Re: [syzbot] [kernfs?] [ext4?] INFO: task hung in sb_start_write (2)

On Tue, Aug 11, 2026 at 1:21 PM 'Christian Brauner' via syzkaller-bugs
<[email protected]> wrote:
>
> On Fri, Aug 07, 2026 at 02:21:34AM -0700, syzbot wrote:
> > syzbot has found a reproducer for the following issue on:
> >
> > HEAD commit:    f9a2394a2348 Merge tag 'mm-hotfixes-stable-2026-08-06-18-4..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1379cfb9580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=98da55a882774dfe
> > dashboard link: https://syzkaller.appspot.com/bug?extid=b3fba2e269970207b61d
> > compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=14b9d7b9580000
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: [email protected]
> >
> > INFO: task syz-executor328:5959 blocked for more than 15 seconds.
> >       Not tainted syzkaller #0
> > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > task:syz-executor328 state:D stack:28328 pid:5959  tgid:5959  ppid:5947   task_flags:0x400040 flags:0x00080000
> > Call Trace:
> >  <TASK>
> >  context_switch kernel/sched/core.c:5510 [inline]
> >  __schedule+0x16dc/0x5500 kernel/sched/core.c:7234
> >  __schedule_loop kernel/sched/core.c:7311 [inline]
> >  schedule+0x164/0x2b0 kernel/sched/core.c:7326
> >  percpu_rwsem_wait+0x32d/0x4a0 kernel/locking/percpu-rwsem.c:164
> >  __percpu_down_read+0xf8/0x140 kernel/locking/percpu-rwsem.c:180
> >  percpu_down_read_internal include/linux/percpu-rwsem.h:67 [inline]
> >  percpu_down_read_freezable include/linux/percpu-rwsem.h:83 [inline]
> >  __sb_start_write include/linux/fs/super.h:19 [inline]
> >  sb_start_write+0x18e/0x1c0 include/linux/fs/super.h:125
> >  mnt_want_write+0x41/0x90 fs/namespace.c:494
> >  do_tmpfile+0x6c/0x240 fs/namei.c:4817
> >  path_openat+0x3095/0x3850 fs/namei.c:4854
> >  do_file_open+0x23e/0x4a0 fs/namei.c:4892
> >  do_sys_openat2+0x115/0x200 fs/open.c:1368
> >  do_sys_open fs/open.c:1374 [inline]
> >  __do_sys_openat fs/open.c:1390 [inline]
> >  __se_sys_openat fs/open.c:1385 [inline]
> >  __x64_sys_openat+0x138/0x170 fs/open.c:1385
> >  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >  do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > RIP: 0033:0x7f3fb490aaf7
> > RSP: 002b:00007fff7f2f6f10 EFLAGS: 00000202 ORIG_RAX: 0000000000000101
> > RAX: ffffffffffffffda RBX: 0000555578a51400 RCX: 00007f3fb490aaf7
> > RDX: 0000000000410001 RSI: 00007f3fb494a764 RDI: ffffffffffffff9c
> > RBP: 00007f3fb494a764 R08: 0000000000000000 R09: 0000000000000000
> > R10: 00000000000001b6 R11: 0000000000000202 R12: 00007fff7f2f70d8
> > R13: 0000000000000002 R14: 00007f3fb4970c80 R15: 0000000000000002
> >  </TASK>
> >
> > Showing all locks held in the system:
> > 1 lock held by khungtaskd/38:
> >  #0: ffffffff8e1c3000 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
> >  #0: ffffffff8e1c3000 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:840 [inline]
> >  #0: ffffffff8e1c3000 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x2e/0x180 kernel/locking/lockdep.c:6775
> > 2 locks held by getty/5356:
> >  #0: ffff88803688d0a0 (&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+0x460/0x1360 drivers/tty/n_tty.c:2211
> > 1 lock held by syz-executor328/5959:
> >  #0: ffff888035a88500 (sb_writers#4){++++}-{0:0}, at: mnt_want_write+0x41/0x90 fs/namespace.c:494
> >
> > =============================================
> >
> > NMI backtrace for cpu 1
> > CPU: 1 UID: 0 PID: 38 Comm: khungtaskd Not tainted syzkaller #0 PREEMPT_{RT,(full)}
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
> > Call Trace:
> >  <TASK>
> >  dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
> >  nmi_cpu_backtrace+0x274/0x2d0 lib/nmi_backtrace.c:122
> >  nmi_trigger_cpumask_backtrace+0x17a/0x380 lib/nmi_backtrace.c:65
> >  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+0xfd7/0x1030 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 1 to CPUs 0:
> > NMI backtrace for cpu 0
> > CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
> > RIP: 0010:pv_native_safe_halt+0xf/0x20 arch/x86/kernel/paravirt.c:64
> > Code: cb 6e 02 e9 13 cf 03 00 cc cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 66 90 0f 00 2d 33 44 24 00 fb f4 <c3> cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc cc 90 90 90 90 90
> > RSP: 0018:ffffffff8de07de0 EFLAGS: 00000242
> > RAX: 000000000009a1e9 RBX: ffffffff81998590 RCX: 0000000080000001
> > RDX: 0000000000000001 RSI: ffffffff8d887e30 RDI: ffffffff8bca6d80
> > RBP: ffffffff8de07eb8 R08: ffff8880b8633d5b R09: 1ffff110170c67ab
> > R10: dffffc0000000000 R11: ffffed10170c67ac R12: 0000000000000000
> > R13: 1ffffffff1bdede8 R14: 1ffffffff1bc0fc4 R15: dffffc0000000000
> > FS:  0000000000000000(0000) GS:ffff888125c36000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 0000563216b981d0 CR3: 000000000dfb0000 CR4: 00000000003526f0
> > Call Trace:
> >  <TASK>
> >  arch_safe_halt arch/x86/kernel/process.c:767 [inline]
> >  default_idle+0x9/0x20 arch/x86/kernel/process.c:768
> >  default_idle_call+0x72/0xb0 kernel/sched/idle.c:122
> >  cpuidle_idle_call kernel/sched/idle.c:199 [inline]
> >  do_idle+0x2e0/0x540 kernel/sched/idle.c:355
> >  cpu_startup_entry+0x43/0x60 kernel/sched/idle.c:454
> >  rest_init+0x2de/0x300 init/main.c:717
> >  start_kernel+0x392/0x3e0 init/main.c:1175
> >  x86_64_start_reservations+0x24/0x30 arch/x86/kernel/head64.c:310
> >  x86_64_start_kernel+0x137/0x1b0 arch/x86/kernel/head64.c:291
> >  common_startup_64+0x13e/0x157
> >  </TASK>
> >
> >
> > ---
> > If you want syzbot to run the reproducer, reply with:
> > #syz test: git://repo/address.git branch-or-commit-hash
> > If you attach or paste a git patch, syzbot will apply it before testing.
> >
>
> The reproducer freezes the root filesystem with FIFREEZE and then opens
> O_TMPFILE on it from a child. Excellent. In addition to that it also
> lowers hung_task_timeout_secs to 15 before freezing and only thaws after
> 60 seconds... Nothing is stuck and the machine recovers.

Thanks for sharing the analysis!

>
> The older crashes look like the same thing. So the fuzzer freezes a
> filesystem and some other program writes to it. That also explains why
> there's only ever the one blocked task and nothing else in the system
> holding anything.
>
> So maybe gate that ioctl for syzkaller?

Normally, we prohibit using FIFREEZE during fuzzing, so the original
finding shouldn't have been caused by it.

But the C reproducer was indeed generated by an LLM, which was not
subject to such restrictions.
I've filed the issue and we'll find a way to address it.

>
> And the LLM thing that is attached to the report is wrong.
>
> #syz invalid
>

FWIW it's better to just keep such bugs open.
"syz invalid" indicates that the issue is a rare or already addressed
false positive and is no longer relevant. In this case, however,
syzbot keeps observing the crash daily, so it will try to re-report it
the next time it observes it.

Something like "#syz set prio: low" and/or "#syz set no-reminders"
would have been a better fit here:
https://github.com/google/syzkaller/blob/master/docs/syzbot.md#bug-labels

-- 
Aleksandr