stack_lvl+0x1a7/0x208 lib/dump_stack.c:118
 print_address_description+0x7f/0x2c0 mm/kasan/report.c:248
 __kasan_report mm/kasan/report.c:435 [inline]
 kasan_report+0x100/0x140 mm/kasan/report.c:452
 check_region_inline mm/kasan/generic.c:-1 [inline]
 kasan_check_range+0x249/0x2a0 mm/kasan/generic.c:189
 __kasan_check_write+0x14/0x20 mm/kasan/shadow.c:37
 instrument_atomic_read_write include/linux/instrumented.h:101 [inline]
 atomic_try_cmpxchg_acquire include/asm-generic/atomic-instrumented.h:705 [inline]
 queued_spin_lock include/asm-generic/qspinlock.h:82 [inline]
 do_raw_spin_lock_flags include/linux/spinlock.h:196 [inline]
 __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:119 [inline]
 _raw_spin_lock_irqsave+0xb5/0x130 kernel/locking/spinlock.c:159
 skb_dequeue+0x2f/0x180 net/core/skbuff.c:3156
 l2tp_session_queue_purge net/l2tp/l2tp_core.c:799 [inline]
 l2tp_session_delete+0x31f/0x4a0 net/l2tp/l2tp_core.c:1585
 l2tp_tunnel_closeall net/l2tp/l2tp_core.c:1235 [inline]
 l2tp_tunnel_del_work+0x180/0x3d0 net/l2tp/l2tp_core.c:1273
 process_one_work+0x6fd/0xbc0 kernel/workqueue.c:2301
 worker_thread+0xa8e/0x13c0 kernel/workqueue.c:2447
 kthread+0x324/0x3b0 kernel/kthread.c:313
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:298

Allocated by task 3824:
 kasan_save_stack mm/kasan/common.c:38 [inline]
 kasan_set_track mm/kasan/common.c:45 [inline]
 set_alloc_info mm/kasan/common.c:430 [inline]
 ____kasan_kmalloc mm/kasan/common.c:509 [inline]
 __kasan_kmalloc+0xd4/0x100 mm/kasan/common.c:518
 kasan_kmalloc include/linux/kasan.h:254 [inline]
 __kmalloc+0x19f/0x330 mm/slub.c:4038
 kmalloc include/linux/slab.h:560 [inline]
 kzalloc include/linux/slab.h:667 [inline]
 l2tp_session_create+0x39/0xb60 net/l2tp/l2tp_core.c:1616
 pppol2tp_connect+0xbf5/0x1640 net/l2tp/l2tp_ppp.c:772
 __sys_connect_file net/socket.c:1867 [inline]
 __sys_connect+0x3ce/0x450 net/socket.c:1884
 __do_sys_connect net/socket.c:1894 [inline]
 __se_sys_connect net/socket.c:1891 [inline]
 __x64_sys_connect+0x7a/0x90 net/socket.c:1891
 do_syscall_64+0x31/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x61/0xcb

Freed by task 3824:
 kasan_save_stack mm/kasan/common.c:38 [inline]
 kasan_set_track+0x4a/0x70 mm/kasan/common.c:45
 kasan_set_free_info+0x23/0x40 mm/kasan/generic.c:370
 ____kasan_slab_free+0x125/0x160 mm/kasan/common.c:362
 __kasan_slab_free+0x11/0x20 mm/kasan/common.c:370
 kasan_slab_free include/linux/kasan.h:220 [inline]
 slab_free_hook mm/slub.c:1600 [inline]
 slab_free_freelist_hook+0xc5/0x190 mm/slub.c:1626
 slab_free mm/slub.c:3208 [inline]
 kfree+0xc0/0x270 mm/slub.c:4196
 l2tp_session_free net/l2tp/l2tp_core.c:168 [inline]
 l2tp_session_put+0xb2/0x1a0 net/l2tp/l2tp_core.c:193
 pppol2tp_ioctl+0x748/0x960 net/l2tp/l2tp_ppp.c:-1
 pppox_ioctl+0x1e4/0x320 drivers/net/ppp/pppox.c:93
 sock_do_ioctl+0xda/0x380 net/socket.c:1068
 sock_ioctl+0x472/0x740 net/socket.c:1208
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl+0x121/0x1a0 fs/ioctl.c:739
 __x64_sys_ioctl+0x7b/0x90 fs/ioctl.c:739
 do_syscall_64+0x31/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x61/0xcb

The buggy address belongs to the object at ffff88810d7e0c00
 which belongs to the cache kmalloc-512 of size 512
The buggy address is located 92 bytes inside of
 512-byte region [ffff88810d7e0c00, ffff88810d7e0e00)
The buggy address belongs to the page:
page:ffffea000435f800 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x10d7e0
head:ffffea000435f800 order:2 compound_mapcount:0 compound_pincount:0
flags: 0x4000000000010200(slab|head)
raw: 4000000000010200 ffffea0004af8e00 0000000600000006 ffff888100043080
raw: 0000000000000000 0000000080100010 00000001ffffffff 0000000000000000
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 2, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 83, ts 3902880137, free_ts 0
 set_page_owner include/linux/page_owner.h:35 [inline]
 post_alloc_hook mm/page_alloc.c:2456 [inline]
 prep_new_page+0x176/0x190 mm/page_alloc.c:2462
 get_page_from_freelist+0x225f/0x23f0 mm/page_alloc.c:4254
 __alloc_pages_nodemask+0x29a/0x640 mm/page_alloc.c:5384
 alloc_slab_page mm/slub.c:-1 [inline]
 allocate_slab mm/slub.c:1813 [inline]
 new_slab+0x84/0x3f0 mm/slub.c:1874
 new_slab_objects mm/slub.c:2632 [inline]
 ___slab_alloc+0x2f8/0x4c0 mm/slub.c:2796
 __slab_alloc+0x63/0xa0 mm/slub.c:2836
 slab_alloc_node mm/slub.c:2918 [inline]
 slab_alloc mm/slub.c:2960 [inline]
 __kmalloc_track_caller+0x1e4/0x310 mm/slub.c:4541
 __kmalloc_reserve net/core/skbuff.c:144 [inline]
 __alloc_skb+0xdc/0x520 net/core/skbuff.c:212
 alloc_skb include/linux/skbuff.h:1126 [inline]
 alloc_skb_with_frags+0xa3/0x560 net/core/skbuff.c:6006
 sock_alloc_send_pskb+0x87f/0x9a0 net/core/sock.c:2393
 unix_dgram_sendmsg+0x6f3/0x18f0 net/unix/af_unix.c:1725
 sock_sendmsg_nosec net/socket.c:652 [inline]
 __sock_sendmsg net/socket.c:664 [inline]
 __sys_sendto+0x467/0x620 net/socket.c:2007
 __do_sys_sendto net/socket.c:2019 [inline]
 __se_sys_sendto net/socket.c:2015 [inline]
 __x64_sys_sendto+0xe5/0x100 net/socket.c:2015
 do_syscall_64+0x31/0x40 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x61/0xcb
page_owner free stack trace missing

Memory state around the buggy address:
 ffff88810d7e0b00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff88810d7e0b80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff88810d7e0c00: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                                    ^
 ffff88810d7e0c80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff88810d7e0d00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 356 at lib/refcount.c:28 refcount_warn_saturate+0x120/0x1a0 lib/refcount.c:28
Modules linked in:

CPU: 0 PID: 356 Comm: kworker/u4:3 Tainted: G    B   W         syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
Workqueue: l2tp l2tp_tunnel_del_work
RIP: 0010:refcount_warn_saturate+0x120/0x1a0 lib/refcount.c:28
Code: 04 01 48 c7 c7 20 48 40 85 e8 5c bd 3a 02 0f 0b eb c3 e8 63 92 26 ff c6 05 c1 7d 82 04 01 48 c7 c7 80 48 40 85 e8 40 bd 3a 02 <0f> 0b eb a7 e8 47 92 26 ff c6 05 a2 7d 82 04 01 48 c7 c7 a0 47 40
RSP: 0018:ffffc90000d07c58 EFLAGS: 00010246
RAX: b767a491eebb6500 RBX: 0000000000000003 RCX: ffff88812b058000
RDX: 0000000000000000 RSI: 0000000080000000 RDI: 0000000000000000
RBP: ffffc90000d07c68 R08: ffff8881f7052fbf R09: 1ffff1103ee0a5f7
R10: dffffc0000000000 R11: ffffed103ee0a5f8 R12: ffff88812c7f2020
R13: dffffc0000000000 R14: 0000000000000003 R15: 0000000000000000
FS:  0000000000000000(0000) GS:ffff8881f7000000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fdb67d242f8 CR3: 000000010d426000 CR4: 00000000003526b0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __refcount_sub_and_test include/linux/refcount.h:283 [inline]
 __refcount_dec_and_test include/linux/refcount.h:315 [inline]
 refcount_dec_and_test include/linux/refcount.h:333 [inline]
 l2tp_session_put+0xcb/0x1a0 net/l2tp/l2tp_core.c:192
 l2tp_session_delete+0x3a9/0x4a0 net/l2tp/l2tp_core.c:1589
 l2tp_tunnel_closeall net/l2tp/l2tp_core.c:1235 [inline]
 l2tp_tunnel_del_work+0x180/0x3d0 net/l2tp/l2tp_core.c:1273
 process_one_work+0x6fd/0xbc0 kernel/workqueue.c:2301
 worker_thread+0xa8e/0x13c0 kernel/workqueue.c:2447
 kthread+0x324/0x3b0 kernel/kthread.c:313
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:298
---[ end trace 44e067ba534ad652 ]---

Crashes (1):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/07/07 22:40 android13-5.10-lts e7cdd7d65ada 76cdc0c0 .config console log report info [disk image] [vmlinux] [kernel image] ci2-android-5-10 KASAN: use-after-free Write in skb_dequeue
* Struck through repros no longer work on HEAD.