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Thu, 12 Nov 2026 15:43:56 -0800 (PST) Date: Thu, 12 Nov 2026 15:43:56 -0800 X-Google-Appengine-App-Id: s~syzkaller X-Google-Appengine-App-Id-Alias: syzkaller Message-ID: <6af64f3c.29bc419b.2c18ce.0002.GAE@google.com> Subject: [syzbot] [kernel?] INFO: task hung in vhost_task_stop (3) From: syzbot To: john.ogness@linutronix.de, linux-kernel@vger.kernel.org, pmladek@suse.com, printk@lists.linux.dev, rostedt@goodmis.org, senozhatsky@chromium.org, syzkaller-bugs@googlegroups.com Content-Type: text/plain; charset="UTF-8" Hello, syzbot found the following issue on: HEAD commit: fa4ad798f6f6 Merge tag 'for-7.4-rc1-tag' of git://git.kern.. git tree: upstream console output: https://syzkaller.appspot.com/x/log.txt?x=101e0b69580000 kernel config: https://syzkaller.appspot.com/x/.config?x=d44b06612b68f56 dashboard link: https://syzkaller.appspot.com/bug?extid=f59eab881ec20d423ab0 compiler: Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8 userspace arch: i386 syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1382f8ad580000 C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1782f8ad580000 Downloadable assets: disk image: https://storage.googleapis.com/syzbot-assets/2bde23f56ff1/disk-fa4ad798.raw.xz vmlinux: https://storage.googleapis.com/syzbot-assets/48017a7575e9/vmlinux-fa4ad798.xz kernel image: https://storage.googleapis.com/syzbot-assets/c5dba52ab74d/bzImage-fa4ad798.xz AI-generated root cause summary: When a KVM VM is initialized, kvm_mmu_start_lpage_recovery() creates the "kvm-nx-lpage-recovery" worker thread via vhost_task_create(). Because vhost_task_create() clones the calling process with CLONE_THREAD and .user_worker = 1 (PF_USER_WORKER) without PF_NOFREEZE, the worker inherits the caller's thread group and cgroup freezer state. If the cgroup is subsequently frozen (e.g. via cgroup1 freezer.state), the worker thread is trapped in __refrigerator() in the TASK_FROZEN state. When the VM file descriptor is closed, kvm_mmu_pre_destroy_vm() calls vhost_task_stop(), which sets VHOST_TASK_FLAGS_STOP, attempts to wake the worker using wake_up_process() (which only targets TASK_NORMAL), and blocks in wait_for_completion(&vtsk->exited). Because wake_up_process() cannot wake a task in TASK_FROZEN and freezing(current) remains true, the worker never exits, causing vhost_task_stop() to hang indefinitely in uninterruptible sleep. full analysis: https://syzkaller.appspot.com/x/ai_root_cause?x=f59eab881ec20d423ab0 IMPORTANT: if you fix the issue, please add the following tag to the commit: Reported-by: syzbot+f59eab881ec20d423ab0@syzkaller.appspotmail.com INFO: task syz.0.17:5888 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:26216 pid:5888 tgid:5888 ppid:5825 task_flags:0x400040 flags:0x08080002 Call Trace: __schedu context_switch kernel/sched/core.c:5583 [inline] __schedule+0x17dd/0x5830 kernel/sched/core.c:7382 __schedule_loop kernel/sched/core.c:7459 [inline] schedule+0x164/0x2b0 kernel/sched/core.c:7474 schedule_timeout+0xc0/0x2c0 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+0x2c5/0x5d0 kernel/sched/completion.c:153 vhost_task_stop+0xc0/0x170 kernel/vhost_task.c:102 kvm_arch_pre_destroy_vm+0x68/0x80 arch/x86/kvm/x86.c:10107 kvm_destroy_vm virt/kvm/kvm_main.c:1284 [inline] kvm_put_kvm+0x377/0xb40 virt/kvm/kvm_main.c:1350 kvm_vm_release+0x6b/0x80 virt/kvm/kvm_main.c:1375 __fput+0x408/0xa40 fs/file_table.c:512 fput_close_sync+0x11f/0x240 fs/file_table.c:618 filp_close_sync+0x6e/0xa0 fs/open.c:1574 __range_close fs/file.c:919 [inline] __do_sys_close_range fs/file.c:1003 [inline] __se_sys_close_range+0xa77/0xef0 fs/file.c:954 do_syscall_32_irqs_on arch/x86/entry/syscall_32.c:79 [inline] __do_fast_syscall_32+0x28b/0x6c0 arch/x86/entry/syscall_32.c:282 do_fast_syscall_32+0x33/0x70 arch/x86/entry/syscall_32.c:307 entry_SYSENTER_compat_after_hwframe+0x84/0x8e RIP: 0023:0xf7fb3f1c RSP: 002b:00000000ffe837dc EFLAGS: 00000206 ORIG_RAX: 00000000000001b4 RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 000000000000001e RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000 RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000000000 R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000 Showing all locks held in the system: locks held by khungtaskd/33: 1, last CPU#0: #0: ffffffff9015a260 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline] #0: ffffffff9015a260 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline] #0: ffffffff9015a260 (rcu_read_lock){....}-{1:3}, at: debug_show_all_locks+0x2e/0x180 kernel/locking/lockdep.c:6871 locks held by kworker/u8:9/1115: 4, last CPU#0: #0: ffff88813fe4c148 ((wq_completion)events_unbound){+.+.}-{0:0}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline] #0: ffff88813fe4c148 ((wq_completion)events_unbound){+.+.}-{0:0}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline] #0: ffff88813fe4c148 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3460 [inline] #0: ffff88813fe4c148 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_scheduled_works+0x97a/0x1630 kernel/workqueue.c:3578 #1: ffff8880b8224708 (psi_seq){-.-.}-{0:0}, at: psi_task_switch+0x57/0x7d0 kernel/sched/psi.c:928 #2: ffff8880b8226498 (&base->lock){-.-.}-{2:2}, at: lock_timer_base kernel/time/timer.c:1005 [inline] #2: ffff8880b8226498 (&base->lock){-.-.}-{2:2}, at: __mod_timer+0x1a9/0xed0 kernel/time/timer.c:1086 #3: ffffffff9c12ba00 (&obj_hash[i].lock){-.-.}-{2:2}, at: debug_object_activate+0xb7/0x420 lib/debugobjects.c:873 locks held by kworker/u8:12/1168: 3, last CPU#0: #0: ffff88813fe4c148 ((wq_completion)events_unbound){+.+.}-{0:0}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline] #0: ffff88813fe4c148 ((wq_completion)events_unbound){+.+.}-{0:0}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline] #0: ffff88813fe4c148 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3460 [inline] #0: ffff88813fe4c148 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_scheduled_works+0x97a/0x1630 kernel/workqueue.c:3578 #1: ffffc900050f7c40 ((work_completion)(&(&nsim_dev->trap_data->trap_report_dw)->work)){+.+.}-{0:0}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline] #1: ffffc900050f7c40 ((work_completion)(&(&nsim_dev->trap_data->trap_report_dw)->work)){+.+.}-{0:0}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline] #1: ffffc900050f7c40 ((work_completion)(&(&nsim_dev->trap_data->trap_report_dw)->work)){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3460 [inline] #1: ffffc900050f7c40 ((work_completion)(&(&nsim_dev->trap_data->trap_report_dw)->work)){+.+.}-{0:0}, at: process_scheduled_works+0x97a/0x1630 kernel/workqueue.c:3578 #2: ffff888037387258 (&devlink->lock_key#9){+.+.}-{4:4}, at: nsim_dev_trap_report_work+0x57/0xb30 drivers/net/netdevsim/dev.c:834 locks held by kworker/0:2/1216: 3, last CPU#0: #0: ffff88813fe0dd48 ((wq_completion)events_power_efficient){+.+.}-{0:0}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline] #0: ffff88813fe0dd48 ((wq_completion)events_power_efficient){+.+.}-{0:0}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline] #0: ffff88813fe0dd48 ((wq_completion)events_power_efficient){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3460 [inline] #0: ffff88813fe0dd48 ((wq_completion)events_power_efficient){+.+.}-{0:0}, at: process_scheduled_works+0x97a/0x1630 kernel/workqueue.c:3578 #1: ffffc90005487c40 ((work_completion)(&(&gc_work->dwork)->work)){+.+.}-{0:0}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline] #1: ffffc90005487c40 ((work_completion)(&(&gc_work->dwork)->work)){+.+.}-{0:0}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline] #1: ffffc90005487c40 ((work_completion)(&(&gc_work->dwork)->work)){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3460 [inline] #1: ffffc90005487c40 ((work_completion)(&(&gc_work->dwork)->work)){+.+.}-{0:0}, at: process_scheduled_works+0x97a/0x1630 kernel/workqueue.c:3578 #2: ffffffff9015a260 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire include/linux/rcupdate.h:309 [inline] #2: ffffffff9015a260 (rcu_read_lock){....}-{1:3}, at: rcu_read_lock include/linux/rcupdate.h:849 [inline] #2: ffffffff9015a260 (rcu_read_lock){....}-{1:3}, at: gc_worker+0x274/0x1890 net/netfilter/nf_conntrack_core.c:1530 locks held by getty/5391: 2, on CPU#1: #0: ffff8880385230a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref_wait+0x25/0x70 drivers/tty/tty_ldisc.c:243 #1: ffffc900032362e8 (&ldata->atomic_read_lock){+.+.}-{4:4}, at: n_tty_read+0x45a/0x1360 drivers/tty/n_tty.c:2211 ============================================= NMI backtrace for cpu 1 CPU: 1 UID: 0 PID: 33 Comm: khungtaskd Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/15/2026 Call Trace: dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120 nmi_cpu_backtrace+0x274/0x2d0 lib/nmi_backtrace.c:123 nmi_trigger_cpumask_backtrace+0x17d/0x390 lib/nmi_backtrace.c:66 trigger_all_cpu_backtrace include/linux/nmi.h:164 [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:377 [inline] watchdog+0x1073/0x10f0 kernel/hung_task.c:598 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 Sending NMI from CPU 1 to CPUs 0: NMI backtrace for cpu 0 CPU: 0 UID: 0 PID: 32 Comm: pr/ttyS0 Not tainted syzkaller #0 PREEMPT(full) Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/15/2026 RIP: 0010:io_serial_in+0x77/0xc0 drivers/tty/serial/8250/8250_port.c:425 Code: e8 2e 6a 43 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 2f f3 ba fc 41 03 1e 89 da ec <0f> b6 c0 5b 41 5c 41 5e 41 5f e9 9a 14 83 06 cc 44 89 f9 80 e1 07 RSP: 0018:ffffc90000a878b8 EFLAGS: 00000002 RAX: 1ffffffff3840b00 RBX: 00000000000003fd RCX: 0000000000000000 RDX: 00000000000003fd RSI: 0000000000000000 RDI: 0000000000000020 RBP: dffffc0000000000 R08: 0000000000000003 R09: 0000000000000004 R10: dffffc0000000000 R11: ffffffff8585a440 R12: dffffc0000000000 R13: 0000000000000000 R14: ffffffff9c205ec0 R15: 0000000000000000 FS: 0000000000000000(0000) GS:ffff888123409000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000563e7aa97d08 CR3: 000000000ff48000 CR4: 00000000003526f0 Call Trace: serial_in drivers/tty/serial/8250/8250.h:134 [inline] serial_lsr_in drivers/tty/serial/8250/8250.h:156 [inline] wait_for_lsr+0x184/0x2f0 drivers/tty/serial/8250/8250_port.c:2036 serial8250_fifo_wait_for_lsr_thre drivers/tty/serial/8250/8250_port.c:3294 [inline] __serial8250_console_fifo_write+0xcd/0x3e0 drivers/tty/serial/8250/8250_port.c:3369 serial8250_console_fifo_write drivers/tty/serial/8250/8250_port.c:3402 [inline] __serial8250_console_write drivers/tty/serial/8250/8250_port.c:3456 [inline] serial8250_console_write+0xa7e/0x1200 drivers/tty/serial/8250/8250_port.c:3520 nbcon_emit_next_record+0xef5/0x1a90 kernel/printk/nbcon.c:-1 nbcon_emit_one kernel/printk/nbcon.c:1157 [inline] nbcon_kthread_func+0x6d9/0x8d0 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 --- This report is generated by a bot. It may contain errors. See https://goo.gl/tpsmEJ for more information about syzbot. syzbot engineers can be reached at syzkaller@googlegroups.com. syzbot will keep track of this issue. See: https://goo.gl/tpsmEJ#status for how to communicate with syzbot. If the report is already addressed, let syzbot know by replying with: #syz fix: exact-commit-title If you want to overwrite report's subsystems, reply with: #syz set subsystems: new-subsystem (See the list of subsystem names on the web dashboard) If the report is a duplicate of another one, reply with: #syz dup: exact-subject-of-another-report If you want to undo deduplication, reply with: #syz undup