[syzbot] [kernfs?] INFO: task hung in do_epoll_ctl_file

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: c1ecb239fa34 Add linux-next specific files for 20260522
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=123e2ac8580000
kernel config: https://syzkaller.appspot.com/x/.config?x=e0299bf0261ddd5
dashboard link: https://syzkaller.appspot.com/bug?extid=f83fa2cf571bd7650422
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=11997cec580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=12672cec580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/b8845b668755/disk-c1ecb239.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/517b638d908a/vmlinux-c1ecb239.xz
kernel image: https://storage.googleapis.com/syzbot-assets/dd5e3b587fce/bzImage-c1ecb239.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+f83fa2...@syzkaller.appspotmail.com

INFO: tas[ 358.361221][ T39] INFO: task syz.1.18:6030 blocked for more than 143 seconds.
Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.1.18 state:D stack:26808 pid:6030 tgid:6030 ppid:5779 task_flags:0x400140 flags:0x00080002
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5402 [inline]
__schedule+0x16f9/0x5500 kernel/sched/core.c:7204
__schedule_loop kernel/sched/core.c:7283 [inline]
rt_mutex_schedule+0x76/0xf0 kernel/sched/core.c:7579
rt_mutex_slowlock_block+0x508/0x680 kernel/locking/rtmutex.c:1666
__rt_mutex_slowlock kernel/locking/rtmutex.c:1743 [inline]
__rt_mutex_slowlock_locked kernel/locking/rtmutex.c:1783 [inline]
rt_mutex_slowlock+0x2dc/0x780 kernel/locking/rtmutex.c:1823
__rt_mutex_lock kernel/locking/rtmutex.c:1838 [inline]
__mutex_lock_common kernel/locking/rtmutex_api.c:560 [inline]
mutex_lock_nested+0x168/0x1d0 kernel/locking/rtmutex_api.c:578
epoll_mutex_lock fs/eventpoll.c:2512 [inline]
ep_ctl_lock fs/eventpoll.c:2559 [inline]
do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
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/0x560 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fa0ef9ece59
RSP: 002b:00007ffe81e0f328 EFLAGS: 00000246 ORIG_RAX: 00000000000000e9
RAX: ffffffffffffffda RBX: 00007fa0efc65fa0 RCX: 00007fa0ef9ece59
RDX: 0000000000000004 RSI: 0000000000000001 RDI: 0000000000000005
RBP: 00007fa0efa82d6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000200000000100 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fa0efc65fac R14: 00007fa0efc65fa0 R15: 00007fa0efc65fa0
</TASK>
INFO: task syz.2.19:6031 blocked for more than 143 seconds.
Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.2.19 state:D stack:26808 pid:6031 tgid:6031 ppid:5786 task_flags:0x400140 flags:0x00080002
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5402 [inline]
__schedule+0x16f9/0x5500 kernel/sched/core.c:7204
__schedule_loop kernel/sched/core.c:7283 [inline]
rt_mutex_schedule+0x76/0xf0 kernel/sched/core.c:7579
rt_mutex_slowlock_block+0x508/0x680 kernel/locking/rtmutex.c:1666
__rt_mutex_slowlock kernel/locking/rtmutex.c:1743 [inline]
__rt_mutex_slowlock_locked kernel/locking/rtmutex.c:1783 [inline]
rt_mutex_slowlock+0x2dc/0x780 kernel/locking/rtmutex.c:1823
__rt_mutex_lock kernel/locking/rtmutex.c:1838 [inline]
__mutex_lock_common kernel/locking/rtmutex_api.c:560 [inline]
mutex_lock_nested+0x168/0x1d0 kernel/locking/rtmutex_api.c:578
epoll_mutex_lock fs/eventpoll.c:2512 [inline]
ep_ctl_lock fs/eventpoll.c:2559 [inline]
do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
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/0x560 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f4ea623ce59
RSP: 002b:00007ffc16d0ea78 EFLAGS: 00000246 ORIG_RAX: 00000000000000e9
RAX: ffffffffffffffda RBX: 00007f4ea64b5fa0 RCX: 00007f4ea623ce59
RDX: 0000000000000004 RSI: 0000000000000001 RDI: 0000000000000005
RBP: 00007f4ea62d2d6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000200000000100 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f4ea64b5fac R14: 00007f4ea64b5fa0 R15: 00007f4ea64b5fa0
</TASK>
INFO: task syz.0.17:6032 blocked for more than 143 seconds.
Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.0.17 state:D stack:26808 pid:6032 tgid:6032 ppid:5782 task_flags:0x400140 flags:0x00080002
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5402 [inline]
__schedule+0x16f9/0x5500 kernel/sched/core.c:7204
__schedule_loop kernel/sched/core.c:7283 [inline]
rt_mutex_schedule+0x76/0xf0 kernel/sched/core.c:7579
rt_mutex_slowlock_block+0x508/0x680 kernel/locking/rtmutex.c:1666
__rt_mutex_slowlock kernel/locking/rtmutex.c:1743 [inline]
__rt_mutex_slowlock_locked kernel/locking/rtmutex.c:1783 [inline]
rt_mutex_slowlock+0x2dc/0x780 kernel/locking/rtmutex.c:1823
__rt_mutex_lock kernel/locking/rtmutex.c:1838 [inline]
__mutex_lock_common kernel/locking/rtmutex_api.c:560 [inline]
mutex_lock_nested+0x168/0x1d0 kernel/locking/rtmutex_api.c:578
epoll_mutex_lock fs/eventpoll.c:2512 [inline]
ep_ctl_lock fs/eventpoll.c:2559 [inline]
do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
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/0x560 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fc5b492ce59
RSP: 002b:00007ffcdf2d99c8 EFLAGS: 00000246 ORIG_RAX: 00000000000000e9
RAX: ffffffffffffffda RBX: 00007fc5b4ba5fa0 RCX: 00007fc5b492ce59
RDX: 0000000000000004 RSI: 0000000000000001 RDI: 0000000000000005
RBP: 00007fc5b49c2d6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000200000000100 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fc5b4ba5fac R14: 00007fc5b4ba5fa0 R15: 00007fc5b4ba5fa0
</TASK>
INFO: task syz.4.21:6033 blocked for more than 143 seconds.
Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.4.21 state:D stack:26808 pid:6033 tgid:6033 ppid:5790 task_flags:0x400140 flags:0x00080002
Call Trace:
<TASK>
context_switch kernel/sched/core.c:5402 [inline]
__schedule+0x16f9/0x5500 kernel/sched/core.c:7204
__schedule_loop kernel/sched/core.c:7283 [inline]
rt_mutex_schedule+0x76/0xf0 kernel/sched/core.c:7579
rt_mutex_slowlock_block+0x508/0x680 kernel/locking/rtmutex.c:1666
__rt_mutex_slowlock kernel/locking/rtmutex.c:1743 [inline]
__rt_mutex_slowlock_locked kernel/locking/rtmutex.c:1783 [inline]
rt_mutex_slowlock+0x2dc/0x780 kernel/locking/rtmutex.c:1823
__rt_mutex_lock kernel/locking/rtmutex.c:1838 [inline]
__mutex_lock_common kernel/locking/rtmutex_api.c:560 [inline]
mutex_lock_nested+0x168/0x1d0 kernel/locking/rtmutex_api.c:578
epoll_mutex_lock fs/eventpoll.c:2512 [inline]
ep_ctl_lock fs/eventpoll.c:2559 [inline]
do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
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/0x560 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f3f07c9ce59
RSP: 002b:00007ffc9dc2e578 EFLAGS: 00000246 ORIG_RAX: 00000000000000e9
RAX: ffffffffffffffda RBX: 00007f3f07f15fa0 RCX: 00007f3f07c9ce59
RDX: 0000000000000004 RSI: 0000000000000001 RDI: 0000000000000005
RBP: 00007f3f07d32d6f R08: 0000000000000000 R09: 0000000000000000
R10: 0000200000000100 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f3f07f15fac R14: 00007f3f07f15fa0 R15: 00007f3f07f15fa0
</TASK>

Showing all locks held in the system:
6 locks held by ktimers/1/30:
#0: ffffffff8de61600 (local_bh){.+.+}-{1:3}, at: __local_bh_disable_ip+0x3c/0x420 kernel/softirq.c:163
#1: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: __local_bh_disable_ip+0x3c/0x420 kernel/softirq.c:163
#2: ffff8880b8726220 (&base->expiry_lock){+...}-{3:3}, at: spin_lock include/linux/spinlock_rt.h:45 [inline]
#2: ffff8880b8726220 (&base->expiry_lock){+...}-{3:3}, at: timer_base_lock_expiry kernel/time/timer.c:1502 [inline]
#2: ffff8880b8726220 (&base->expiry_lock){+...}-{3:3}, at: __run_timer_base+0x120/0x9f0 kernel/time/timer.c:2384
#3: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
#3: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
#3: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: __rt_spin_lock kernel/locking/spinlock_rt.c:50 [inline]
#3: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: rt_spin_lock+0x1e0/0x400 kernel/locking/spinlock_rt.c:57
#4: ffffc90000a4fa80 ((&ndev->rs_timer)){+...}-{0:0}, at: call_timer_fn+0xd4/0x5e0 kernel/time/timer.c:1745
#5: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
#5: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
#5: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: ndisc_send_skb+0x215/0x1670 net/ipv6/ndisc.c:482
1 lock held by khungtaskd/39:
#0: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
#0: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
#0: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x2e/0x180 kernel/locking/lockdep.c:6777
8 locks held by kworker/u8:4/67:
#0: ffff88813fe84138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x897/0x1630 kernel/workqueue.c:3293
#1: ffffc9000152fc40 ((work_completion)(&rdev->wiphy_work)){+.+.}-{0:0}, at: process_one_work+0x8be/0x1630 kernel/workqueue.c:3294
#2: ffff8880626608d8 (&rdev->wiphy.mtx){+.+.}-{4:4}, at: class_wiphy_constructor include/net/cfg80211.h:6885 [inline]
#2: ffff8880626608d8 (&rdev->wiphy.mtx){+.+.}-{4:4}, at: cfg80211_wiphy_work+0xb4/0x460 net/wireless/core.c:524
#3: ffffffff8de61600 (local_bh){.+.+}-{1:3}, at: __local_bh_disable_ip+0x3c/0x420 kernel/softirq.c:163
#4: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: __local_bh_disable_ip+0x3c/0x420 kernel/softirq.c:163
#5: ffff888062660228 (&rdev->bss_lock){+...}-{3:3}, at: spin_lock_bh include/linux/spinlock_rt.h:90 [inline]
#5: ffff888062660228 (&rdev->bss_lock){+...}-{3:3}, at: cfg80211_inform_single_bss_data+0xbbf/0x1b80 net/wireless/scan.c:2367
#6: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
#6: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
#6: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: __rt_spin_lock kernel/locking/spinlock_rt.c:50 [inline]
#6: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: rt_spin_lock+0x1e0/0x400 kernel/locking/spinlock_rt.c:57
#7: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
#7: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
#7: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: class_rcu_constructor include/linux/rcupdate.h:1181 [inline]
#7: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: unwind_next_frame+0xa6/0x2550 arch/x86/kernel/unwind_orc.c:495
6 locks held by kworker/u8:7/405:
#0: ffff88813fe84138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x897/0x1630 kernel/workqueue.c:3293
#1: ffffc9000419fc40 ((work_completion)(&rdev->wiphy_work)){+.+.}-{0:0}, at: process_one_work+0x8be/0x1630 kernel/workqueue.c:3294
#2: ffff8880566c08d8 (&rdev->wiphy.mtx){+.+.}-{4:4}, at: class_wiphy_constructor include/net/cfg80211.h:6885 [inline]
#2: ffff8880566c08d8 (&rdev->wiphy.mtx){+.+.}-{4:4}, at: cfg80211_wiphy_work+0xb4/0x460 net/wireless/core.c:524
#3: ffffffff8de61600 (local_bh){.+.+}-{1:3}, at: __local_bh_disable_ip+0x3c/0x420 kernel/softirq.c:163
#4: ffffffff8dfcabc0 (rcu_read_lock){....}-{1:3}, at: __local_bh_disable_ip+0x3c/0x420 kernel/softirq.c:163
#5: ffff8880566c0228 (&rdev->bss_lock){+...}-{3:3}, at: spin_lock_bh include/linux/spinlock_rt.h:90 [inline]
#5: ffff8880566c0228 (&rdev->bss_lock){+...}-{3:3}, at: cfg80211_put_bss+0x5b/0x270 net/wireless/scan.c:3347
4 locks held by kworker/u8:8/1024:
#0: ffff88813fe84138 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x897/0x1630 kernel/workqueue.c:3293
#1: ffffc90005c67c40 ((work_completion)(&rdev->wiphy_work)){+.+.}-{0:0}, at: process_one_work+0x8be/0x1630 kernel/workqueue.c:3294
#2: ffff8880634908d8 (&rdev->wiphy.mtx){+.+.}-{4:4}, at: class_wiphy_constructor include/net/cfg80211.h:6885 [inline]
#2: ffff8880634908d8 (&rdev->wiphy.mtx){+.+.}-{4:4}, at: cfg80211_wiphy_work+0xb4/0x460 net/wireless/core.c:524
#3: ffffffff8de61600 (local_bh){.+.+}-{1:3}, at: __local_bh_disable_ip+0x3c/0x420 kernel/softirq.c:163
2 locks held by getty/5375:
#0: ffff8880361670a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x25/0x70 drivers/tty/tty_ldisc.c:243
#1: ffffc90003cc62e0 (&ldata->atomic_read_lock){+.+.}-{4:4}, at: n_tty_read+0x462/0x13a0 drivers/tty/n_tty.c:2211
2 locks held by syz.3.20/6016:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
#1: ffff888028754058 (&ep->mtx){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#1: ffff888028754058 (&ep->mtx){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2573 [inline]
#1: ffff888028754058 (&ep->mtx){+.+.}-{4:4}, at: do_epoll_ctl_file+0xc69/0xed0 fs/eventpoll.c:2636
1 lock held by syz.1.18/6030:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.2.19/6031:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.0.17/6032:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.4.21/6033:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.5.22/6168:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.6.23/6240:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.7.24/6241:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.8.25/6245:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.9.26/6246:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.3.27/6289:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.1.28/6406:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.2.29/6467:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.0.30/6476:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.4.31/6481:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.5.32/6482:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.6.33/6535:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.7.34/6704:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.8.35/6724:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.9.36/6735:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.3.37/6737:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
1 lock held by syz.1.38/6739:
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: epoll_mutex_lock fs/eventpoll.c:2512 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: ep_ctl_lock fs/eventpoll.c:2559 [inline]
#0: ffffffff8e157958 (epnested_mutex){+.+.}-{4:4}, at: do_epoll_ctl_file+0x9d1/0xed0 fs/eventpoll.c:2636
2 locks held by syz-executor/6744:
#0: ffffffff8eae37a8 (&ops->srcu#2){.+.+}-{0:0}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
#0: ffffffff8eae37a8 (&ops->srcu#2){.+.+}-{0:0}, at: rcu_read_lock include/linux/rcupdate.h:838 [inline]
#0: ffffffff8eae37a8 (&ops->srcu#2){.+.+}-{0:0}, at: rtnl_link_ops_get+0x23/0x250 net/core/rtnetlink.c:570
#1: ffffffff8f37fa78 (rtnl_mutex){+.+.}-{4:4}, at: rtnl_lock net/core/rtnetlink.c:80 [inline]
#1: ffffffff8f37fa78 (rtnl_mutex){+.+.}-{4:4}, at: rtnl_nets_lock net/core/rtnetlink.c:341 [inline]
#1: ffffffff8f37fa78 (rtnl_mutex){+.+.}-{4:4}, at: rtnl_newlink+0x883/0x1bb0 net/core/rtnetlink.c:4122

=============================================

NMI backtrace for cpu 0
CPU: 0 UID: 0 PID: 39 Comm: khungtaskd 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
nmi_cpu_backtrace+0x274/0x2d0 lib/nmi_backtrace.c:119
nmi_trigger_cpumask_backtrace+0x17a/0x380 lib/nmi_backtrace.c:62
trigger_all_cpu_backtrace include/linux/nmi.h:162 [inline]
__sys_info lib/sys_info.c:157 [inline]
sys_info+0x135/0x170 lib/sys_info.c:165
check_hung_uninterruptible_tasks kernel/hung_task.c:353 [inline]
watchdog+0xfd3/0x1030 kernel/hung_task.c:561
kthread+0x388/0x470 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
</TASK>
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 UID: 0 PID: 17 Comm: pr/legacy Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
RIP: 0010:rdtsc_ordered arch/x86/include/asm/tsc.h:57 [inline]
RIP: 0010:delay_tsc+0x71/0xc0 arch/x86/lib/delay.c:72
Code: 00 00 e8 b2 4a 68 f6 65 8b 05 eb c5 47 07 85 c0 74 1c f3 90 bf 01 00 00 00 e8 fb 48 68 f6 e8 76 86 01 00 39 c5 75 0e 0f 01 f9 <66> 90 eb bd e8 f6 c0 07 f6 eb dd 4c 29 fb e8 5c 86 01 00 89 c5 0f
RSP: 0018:ffffc900001679d0 EFLAGS: 00000246
RAX: 00000000499b963d RBX: 0000000000000899 RCX: 0000000000000001
RDX: 00000000000000ba RSI: ffffffff8ba9cd60 RDI: ffffffff8ba9cd20
RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
R10: dffffc0000000000 R11: ffffffff8b2b79f0 R12: 1ffffffff3374446
R13: 00000000ffffffff R14: 000000ba499b91db R15: 000000000000038d
FS: 0000000000000000(0000) GS:ffff888125fec000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fc83a6dee9c CR3: 00000000352e2000 CR4: 00000000003526f0
Call Trace:
<TASK>
udelay include/asm-generic/delay.h:62 [inline]
wait_for_lsr+0x166/0x2f0 drivers/tty/serial/8250/8250_port.c:1981
serial8250_fifo_wait_for_lsr_thre drivers/tty/serial/8250/8250_port.c:3212 [inline]
serial8250_console_fifo_write drivers/tty/serial/8250/8250_port.c:3277 [inline]
serial8250_console_write+0x11c9/0x1b90 drivers/tty/serial/8250/8250_port.c:3362
console_emit_next_record kernel/printk/printk.c:3163 [inline]
console_flush_one_record+0x68b/0xb90 kernel/printk/printk.c:3269
legacy_kthread_func+0x1b6/0x250 kernel/printk/printk.c:3712
kthread+0x388/0x470 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
</TASK>


---
This report is generated by a bot. It may contain errors.
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If the report is already addressed, let syzbot know by replying with:
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[syzbot]

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***

Subject: [PATCH] eventpoll: fix use-after-free in clear_tfile_check_list()
Author: karti...@gmail.com

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

syzbot reports a slab-use-after-free read in
clear_tfile_check_list() during EPOLL_CTL_ADD when ep_insert()
takes an error path that calls ep_remove() before
do_epoll_ctl_file() drains ctx->tfile_check_list.

ep_remove_file() decides whether to kmem_cache_free() a struct
epitems_head by testing v->next == NULL, on the convention that
NULL means "this head is not linked on any check list".
list_file() pushes a head onto ctx->tfile_check_list by storing
the previous list head into head->next; when the list is empty
that store is NULL, so the head that ends up at the tail of the
check list also has next == NULL. ep_remove_file() then misreads
the tail as "not linked" and frees it. clear_tfile_check_list()
later walks the list and dereferences head->next on the freed
object:

BUG: KASAN: slab-use-after-free in
clear_tfile_check_list+0x114/0x380 fs/eventpoll.c:2443
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
Freed by task 5985:
ep_remove+0x155/0x2a0 fs/eventpoll.c:1135
ep_insert+0x1372/0x1820

Terminate ctx->tfile_check_list with a non-NULL sentinel
(EP_TFILE_LIST_END) so that next == NULL unambiguously means
"not on any check list". list_file() stores the sentinel when
the list is empty, reverse_path_check() and
clear_tfile_check_list() stop the walk on the sentinel, and
do_epoll_ctl_file() seeds ctx->tfile_check_list with it. The
guard in ep_remove_file() then correctly refuses to free a head
while it remains on the check list.

Reported-by: syzbot+f83fa2...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=f83fa2cf571bd7650422
Signed-off-by: Deepanshu Kartikey <karti...@gmail.com>
---
fs/eventpoll.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index a569e98d4a99..349d79e8ddc8 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -222,6 +222,15 @@

#define EP_UNACTIVE_PTR ((void *) -1L)

+/* Non-NULL sentinel terminating ctx->tfile_check_list, so that
+ * "head->next == NULL" unambiguously means "this head is not on any
+ * check list" -- the invariant ep_remove_file() and list_file() rely
+ * on. Without this, the tail of the check list aliases the
+ * "not linked" state and ep_remove_file() may free a head that
+ * clear_tfile_check_list() still references.
+ */
+#define EP_TFILE_LIST_END ((struct epitems_head *)EP_UNACTIVE_PTR)
+
#define EP_ITEM_COST (sizeof(struct epitem) + sizeof(struct eppoll_entry))

/* Wait structure used by the poll hooks */
@@ -472,7 +481,7 @@ static void list_file(struct file *file, struct ep_ctl_ctx *ctx)

head = container_of(file->f_ep, struct epitems_head, epitems);
if (!head->next) {
- head->next = ctx->tfile_check_list;
+ head->next = ctx->tfile_check_list ? : EP_TFILE_LIST_END;
ctx->tfile_check_list = head;
}
}
@@ -1685,7 +1694,7 @@ 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_TFILE_LIST_END; p = p->next) {
int error;
path_count_init(ctx);
rcu_read_lock();
@@ -2438,11 +2447,12 @@ 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_TFILE_LIST_END) {
struct epitems_head *head = ctx->tfile_check_list;
ctx->tfile_check_list = head->next;
unlist_file(head);
}
+ ctx->tfile_check_list = NULL;
rcu_read_unlock();
}

@@ -2601,7 +2611,7 @@ 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_TFILE_LIST_END };

/* The target file descriptor must support poll */
if (!file_can_poll(tf->file))
--
2.43.0

[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=1266452e580000
kernel config: https://syzkaller.appspot.com/x/.config?x=2f6cdf8f0e18fea

dashboard link: https://syzkaller.appspot.com/bug?extid=f83fa2cf571bd7650422
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=15f1b6ec580000