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

Status: auto-obsoleted due to no activity on 2026/05/21 19:54
Subsystems: kernel
[Documentation on labels]
Reported-by: syzbot+6c1861115b4253e45969@syzkaller.appspotmail.com
First crash: 165d, last: 165d
▶ ▼ Discussions (2)
Title Replies (including bot) Last reply
[REPORT] futex: private hash refcount corruption and UAF via CLONE_VM PR_FUTEX_HASH race 1 (1) 2026/04/30 09:38
[syzbot] [kernel?] KASAN: slab-use-after-free Read in futex_unqueue 5 (11) 2026/02/11 09:46
▶ ▼ Last patch testing requests (6)
Created Duration User Patch Repo Result
2026/02/24 19:59 23m retest repro upstream OK log
2026/02/11 09:22 22m hdanton@sina.com patch upstream OK log
2026/02/11 06:31 22m hdanton@sina.com patch upstream OK log
2026/02/11 03:31 16m suunj1331@gmail.com patch upstream report log
2026/02/11 00:38 15m hdanton@sina.com patch upstream report log
2026/02/10 22:52 22m hdanton@sina.com patch upstream report log

Sample crash report:
==================================================================
BUG: KASAN: slab-use-after-free in __raw_spin_lock include/linux/spinlock_api_smp.h:133 [inline]
BUG: KASAN: slab-use-after-free in _raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:154
Read of size 1 at addr ffff888033ce23e0 by task syz.0.19/6039

CPU: 1 UID: 0 PID: 6039 Comm: syz.0.19 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
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+0x156/0x4c9 mm/kasan/report.c:482
 kasan_report+0xdf/0x1a0 mm/kasan/report.c:595
 __kasan_check_byte+0x36/0x50 mm/kasan/common.c:574
 kasan_check_byte include/linux/kasan.h:402 [inline]
 lock_acquire kernel/locking/lockdep.c:5842 [inline]
 lock_acquire+0xf5/0x330 kernel/locking/lockdep.c:5825
 __raw_spin_lock include/linux/spinlock_api_smp.h:133 [inline]
 _raw_spin_lock+0x2e/0x40 kernel/locking/spinlock.c:154
 spin_lock include/linux/spinlock.h:351 [inline]
 futex_unqueue+0xa5/0x2c0 kernel/futex/core.c:938
 __futex_wait+0x1cc/0x300 kernel/futex/waitwake.c:690
 futex_wait+0xed/0x380 kernel/futex/waitwake.c:715
 do_futex+0x1ef/0x350 kernel/futex/syscalls.c:130
 __do_sys_futex kernel/futex/syscalls.c:207 [inline]
 __se_sys_futex kernel/futex/syscalls.c:188 [inline]
 __x64_sys_futex+0x34f/0x4d0 kernel/futex/syscalls.c:188
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc9/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f6f87b9bf79
Code: Unable to access opcode bytes at 0x7f6f87b9bf4f.
RSP: 002b:00007f6f889c40e8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: ffffffffffffffda RBX: 00007f6f87e15fa8 RCX: 00007f6f87b9bf79
RDX: 0000000000000000 RSI: 0000000000000080 RDI: 00007f6f87e15fa8
RBP: 00007f6f87e15fa0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f6f87e16038 R14: 00007ffe2ba93260 R15: 00007ffe2ba93348
 </TASK>

Allocated by task 6038:
 kasan_save_stack+0x30/0x50 mm/kasan/common.c:57
 kasan_save_track+0x14/0x30 mm/kasan/common.c:78
 poison_kmalloc_redzone mm/kasan/common.c:398 [inline]
 __kasan_kmalloc+0xaa/0xb0 mm/kasan/common.c:415
 kasan_kmalloc include/linux/kasan.h:263 [inline]
 __do_kmalloc_node mm/slub.c:5657 [inline]
 __kvmalloc_node_noprof+0x34d/0xac0 mm/slub.c:7144
 futex_hash_allocate+0x40b/0x1090 kernel/futex/core.c:1812
 futex_hash_allocate_default+0x2ca/0x5b0 kernel/futex/core.c:1921
 copy_process+0x4eb5/0x79b0 kernel/fork.c:2344
 kernel_clone+0xfc/0x930 kernel/fork.c:2654
 __do_sys_clone3+0x214/0x290 kernel/fork.c:2956
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc9/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 6038:
 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:2540 [inline]
 slab_free mm/slub.c:6674 [inline]
 kfree+0x1c7/0x690 mm/slub.c:6886
 futex_hash_free+0x98/0xc0 kernel/futex/core.c:1739
 __mmput+0x30c/0x410 kernel/fork.c:1185
 mmput+0x67/0x80 kernel/fork.c:1197
 exit_mm kernel/exit.c:581 [inline]
 do_exit+0x78a/0x2a30 kernel/exit.c:959
 do_group_exit+0xd5/0x2a0 kernel/exit.c:1112
 __do_sys_exit_group kernel/exit.c:1123 [inline]
 __se_sys_exit_group kernel/exit.c:1121 [inline]
 __x64_sys_exit_group+0x3e/0x50 kernel/exit.c:1121
 x64_sys_call+0x14fd/0x1510 arch/x86/include/generated/asm/syscalls_64.h:232
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc9/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The buggy address belongs to the object at ffff888033ce2000
 which belongs to the cache kmalloc-cg-4k of size 4096
The buggy address is located 992 bytes inside of
 freed 4096-byte region [ffff888033ce2000, ffff888033ce3000)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x33ce0
head: order:3 mapcount:0 entire_mapcount:0 nr_pages_mapped:0 pincount:0
memcg:ffff888076582001
flags: 0xfff00000000040(head|node=0|zone=1|lastcpupid=0x7ff)
page_type: f5(slab)
raw: 00fff00000000040 ffff88813fe30500 dead000000000122 0000000000000000
raw: 0000000000000000 0000000080040004 00000000f5000000 ffff888076582001
head: 00fff00000000040 ffff88813fe30500 dead000000000122 0000000000000000
head: 0000000000000000 0000000080040004 00000000f5000000 ffff888076582001
head: 00fff00000000003 ffffea0000cf3801 00000000ffffffff 00000000ffffffff
head: ffffffffffffffff 0000000000000000 00000000ffffffff 0000000000000008
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 3, migratetype Unmovable, gfp_mask 0xd20c0(__GFP_IO|__GFP_FS|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 5887, tgid 5887 (udevd), ts 101745764711, free_ts 101704527247
 set_page_owner include/linux/page_owner.h:32 [inline]
 post_alloc_hook+0x1e1/0x250 mm/page_alloc.c:1884
 prep_new_page mm/page_alloc.c:1892 [inline]
 get_page_from_freelist+0xe3d/0x2e10 mm/page_alloc.c:3945
 __alloc_frozen_pages_noprof+0x26c/0x2410 mm/page_alloc.c:5240
 alloc_pages_mpol+0x1fb/0x550 mm/mempolicy.c:2486
 alloc_slab_page mm/slub.c:3075 [inline]
 allocate_slab mm/slub.c:3248 [inline]
 new_slab+0x2c4/0x440 mm/slub.c:3302
 ___slab_alloc+0xda3/0x1ca0 mm/slub.c:4656
 __slab_alloc.isra.0+0x63/0x110 mm/slub.c:4779
 __slab_alloc_node mm/slub.c:4855 [inline]
 slab_alloc_node mm/slub.c:5251 [inline]
 __do_kmalloc_node mm/slub.c:5656 [inline]
 __kvmalloc_node_noprof+0x749/0xac0 mm/slub.c:7144
 seq_buf_alloc fs/seq_file.c:38 [inline]
 seq_read_iter+0x819/0x1270 fs/seq_file.c:210
 kernfs_fop_read_iter+0x46c/0x610 fs/kernfs/file.c:297
 new_sync_read fs/read_write.c:493 [inline]
 vfs_read+0x825/0xb30 fs/read_write.c:574
 ksys_read+0x12a/0x250 fs/read_write.c:717
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc9/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
page last free pid 5933 tgid 5933 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 free_pages_prepare mm/page_alloc.c:1433 [inline]
 __free_frozen_pages+0x822/0x1130 mm/page_alloc.c:2973
 discard_slab mm/slub.c:3346 [inline]
 __put_partials+0x127/0x160 mm/slub.c:3886
 qlink_free mm/kasan/quarantine.c:163 [inline]
 qlist_free_all+0x47/0xe0 mm/kasan/quarantine.c:179
 kasan_quarantine_reduce+0x1a0/0x1f0 mm/kasan/quarantine.c:286
 __kasan_slab_alloc+0x69/0x90 mm/kasan/common.c:350
 kasan_slab_alloc include/linux/kasan.h:253 [inline]
 slab_post_alloc_hook mm/slub.c:4953 [inline]
 slab_alloc_node mm/slub.c:5263 [inline]
 __kmalloc_cache_noprof+0x2e1/0x810 mm/slub.c:5775
 kmalloc_noprof include/linux/slab.h:957 [inline]
 kzalloc_noprof include/linux/slab.h:1094 [inline]
 nsim_fib4_rt_create drivers/net/netdevsim/fib.c:280 [inline]
 nsim_fib4_rt_insert drivers/net/netdevsim/fib.c:426 [inline]
 nsim_fib4_event drivers/net/netdevsim/fib.c:464 [inline]
 nsim_fib_event drivers/net/netdevsim/fib.c:884 [inline]
 nsim_fib_event_work+0xfeb/0x63b0 drivers/net/netdevsim/fib.c:1493
 process_one_work+0x9c2/0x1840 kernel/workqueue.c:3257
 process_scheduled_works kernel/workqueue.c:3340 [inline]
 worker_thread+0x5da/0xe40 kernel/workqueue.c:3421
 kthread+0x370/0x450 kernel/kthread.c:467
 ret_from_fork+0x754/0xaf0 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246

Memory state around the buggy address:
 ffff888033ce2280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888033ce2300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>ffff888033ce2380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                                       ^
 ffff888033ce2400: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff888033ce2480: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
==================================================================

Crashes (1):
Time Kernel Commit Syzkaller Config Log Report Syz repro C repro VM info Assets (help?) Manager Title
2026/02/10 19:54 upstream 72c395024dac a076df6f .config console log report syz / log C [disk image] [vmlinux] [kernel image] ci-qemu-gce-upstream-auto KASAN: slab-use-after-free Read in futex_unqueue
* Struck through repros no longer work on HEAD.