Re: [syzbot] [lsm] [kernel] INFO: task hung in restrict_one_thread_callback

[syzbot <syzbot+7ea2f5e9dfd468201817@syzkaller.appspotmail.com>]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
INFO: task hung in landlock_restrict_sibling_threads

INFO: task syz.0.146:6909 blocked for more than 143 seconds.
      Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.0.146       state:D stack:28832 pid:6909  tgid:6908  ppid:6445   task_flags:0x400140 flags:0x00080006
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:5295 [inline]
 __schedule+0x1585/0x5340 kernel/sched/core.c:6907
 __schedule_loop kernel/sched/core.c:6989 [inline]
 schedule+0x164/0x360 kernel/sched/core.c:7004
 schedule_timeout+0xc3/0x2c0 kernel/time/sleep_timeout.c:75
 do_wait_for_common kernel/sched/completion.c:100 [inline]
 __wait_for_common kernel/sched/completion.c:121 [inline]
 wait_for_common kernel/sched/completion.c:132 [inline]
 wait_for_completion+0x2cc/0x5e0 kernel/sched/completion.c:153
 landlock_restrict_sibling_threads+0xf3d/0x11f0 security/landlock/tsync.c:549
 __do_sys_landlock_restrict_self security/landlock/syscalls.c:574 [inline]
 __se_sys_landlock_restrict_self+0x540/0x810 security/landlock/syscalls.c:482
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f81f479bf79
RSP: 002b:00007f81f559d028 EFLAGS: 00000246 ORIG_RAX: 00000000000001be
RAX: ffffffffffffffda RBX: 00007f81f4a15fa0 RCX: 00007f81f479bf79
RDX: 0000000000000000 RSI: 000000000000000e RDI: 0000000000000003
RBP: 00007f81f48327e0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f81f4a16038 R14: 00007f81f4a15fa0 R15: 00007ffd3d3acd98
 </TASK>
INFO: task syz.0.146:6910 blocked for more than 143 seconds.
      Not tainted syzkaller #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz.0.146       state:D stack:28616 pid:6910  tgid:6908  ppid:6445   task_flags:0x400140 flags:0x00080006
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:5295 [inline]
 __schedule+0x1585/0x5340 kernel/sched/core.c:6907
 __schedule_loop kernel/sched/core.c:6989 [inline]
 schedule+0x164/0x360 kernel/sched/core.c:7004
 schedule_timeout+0xc3/0x2c0 kernel/time/sleep_timeout.c:75
 do_wait_for_common kernel/sched/completion.c:100 [inline]
 __wait_for_common kernel/sched/completion.c:121 [inline]
 wait_for_common kernel/sched/completion.c:132 [inline]
 wait_for_completion+0x2cc/0x5e0 kernel/sched/completion.c:153
 landlock_restrict_sibling_threads+0xf3d/0x11f0 security/landlock/tsync.c:549
 __do_sys_landlock_restrict_self security/landlock/syscalls.c:574 [inline]
 __se_sys_landlock_restrict_self+0x540/0x810 security/landlock/syscalls.c:482
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f81f479bf79
RSP: 002b:00007f81f557c028 EFLAGS: 00000246 ORIG_RAX: 00000000000001be
RAX: ffffffffffffffda RBX: 00007f81f4a16090 RCX: 00007f81f479bf79
RDX: 0000000000000000 RSI: 000000000000000e RDI: 0000000000000003
RBP: 00007f81f48327e0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f81f4a16128 R14: 00007f81f4a16090 R15: 00007ffd3d3acd98
 </TASK>

Showing all locks held in the system:
1 lock held by khungtaskd/31:
 #0: ffffffff8e9602e0 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:312 [inline]
 #0: ffffffff8e9602e0 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:850 [inline]
 #0: ffffffff8e9602e0 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x2e/0x180 kernel/locking/lockdep.c:6775
5 locks held by kworker/u8:6/411:
2 locks held by getty/5575:
 #0: ffff888036de80a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x25/0x70 drivers/tty/tty_ldisc.c:243
 #1: ffffc900033332f0 (&ldata->atomic_read_lock){+.+.}-{4:4}, at: n_tty_read+0x45c/0x13c0 drivers/tty/n_tty.c:2211

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

NMI backtrace for cpu 0
CPU: 0 UID: 0 PID: 31 Comm: khungtaskd Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 nmi_cpu_backtrace+0x274/0x2d0 lib/nmi_backtrace.c:113
 nmi_trigger_cpumask_backtrace+0x17a/0x300 lib/nmi_backtrace.c:62
 trigger_all_cpu_backtrace include/linux/nmi.h:161 [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:346 [inline]
 watchdog+0xfd9/0x1030 kernel/hung_task.c:515
 kthread+0x388/0x470 kernel/kthread.c:467
 ret_from_fork+0x51e/0xb90 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: 411 Comm: kworker/u8:6 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
Workqueue:  0x0 (events_unbound)
RIP: 0010:lockdep_recursion_finish kernel/locking/lockdep.c:470 [inline]
RIP: 0010:lock_is_held_type+0xda/0x150 kernel/locking/lockdep.c:5941
Code: f9 eb cb 31 db eb 1c 83 fd ff 74 12 31 c0 f6 43 22 03 0f 95 c0 31 db 39 c5 0f 94 c3 eb 05 bb 01 00 00 00 48 c7 c7 0f 1d 28 8e <e8> 21 18 00 00 b8 ff ff ff ff 65 0f c1 05 04 f0 88 07 83 f8 01 75
RSP: 0018:ffffc900034afa88 EFLAGS: 00000046
RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000080000001
RDX: 0000000000000000 RSI: ffffffff8e028949 RDI: ffffffff8e281d0f
RBP: 00000000ffffffff R08: ffffffff903384b7 R09: 1ffffffff2067096
R10: dffffc0000000000 R11: fffffbfff2067097 R12: 0000000000000246
R13: ffff888024c8dac0 R14: ffffffff8e9603a0 R15: 1ffff110170e75b3
FS:  0000000000000000(0000) GS:ffff888125311000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055558fd504e8 CR3: 000000000e74c000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 lock_is_held include/linux/lockdep.h:249 [inline]
 schedule_debug kernel/sched/core.c:5874 [inline]
 __schedule+0x262/0x5340 kernel/sched/core.c:6783
 __schedule_loop kernel/sched/core.c:6989 [inline]
 schedule+0x164/0x360 kernel/sched/core.c:7004
 worker_thread+0xc1e/0xfc0 kernel/workqueue.c:3454
 kthread+0x388/0x470 kernel/kthread.c:467
 ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>


Tested on:

commit:         7dff99b3 Remove WARN_ALL_UNSEEDED_RANDOM kernel config..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11e97c4a580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=56c0294cb1688557
dashboard link: https://syzkaller.appspot.com/bug?extid=7ea2f5e9dfd468201817
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=13c17c4a580000