Re: [syzbot] [kvm?] INFO: task hung in kvm_gmem_release

[Hillf Danton <hdanton@sina.com>]

On Mon, 31 Aug 2026 08:15:44 -0700 Sean Christopherson wrote:
>On Mon, Aug 31, 2026, syzbot wrote:
>> Hello,
>> 
>> syzbot found the following issue on:
>> 
>> HEAD commit:    08dbfad3f504 Merge tag 'for-linus' of git://git.kernel.org..
>> git tree:       upstream
>> console+strace: https://syzkaller.appspot.com/x/log.txt?x=12ee0349580000
>> kernel config:  https://syzkaller.appspot.com/x/.config?x=5e4e3a0e188a497e
>> dashboard link: https://syzkaller.appspot.com/bug?extid=6895ff73d58c59e653ba
>> compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1153ee25580000
>> 
>> Downloadable assets:
>> disk image: https://storage.googleapis.com/syzbot-assets/ddff5719ec3b/disk-08dbfad3.raw.xz
>> vmlinux: https://storage.googleapis.com/syzbot-assets/4b5e39b20fa5/vmlinux-08dbfad3.xz
>> kernel image: https://storage.googleapis.com/syzbot-assets/eb1f70eed57f/bzImage-08dbfad3.xz
>> 
>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>> Reported-by: syzbot+6895ff73d58c59e653ba@syzkaller.appspotmail.com
>> 
>> INFO: task syz.0.17:6104 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:25544 pid:6104  tgid:6104  ppid:5900   task_flags:0x400040 flags:0x00080003
>> Call Trace:
>>  <TASK>
>>  context_switch kernel/sched/core.c:5520 [inline]
>>  __schedule+0x17d4/0x5820 kernel/sched/core.c:7270
>>  __schedule_loop kernel/sched/core.c:7347 [inline]
>>  schedule+0x164/0x2b0 kernel/sched/core.c:7362
>>  schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7419
>>  __mutex_lock+0
>>  __mutex_lock_common kernel/locking/mutex.c:726 [inline]
>>  __mutex_lock+0x7c1/0x1550 kernel/locking/mutex.c:821
>>  kvm_gmem_release+0xdf/0x3e0 virt/kvm/guest_memfd.c:310
>>  __fput+0x418/0xa50 fs/file_table.c:512
>>  task_work_run+0x1d9/0x270 kernel/task_work.c:233
>>  ptrace_notify+0x27c/0x2b0 kernel/signal.c:2524
>>  ptrace_report_syscall include/linux/ptrace.h:416 [inline]
>>  ptrace_report_syscall_exit+0x2d/0x50 include/linux/ptrace.h:477
>>  arch_ptrace_report_syscall_exit include/linux/entry-common.h:233 [inline]
>>  syscall_exit_work include/linux/entry-common.h:266 [inline]
>>  syscall_exit_to_user_mode_work include/linux/entry-common.h:297 [inline]
>>  syscall_exit_to_user_mode include/linux/entry-common.h:334 [inline]
>>  do_syscall_64+0x4a0/0x520 arch/x86/entry/syscall_64.c:89
>>  entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
>...
>
>> CPU: 1 UID: 0 PID: 16 Comm: pr/ttyS0 Not tainted syzkaller #0 PREEMPT(full) 
>> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
>> RIP: 0010:io_serial_in+0x77/0xc0 drivers/tty/serial/8250/8250_port.c:409
>> Code: e8 4e e8 55 fc 44 89 f9 d3 e3 49 83 c6 40 4c 89 f0 48 c1 e8 03 42 80 3c 20 00 74 08 4c 89 f7 e8 1f b0 c5 fc 41 03 1e 89 da ec <0f> b6 c0 5b 41 5c 41 5e 41 5f e9 5a cc 74 06 cc 44 89 f9 80 e1 07
>> RSP: 0018:ffffc900001578b8 EFLAGS: 00000002
>> RAX: 1ffffffff35a5e00 RBX: 00000000000003fd RCX: 0000000000000000
>> RDX: 00000000000003fd RSI: 0000000000000000 RDI: 0000000000000020
>> RBP: dffffc0000000000 R08: 0000000000000003 R09: 0000000000000004
>> R10: dffffc0000000000 R11: ffffffff8571cb00 R12: dffffc0000000000
>> R13: 0000000000000000 R14: ffffffff9ad2f0c0 R15: 0000000000000000
>> FS:  0000000000000000(0000) GS:ffff888124ddc000(0000) knlGS:0000000000000000
>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 0000557adc994a38 CR3: 00000000772ec000 CR4: 00000000003526f0
>> Call Trace:
>>  <TASK>
>>  serial_in drivers/tty/serial/8250/8250.h:128 [inline]
>>  serial_lsr_in drivers/tty/serial/8250/8250.h:150 [inline]
>>  wait_for_lsr+0x184/0x2f0 drivers/tty/serial/8250/8250_port.c:2013
>>  serial8250_fifo_wait_for_lsr_thre drivers/tty/serial/8250/8250_port.c:3270 [inline]
>>  __serial8250_console_fifo_write+0xcd/0x3e0 drivers/tty/serial/8250/8250_port.c:3345
>>  serial8250_console_fifo_write drivers/tty/serial/8250/8250_port.c:3378 [inline]
>>  __serial8250_console_write drivers/tty/serial/8250/8250_port.c:3429 [inline]
>>  serial8250_console_write+0xaa1/0x1200 drivers/tty/serial/8250/8250_port.c:3493
>>  nbcon_emit_next_record+0xef5/0x1a90 kernel/printk/nbcon.c:-1
>>  nbcon_emit_one kernel/printk/nbcon.c:1157 [inline]
>>  nbcon_kthread_func+0x679/0x880 kernel/printk/nbcon.c:1271
>>  kthread+0x38b/0x480 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>
>
> Marking this invalid, as I'm 99.9% certain guest_memfd is an innocent bystander.
> CPU1 is stuck on an IN instruction (the actual port I/O), and there are a handful
> of identical failures in random subsystems that pop up a few times a month[*].
> My guess is the host side of the serial port is going out to lunch intermittently.
> 
> [*] https://lore.kernel.org/all/?q=%22io_serial_in%22%20syzkaller
> 
> #syz invalid
>
The 0.1% uncertainty hints this invalid mark is NOT true or at least earlier
than thought, because a) the IN instruction is not kvm specific [11] and b) the
i2c hang [11] is i2c specific [12].

[11] Subject: [syzbot] [i2c?] [usb?] INFO: task hung in em28xx_i2c_unregister
https://lore.kernel.org/lkml/6a95ef74.764875ec.b345.0003.GAE@google.com/

[12] Subject: [syzbot] [i2c?] [usb?] INFO: task hung in em28xx_i2c_unregister
https://lore.kernel.org/lkml/6a9676c7.764875ec.b345.0007.GAE@google.com/

That said, extra evidence is needed to prove that the serial port is going out
to lunch on the host side.