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

[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+e70e1b6cba8714543f7c@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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If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want syzbot to run the reproducer, reply with:
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If you want to overwrite report's subsystems, reply with:
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#syz undup

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

[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)
--

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

[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

Forwarded: [PATCH] eventpoll: restore EP_UNACTIVE_PTR sentinel for ctx->tfile_check_list

[syzbot]

For archival purposes, forwarding an incoming command email to
linux-kernel@vger.kernel.org.

***

Subject: [PATCH] eventpoll: restore EP_UNACTIVE_PTR sentinel for ctx->tfile_check_list
Author: zhanwei919@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master

commit e09c77d94003 ("eventpoll: hoist CTL_ADD scratch state into
struct ep_ctl_ctx") moved tfile_check_list from file-scope global
to stack-allocated struct ep_ctl_ctx, replacing the EP_UNACTIVE_PTR
sentinel with NULL because "NULL is the obvious 'empty'
value and the zero-init handle it for free", and describe the
change "No functional change". but its not.

epitems_head->next is overload:

  1: as a linked-list next pointer for heads on ctx->tfile_check_list,
  2: as a membership flag: ep_remove_file() uses
    !smp_load_acquire(&v->next) to mean "this head is not on any
    pending ctx->tfile_check_list and is safe to free".

Before e09c77d94003, the EP_UNACTIVE_PTR sentinel made the two role
disjoint: a head on tfile_check_list always has a non-NULL next
(another head, or the sentinel), so v->next == NULL was equivalent
to never list. With the sentinel gone the list is NULL-terminated
and the tail head's ->next is NULL also. ep_remove_file()'s gate
no longer tell never list from list at the tail, and
misfires on the tail.

The reader hold epnested_mutex + rcu_read_lock; the freer hold
ep->mtx + file->f_lock. There is no sharing mutex between them; the
sentinel was the invariant the gate relied on to skip the read side.

The syzbot reproducer hit this within seconds on a multi-CPU VM.

Restore the sentinel: initialize ctx.tfile_check_list to
EP_UNACTIVE_PTR in do_epoll_ctl_file(), and walk it with
"!= EP_UNACTIVE_PTR" termination in reverse_path_check() and
clear_tfile_check_list(). The gate in ep_remove_file() regains its
never list exclusivity and stop misfiring on the tail.
ep_remove_file() itself does not change.

This restores the invariant the file-scope tfile_check_list relied
on before e09c77d94003, preserving the ctx packaging that commit
introduced.

Reported-by: syzbot+e70e1b6cba8714543f7c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e70e1b6cba8714543f7c
Fixes: e09c77d94003 ("eventpoll: hoist CTL_ADD scratch state into struct ep_ctl_ctx")
Suggested-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/all/20260528-rotwild-summt-kuhhandel-7276ef4c33b7@brauner.io/

Signed-off-by: Zhan Wei <zhanwei919@gmail.com>
---
 fs/eventpoll.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index a569e98d4a99..4973a5a5a3e1 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1685,7 +1685,7 @@ static int reverse_path_check(struct ep_ctl_ctx *ctx)
 {
 	struct epitems_head *p;
 
-	for (p = ctx->tfile_check_list; p; p = p->next) {
+	for (p = ctx->tfile_check_list; p != EP_UNACTIVE_PTR; p = p->next) {
 		int error;
 		path_count_init(ctx);
 		rcu_read_lock();
@@ -2438,7 +2438,7 @@ static int ep_loop_check(struct ep_ctl_ctx *ctx, struct eventpoll *ep,
 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;
 		unlist_file(head);
@@ -2601,7 +2601,9 @@ int do_epoll_ctl_file(struct file *f, int op, struct epoll_key *tf,
 	int full_check;
 	struct eventpoll *ep;
 	struct epitem *epi;
-	struct ep_ctl_ctx ctx = { };
+	struct ep_ctl_ctx ctx = {
+		.tfile_check_list = EP_UNACTIVE_PTR,
+	};
 
 	/* The target file descriptor must support poll */
 	if (!file_can_poll(tf->file))
-- 
2.43.0

Forwarded: [PATCH] eventpoll: restore EP_UNACTIVE_PTR sentinel for ctx->tfile_check_list

[syzbot]

For archival purposes, forwarding an incoming command email to
linux-kernel@vger.kernel.org.

***

Subject: [PATCH] eventpoll: restore EP_UNACTIVE_PTR sentinel for ctx->tfile_check_list
Author: zhanwei919@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git master

commit e09c77d94003 ("eventpoll: hoist CTL_ADD scratch state into
struct ep_ctl_ctx") moved tfile_check_list from file-scope global
to stack-allocated struct ep_ctl_ctx, replacing the EP_UNACTIVE_PTR
sentinel with NULL because "NULL is the obvious 'empty'
value and the zero-init handle it for free", and describe the
change "No functional change". but its not.

epitems_head->next is overload:

  1: as a linked-list next pointer for heads on ctx->tfile_check_list,
  2: as a membership flag: ep_remove_file() uses
    !smp_load_acquire(&v->next) to mean "this head is not on any
    pending ctx->tfile_check_list and is safe to free".

Before e09c77d94003, the EP_UNACTIVE_PTR sentinel made the two role
disjoint: a head on tfile_check_list always has a non-NULL next
(another head, or the sentinel), so v->next == NULL was equivalent
to never list. With the sentinel gone the list is NULL-terminated
and the tail head's ->next is NULL also. ep_remove_file()'s gate
no longer tell never list from list at the tail, and
misfires on the tail.

The reader hold epnested_mutex + rcu_read_lock; the freer hold
ep->mtx + file->f_lock. There is no sharing mutex between them; the
sentinel was the invariant the gate relied on to skip the read side.

The syzbot reproducer hit this within seconds on a multi-CPU VM.

Restore the sentinel: initialize ctx.tfile_check_list to
EP_UNACTIVE_PTR in do_epoll_ctl_file(), and walk it with
"!= EP_UNACTIVE_PTR" termination in reverse_path_check() and
clear_tfile_check_list(). The gate in ep_remove_file() regains its
never list exclusivity and stop misfiring on the tail.
ep_remove_file() itself does not change.

This restores the invariant the file-scope tfile_check_list relied
on before e09c77d94003, preserving the ctx packaging that commit
introduced.

Reported-by: syzbot+e70e1b6cba8714543f7c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e70e1b6cba8714543f7c
Fixes: e09c77d94003 ("eventpoll: hoist CTL_ADD scratch state into struct ep_ctl_ctx")
Suggested-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/all/20260528-rotwild-summt-kuhhandel-7276ef4c33b7@brauner.io/

Signed-off-by: Zhan Wei <zhanwei919@gmail.com>
---
 fs/eventpoll.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index a569e98d4a99..4973a5a5a3e1 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1685,7 +1685,7 @@ static int reverse_path_check(struct ep_ctl_ctx *ctx)
 {
 	struct epitems_head *p;
 
-	for (p = ctx->tfile_check_list; p; p = p->next) {
+	for (p = ctx->tfile_check_list; p != EP_UNACTIVE_PTR; p = p->next) {
 		int error;
 		path_count_init(ctx);
 		rcu_read_lock();
@@ -2438,7 +2438,7 @@ static int ep_loop_check(struct ep_ctl_ctx *ctx, struct eventpoll *ep,
 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;
 		unlist_file(head);
@@ -2601,7 +2601,9 @@ int do_epoll_ctl_file(struct file *f, int op, struct epoll_key *tf,
 	int full_check;
 	struct eventpoll *ep;
 	struct epitem *epi;
-	struct ep_ctl_ctx ctx = { };
+	struct ep_ctl_ctx ctx = {
+		.tfile_check_list = EP_UNACTIVE_PTR,
+	};
 
 	/* The target file descriptor must support poll */
 	if (!file_can_poll(tf->file))
-- 
2.43.0

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

[syzbot]

Hello,

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



Tested on:

commit:         f7af91ad Add linux-next specific files for 20260528
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=160117a6580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=938b3c3730e59c71
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=1044cd5e580000

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

[syzbot]

Hello,

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



Tested on:

commit:         e7d700e1 Add linux-next specific files for 20260527
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=178429ec580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2c4840d777495ef2
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=12ac46f2580000