[syzbot] [fs?] KASAN: slab-use-after-free Read in reverse_path_check_proc

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 80dd246accce Add linux-next specific files for 20260518
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=113cba73980000
kernel config: https://syzkaller.appspot.com/x/.config?x=2a0b5969e136a632
dashboard link: https://syzkaller.appspot.com/bug?extid=e70e1b6cba8714543f7c
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14dc9fba580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=10bffcc8580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/5e187606d6dc/disk-80dd246a.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/96a72e613393/vmlinux-80dd246a.xz
kernel image: https://storage.googleapis.com/syzbot-assets/c2905548782c/bzImage-80dd246a.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e70e1b...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: slab-use-after-free in reverse_path_check_proc+0x5b/0x240 fs/eventpoll.c:1663
Read of size 8 at addr ffff88803777f7e0 by task syz.3.20/6001


CPU: 0 UID: 0 PID: 6001 Comm: syz.3.20 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
Call Trace:
<TASK>
dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
print_address_description+0x55/0x1e0 mm/kasan/report.c:378
print_report+0x58/0x70 mm/kasan/report.c:482
kasan_report+0x117/0x150 mm/kasan/report.c:595
reverse_path_check_proc+0x5b/0x240 fs/eventpoll.c:1663
reverse_path_check fs/eventpoll.c:1692 [inline]
ep_insert+0xc6c/0x1820 fs/eventpoll.c:1881
do_epoll_ctl_file+0x8bb/0xed0 fs/eventpoll.c:2651
do_epoll_ctl fs/eventpoll.c:2698 [inline]
__do_sys_epoll_ctl fs/eventpoll.c:2715 [inline]
__se_sys_epoll_ctl+0x14e/0x210 fs/eventpoll.c:2706
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f96281fce59
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:00007f962783d028 EFLAGS: 00000246 ORIG_RAX: 00000000000000e9
RAX: ffffffffffffffda RBX: 00007f9628476090 RCX: 00007f96281fce59
RDX: 0000000000000003 RSI: 0000000000000001 RDI: 0000000000000006
RBP: 00007f9628292d6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000200000000600 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f9628476128 R14: 00007f9628476090 R15: 00007ffcab0f8478
</TASK>

Allocated by task 5997:
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
unpoison_slab_object mm/kasan/common.c:340 [inline]
__kasan_slab_alloc+0x6c/0x80 mm/kasan/common.c:366
kasan_slab_alloc include/linux/kasan.h:253 [inline]
slab_post_alloc_hook mm/slub.c:4583 [inline]
slab_alloc_node mm/slub.c:4912 [inline]
kmem_cache_alloc_noprof+0x33b/0x680 mm/slub.c:4919
ep_attach_file fs/eventpoll.c:1751 [inline]
ep_register_epitem fs/eventpoll.c:1833 [inline]
ep_insert+0x512/0x1820 fs/eventpoll.c:1876
do_epoll_ctl_file+0x8bb/0xed0 fs/eventpoll.c:2651
do_epoll_ctl fs/eventpoll.c:2698 [inline]
__do_sys_epoll_ctl fs/eventpoll.c:2715 [inline]
__se_sys_epoll_ctl+0x14e/0x210 fs/eventpoll.c:2706
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 5997:
kasan_save_stack mm/kasan/common.c:57 [inline]
kasan_save_track+0x3e/0x80 mm/kasan/common.c:78
kasan_save_free_info+0x46/0x50 mm/kasan/generic.c:584
poison_slab_object mm/kasan/common.c:253 [inline]
__kasan_slab_free+0x5c/0x80 mm/kasan/common.c:285
kasan_slab_free include/linux/kasan.h:235 [inline]
slab_free_hook mm/slub.c:2700 [inline]
slab_free mm/slub.c:6284 [inline]
kmem_cache_free+0x187/0x6c0 mm/slub.c:6411
eventpoll_release_file+0xc2/0x240 fs/eventpoll.c:1386
eventpoll_release include/linux/eventpoll.h:61 [inline]
__fput+0x83c/0xa70 fs/file_table.c:501
task_work_run+0x1d9/0x270 kernel/task_work.c:233
resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
__exit_to_user_mode_loop kernel/entry/common.c:67 [inline]
exit_to_user_mode_loop+0xf3/0x4d0 kernel/entry/common.c:98
__exit_to_user_mode_prepare include/linux/irq-entry-common.h:207 [inline]
syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:230 [inline]
syscall_exit_to_user_mode include/linux/entry-common.h:318 [inline]
do_syscall_64+0x33e/0xf80 arch/x86/entry/syscall_64.c:100
entry_SYSCALL_64_after_hwframe+0x77/0x7f

The buggy address belongs to the object at ffff88803777f7e0
which belongs to the cache ep_head of size 16
The buggy address is located 0 bytes inside of
freed 16-byte region [ffff88803777f7e0, ffff88803777f7f0)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff88803777f280 pfn:0x3777f
memcg:ffff88802b46c801
flags: 0x80000000000200(workingset|node=0|zone=1)
page_type: f5(slab)
raw: 0080000000000200 ffff888140ae73c0 ffffea0000e3c510 ffffea0000d70e90
raw: ffff88803777f280 0000000800800024 00000000f5000000 ffff88802b46c801
page dumped because: kasan: bad access detected
page_owner tracks the page as allocated
page last allocated via order 0, migratetype Unmovable, gfp_mask 0xd2cc0(GFP_KERNEL|__GFP_NOWARN|__GFP_NORETRY|__GFP_COMP|__GFP_NOMEMALLOC), pid 4987, tgid 4987 (udevd), ts 71313648267, free_ts 0
set_page_owner include/linux/page_owner.h:32 [inline]
post_alloc_hook+0x1f9/0x250 mm/page_alloc.c:1873
prep_new_page mm/page_alloc.c:1881 [inline]
get_page_from_freelist+0x27d6/0x2850 mm/page_alloc.c:3954
__alloc_frozen_pages_noprof+0x18d/0x380 mm/page_alloc.c:5297
alloc_slab_page mm/slub.c:3289 [inline]
allocate_slab+0x74/0x5e0 mm/slub.c:3404
new_slab mm/slub.c:3447 [inline]
refill_objects+0x33c/0x3d0 mm/slub.c:7226
refill_sheaf mm/slub.c:2827 [inline]
__pcs_replace_empty_main+0x373/0x720 mm/slub.c:4665
alloc_from_pcs mm/slub.c:4763 [inline]
slab_alloc_node mm/slub.c:4897 [inline]
kmem_cache_alloc_noprof+0x433/0x680 mm/slub.c:4919
ep_attach_file fs/eventpoll.c:1751 [inline]
ep_register_epitem fs/eventpoll.c:1833 [inline]
ep_insert+0x512/0x1820 fs/eventpoll.c:1876
do_epoll_ctl_file+0x8bb/0xed0 fs/eventpoll.c:2651
do_epoll_ctl fs/eventpoll.c:2698 [inline]
__do_sys_epoll_ctl fs/eventpoll.c:2715 [inline]
__se_sys_epoll_ctl+0x14e/0x210 fs/eventpoll.c:2706
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
page_owner free stack trace missing

Memory state around the buggy address:
ffff88803777f680: fa fb fc fc fa fb fc fc fa fb fc fc 00 00 fc fc
ffff88803777f700: 00 00 fc fc 00 00 fc fc 00 00 fc fc 00 00 fc fc
>ffff88803777f780: 00 00 fc fc fa fb fc fc 00 00 fc fc fa fb fc fc
^
ffff88803777f800: fa fb fc fc fa fb fc fc fa fb fc fc fa fb fc fc
ffff88803777f880: fa fb fc fc fa fb fc fc fa fb fc fc fa fb fc fc
==================================================================


---
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[Hillf Danton]

> Date: Fri, 22 May 2026 12:48:27 -0700 [thread overview]

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 80dd246accce Add linux-next specific files for 20260518
> git tree: linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=113cba73980000
> kernel config: https://syzkaller.appspot.com/x/.config?x=2a0b5969e136a632
> dashboard link: https://syzkaller.appspot.com/bug?extid=e70e1b6cba8714543f7c
> compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14dc9fba580000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=10bffcc8580000

#syz test

--- x/fs/eventpoll.c
+++ y/fs/eventpoll.c
@@ -456,14 +456,21 @@ static struct kmem_cache *pwq_cache __ro
struct epitems_head {
struct hlist_head epitems;
struct epitems_head *next;
+ struct rcu_head rcu;
};

static struct kmem_cache *ephead_cache __ro_after_init;

+static void __free_ephead(struct rcu_head *r)
+{
+ struct epitems_head *head = container_of(r, struct epitems_head, rcu);
+ kmem_cache_free(ephead_cache, head);
+}
+
static inline void free_ephead(struct epitems_head *head)
{
if (head)
- kmem_cache_free(ephead_cache, head);
+ call_rcu(&head->rcu, __free_ephead);
}

static void list_file(struct file *file, struct ep_ctl_ctx *ctx)
--

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
lost connection to test machine



Tested on:

commit: c1ecb239 Add linux-next specific files for 20260522
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=13a6cac8580000
kernel config: https://syzkaller.appspot.com/x/.config?x=77a9211ff284de54

dashboard link: https://syzkaller.appspot.com/bug?extid=e70e1b6cba8714543f7c
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

patch: https://syzkaller.appspot.com/x/patch.diff?x=15adfa73980000