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

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

[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=17ca9fba580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2a0b5969e136a632
dashboard link: https://syzkaller.appspot.com/bug?extid=69a3d7738ad3aa175caf
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=1598082e580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17ac6596580000

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+69a3d7738ad3aa175caf@syzkaller.appspotmail.com

RAX: ffffffffffffffda RBX: 00007f129c6a5fa0 RCX: 00007f129c42ce59
RDX: 0000000000000005 RSI: 0000000000000001 RDI: 0000000000000003
RBP: 00007ffeb1d5c4f0 R08: 0000000000000000 R09: 0000000000000000
R10: 00002000000004c0 R11: 0000000000000246 R12: 0000000000000001
R13: 00007f129c6a5fac R14: 00007f129c6a5fa0 R15: 00007f129c6a5fa0
 </TASK>
==================================================================
BUG: KASAN: slab-use-after-free in clear_tfile_check_list+0x114/0x380 fs/eventpoll.c:2443
Read of size 8 at addr ffff88803f021568 by task syz.0.74/5985

CPU: 1 UID: 0 PID: 5985 Comm: syz.0.74 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
 clear_tfile_check_list+0x114/0x380 fs/eventpoll.c:2443
 do_epoll_ctl_file+0x8fd/0xed0 fs/eventpoll.c:-1
 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: 0033:0x7f129c42ce59
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:00007ffeb1d5c488 EFLAGS: 00000246 ORIG_RAX: 00000000000000e9
RAX: ffffffffffffffda RBX: 00007f129c6a5fa0 RCX: 00007f129c42ce59
RDX: 0000000000000005 RSI: 0000000000000001 RDI: 0000000000000003
RBP: 00007ffeb1d5c4f0 R08: 0000000000000000 R09: 0000000000000000
R10: 00002000000004c0 R11: 0000000000000246 R12: 0000000000000001
R13: 00007f129c6a5fac R14: 00007f129c6a5fa0 R15: 00007f129c6a5fa0
 </TASK>

Allocated by task 5985:
 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 5985:
 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
 ep_remove+0x155/0x2a0 fs/eventpoll.c:1135
 ep_insert+0x1372/0x1820 fs/eventpoll.c:-1
 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

The buggy address belongs to the object at ffff88803f021560
 which belongs to the cache ep_head of size 16
The buggy address is located 8 bytes inside of
 freed 16-byte region [ffff88803f021560, ffff88803f021570)

The buggy address belongs to the physical page:
page: refcount:0 mapcount:0 mapping:0000000000000000 index:0xffff88803f0213e0 pfn:0x3f021
memcg:ffff88803d044001
flags: 0x80000000000200(workingset|node=0|zone=1)
page_type: f5(slab)
raw: 0080000000000200 ffff888140419c80 ffffea0000a5dc90 ffff888020ecdf88
raw: ffff88803f0213e0 0000000800800078 00000000f5000000 ffff88803d044001
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 4982, tgid 4982 (udevd), ts 45918061529, free_ts 45769000837
 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 last free pid 21 tgid 21 stack trace:
 reset_page_owner include/linux/page_owner.h:25 [inline]
 __free_pages_prepare mm/page_alloc.c:1420 [inline]
 __free_frozen_pages+0x1075/0x11b0 mm/page_alloc.c:2958
 __tlb_remove_table_free mm/mmu_gather.c:228 [inline]
 tlb_remove_table_rcu+0x85/0x100 mm/mmu_gather.c:299
 rcu_do_batch kernel/rcu/tree.c:2617 [inline]
 rcu_core kernel/rcu/tree.c:2869 [inline]
 rcu_cpu_kthread+0x99e/0x1470 kernel/rcu/tree.c:2957
 smpboot_thread_fn+0x541/0xa50 kernel/smpboot.c:160
 kthread+0x389/0x470 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

Memory state around the buggy address:
 ffff88803f021400: fa fb fc fc fa fb fc fc fa fb fc fc fa fb fc fc
 ffff88803f021480: fa fb fc fc fa fb fc fc fa fb fc fc fa fb fc fc
>ffff88803f021500: fa fb fc fc fa fb fc fc fa fb fc fc fa fb fc fc
                                                          ^
 ffff88803f021580: fa fb fc fc fa fb fc fc fa fb fc fc fa fb fc fc
 ffff88803f021600: fa fb fc fc fa fb fc fc fa fb fc fc fa fb fc fc
==================================================================


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

[Edward Adam Davis]

#syz test

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index a569e98d4a99..b2fb707e06aa 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1839,9 +1839,6 @@ static int ep_register_epitem(struct ep_ctl_ctx *ctx, struct eventpoll *ep,
 		return error;
 	}
 
-	if (full_check && !tep)
-		list_file(tfile, ctx);
-
 	ep_rbtree_insert(ep, epi);
 
 	if (tep)
@@ -1863,6 +1860,7 @@ static int ep_insert(struct ep_ctl_ctx *ctx, struct eventpoll *ep,
 	struct epitem *epi;
 	struct ep_pqueue epq;
 	struct eventpoll *tep = NULL;
+	struct file *tfile;
 
 	if (is_file_epoll(tf->file))
 		tep = tf->file->private_data;
@@ -1873,6 +1871,7 @@ static int ep_insert(struct ep_ctl_ctx *ctx, struct eventpoll *ep,
 	if (IS_ERR(epi))
 		return PTR_ERR(epi);
 
+	tfile = epi->ffd.file;
 	error = ep_register_epitem(ctx, ep, epi, tep, full_check);
 	if (error)
 		return error;
@@ -1910,6 +1909,9 @@ static int ep_insert(struct ep_ctl_ctx *ctx, struct eventpoll *ep,
 		return -ENOMEM;
 	}
 
+	if (full_check && !tep)
+		list_file(tfile, ctx);
+
 	/* Drop the new item onto the ready list if it is already ready. */
 	spin_lock_irq(&ep->lock);
 

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

[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=107e6d6a580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=77a9211ff284de54
dashboard link: https://syzkaller.appspot.com/bug?extid=69a3d7738ad3aa175caf
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=134c147e580000

[PATCH next] eventpoll: delay file list memory deallocation until unlisting

[Edward Adam Davis]

Commit e09c77d94003 implicitly sets the head->next value within list_file()
to NULL; this occurs because ctx->tfile_check_list which resides on the stack
is initialized to NULL. This introduces a potential risk to ep_remove_file(),
as the decision to reclaim epitems_head depends solely on whether the next
pointer is NULL. Prior to the introduction of e09c77d94003, the presence of
the sentinel value EP_UNACTIVE_PTR prevented the next pointer from becoming
NULL prematurely; the next value would only be updated to NULL during the
execution of unlist_file(). 
However, following the introduction of e09c77d94003, list_file() also updates
the next value to NULL, which ultimately led to the uaf reported in [1].

To mitigate the risk posed by the potentially NULL next pointer, the memory
reclamation for the file list originally performed within ep_remove_file()
has been deferred to unlist_file().

[1]
BUG: KASAN: slab-use-after-free in clear_tfile_check_list+0x114/0x380 fs/eventpoll.c:2443
Read of size 8 at addr ffff88803f021568 by task syz.0.74/5985
Call Trace:
 clear_tfile_check_list+0x114/0x380 fs/eventpoll.c:2443
 do_epoll_ctl_file+0x8fd/0xed0 fs/eventpoll.c:-1
 
Allocated by task 5985:
 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
 
Freed by task 5985:
 kmem_cache_free+0x187/0x6c0 mm/slub.c:6411
 ep_remove+0x155/0x2a0 fs/eventpoll.c:1135
 ep_insert+0x1372/0x1820 fs/eventpoll.c:-1
 do_epoll_ctl_file+0x8bb/0xed0 fs/eventpoll.c:2651

Fixes: e09c77d94003 ("eventpoll: hoist CTL_ADD scratch state into struct ep_ctl_ctx")
Reported-by: syzbot+69a3d7738ad3aa175caf@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=69a3d7738ad3aa175caf
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 fs/eventpoll.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index a569e98d4a99..66aa4f200909 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1080,6 +1080,8 @@ static void ep_remove_file(struct eventpoll *ep, struct epitem *epi,
 			v = container_of(head, struct epitems_head, epitems);
 			if (!smp_load_acquire(&v->next))
 				to_free = v;
+			if (!hlist_empty(&v->epitems))
+				to_free = NULL;
 		}
 	}
 	hlist_del_rcu(&epi->fllink);
-- 
2.43.0

Re: [PATCH next] eventpoll: delay file list memory deallocation until unlisting

[Christian Brauner]

On Sun, May 24, 2026 at 06:27:59PM +0800, Edward Adam Davis wrote:
> Commit e09c77d94003 implicitly sets the head->next value within list_file()
> to NULL; this occurs because ctx->tfile_check_list which resides on the stack
> is initialized to NULL. This introduces a potential risk to ep_remove_file(),
> as the decision to reclaim epitems_head depends solely on whether the next
> pointer is NULL. Prior to the introduction of e09c77d94003, the presence of
> the sentinel value EP_UNACTIVE_PTR prevented the next pointer from becoming
> NULL prematurely; the next value would only be updated to NULL during the
> execution of unlist_file(). 
> However, following the introduction of e09c77d94003, list_file() also updates
> the next value to NULL, which ultimately led to the uaf reported in [1].
> 
> To mitigate the risk posed by the potentially NULL next pointer, the memory
> reclamation for the file list originally performed within ep_remove_file()
> has been deferred to unlist_file().
> 
> [1]
> BUG: KASAN: slab-use-after-free in clear_tfile_check_list+0x114/0x380 fs/eventpoll.c:2443
> Read of size 8 at addr ffff88803f021568 by task syz.0.74/5985
> Call Trace:
>  clear_tfile_check_list+0x114/0x380 fs/eventpoll.c:2443
>  do_epoll_ctl_file+0x8fd/0xed0 fs/eventpoll.c:-1
>  
> Allocated by task 5985:
>  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
>  
> Freed by task 5985:
>  kmem_cache_free+0x187/0x6c0 mm/slub.c:6411
>  ep_remove+0x155/0x2a0 fs/eventpoll.c:1135
>  ep_insert+0x1372/0x1820 fs/eventpoll.c:-1
>  do_epoll_ctl_file+0x8bb/0xed0 fs/eventpoll.c:2651
> 
> Fixes: e09c77d94003 ("eventpoll: hoist CTL_ADD scratch state into struct ep_ctl_ctx")
> Reported-by: syzbot+69a3d7738ad3aa175caf@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=69a3d7738ad3aa175caf
> Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> ---
>  fs/eventpoll.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index a569e98d4a99..66aa4f200909 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -1080,6 +1080,8 @@ static void ep_remove_file(struct eventpoll *ep, struct epitem *epi,
>  			v = container_of(head, struct epitems_head, epitems);
>  			if (!smp_load_acquire(&v->next))
>  				to_free = v;
> +			if (!hlist_empty(&v->epitems))
> +				to_free = NULL;

Hm, at the singular-node check, hlist_del_rcu hasn't run yet so
!hlist_empty(&v->epitems) is unconditionally true. So this stops freeing
heads in all cases. Heads that were never on ctx->tfile_check_list
(CTL_ADD without full_check, or any CTL_DEL of a previously-unlisted
head) leak permanently.

Wouldn't something like:

  diff --git a/fs/eventpoll.c b/fs/eventpoll.c
  @@ static int reverse_path_check(struct ep_ctl_ctx *ctx)
  -     for (p = ctx->tfile_check_list; p; p = p->next) {
  +     for (p = ctx->tfile_check_list; p != EP_UNACTIVE_PTR; p = p->next) {

  @@ static void clear_tfile_check_list(struct ep_ctl_ctx *ctx)
        rcu_read_lock();
  -     while (ctx->tfile_check_list) {
  +     while (ctx->tfile_check_list != EP_UNACTIVE_PTR) {
                struct epitems_head *head = ctx->tfile_check_list;
                ctx->tfile_check_list = head->next;

  @@ int do_epoll_ctl_file(...)
  -     struct ep_ctl_ctx ctx = { };
  +     struct ep_ctl_ctx ctx = {
  +             .tfile_check_list = EP_UNACTIVE_PTR,
  +     };

be better?