INFO: task syz-executor391:7978 blocked for more than 140 seconds. Not tainted 4.14.281-syzkaller #0 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. syz-executor391 D27040 7978 7977 0x00000004 Call Trace: context_switch kernel/sched/core.c:2811 [inline] __schedule+0x88b/0x1de0 kernel/sched/core.c:3387 schedule+0x8d/0x1b0 kernel/sched/core.c:3431 schedule_timeout+0x80a/0xe90 kernel/time/timer.c:1724 do_wait_for_common kernel/sched/completion.c:91 [inline] __wait_for_common kernel/sched/completion.c:112 [inline] wait_for_common+0x272/0x430 kernel/sched/completion.c:123 flush_work+0x3fe/0x770 kernel/workqueue.c:2894 n_tty_poll+0x4de/0x7d0 drivers/tty/n_tty.c:2412 tty_poll+0x129/0x1a0 drivers/tty/tty_io.c:2102 do_select+0xa83/0x1290 fs/select.c:513 core_sys_select+0x32f/0x6a0 fs/select.c:656 do_pselect fs/select.c:733 [inline] SYSC_pselect6 fs/select.c:774 [inline] SyS_pselect6+0x358/0x3c0 fs/select.c:759 do_syscall_64+0x1d5/0x640 arch/x86/entry/common.c:292 entry_SYSCALL_64_after_hwframe+0x46/0xbb RIP: 0033:0x7f4f60f1e3c9 RSP: 002b:00007fffb4b174f8 EFLAGS: 00000246 ORIG_RAX: 000000000000010e RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f4f60f1e3c9 RDX: 0000000000000000 RSI: 00000000200000c0 RDI: 0000000000000040 RBP: 00007fffb4b17530 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000020000140 R11: 0000000000000246 R12: 00007f4f60ee1990 R13: 431bde82d7b634db R14: 0000000000000000 R15: 0000000000000000 Showing all locks held in the system: 1 lock held by khungtaskd/1532: #0: (tasklist_lock){.+.+}, at: [] debug_show_all_locks+0x7c/0x21a kernel/locking/lockdep.c:4548 1 lock held by syz-executor391/7978: #0: (&tty->ldisc_sem){++++}, at: [] tty_ldisc_ref_wait+0x22/0x80 drivers/tty/tty_ldisc.c:284 ============================================= NMI backtrace for cpu 1 CPU: 1 PID: 1532 Comm: khungtaskd Not tainted 4.14.281-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Call Trace: __dump_stack lib/dump_stack.c:17 [inline] dump_stack+0x1b2/0x281 lib/dump_stack.c:58 nmi_cpu_backtrace.cold+0x57/0x93 lib/nmi_backtrace.c:101 nmi_trigger_cpumask_backtrace+0x13a/0x180 lib/nmi_backtrace.c:62 trigger_all_cpu_backtrace include/linux/nmi.h:140 [inline] check_hung_uninterruptible_tasks kernel/hung_task.c:195 [inline] watchdog+0x5b9/0xb40 kernel/hung_task.c:274 kthread+0x30d/0x420 kernel/kthread.c:232 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:404 Sending NMI from CPU 1 to CPUs 0: NMI backtrace for cpu 0 CPU: 0 PID: 2874 Comm: kworker/u4:4 Not tainted 4.14.281-syzkaller #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Workqueue: events_unbound flush_to_ldisc task: ffff8880ac2ce500 task.stack: ffff8880ac0d8000 RIP: 0010:mark_lock+0xa2/0x1050 kernel/locking/lockdep.c:3168 RSP: 0018:ffff8880ac0df770 EFLAGS: 00000806 RAX: dffffc0000000000 RBX: ffffffff8ba48d30 RCX: 1ffff11015859dce RDX: 1ffffffff17491ac RSI: ffff8880ac2cee50 RDI: ffffffff8ba48d60 RBP: ffff8880ac2cee50 R08: 0000000000000001 R09: 0000000000040458 R10: ffff8880ac2cee50 R11: ffff8880ac2ce500 R12: 0000000000000100 R13: ffff8880ac2ce500 R14: ffff8880ac2cee70 R15: 0000000000000008 FS: 0000000000000000(0000) GS:ffff8880ba400000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f81c3892000 CR3: 00000000a1231000 CR4: 00000000003406f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Call Trace: __lock_acquire+0x5fc/0x3f20 kernel/locking/lockdep.c:3452 lock_acquire+0x170/0x3f0 kernel/locking/lockdep.c:3998 __mutex_lock_common kernel/locking/mutex.c:756 [inline] __mutex_lock+0xc4/0x1310 kernel/locking/mutex.c:893 commit_echoes+0x4c/0x1e0 drivers/tty/n_tty.c:778 n_tty_receive_char_fast drivers/tty/n_tty.c:1449 [inline] n_tty_receive_buf_fast drivers/tty/n_tty.c:1609 [inline] __receive_buf drivers/tty/n_tty.c:1644 [inline] n_tty_receive_buf_common+0x1795/0x25a0 drivers/tty/n_tty.c:1742 tty_ldisc_receive_buf+0x9e/0x170 drivers/tty/tty_buffer.c:455 tty_port_default_receive_buf+0x6e/0xa0 drivers/tty/tty_port.c:37 receive_buf drivers/tty/tty_buffer.c:475 [inline] flush_to_ldisc+0x1e9/0x440 drivers/tty/tty_buffer.c:527 process_one_work+0x793/0x14a0 kernel/workqueue.c:2117 worker_thread+0x5cc/0xff0 kernel/workqueue.c:2251 kthread+0x30d/0x420 kernel/kthread.c:232 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:404 Code: 00 48 69 d8 50 01 00 00 48 81 eb 50 01 00 00 48 81 c3 00 db 9e 8b 48 8d 7b 30 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 07 0c 00 00 4c 89 e0 48 23 43 30 0f 84 9c 00 ---------------- Code disassembly (best guess): 0: 00 48 69 add %cl,0x69(%rax) 3: d8 50 01 fcoms 0x1(%rax) 6: 00 00 add %al,(%rax) 8: 48 81 eb 50 01 00 00 sub $0x150,%rbx f: 48 81 c3 00 db 9e 8b add $0xffffffff8b9edb00,%rbx 16: 48 8d 7b 30 lea 0x30(%rbx),%rdi 1a: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax 21: fc ff df 24: 48 89 fa mov %rdi,%rdx 27: 48 c1 ea 03 shr $0x3,%rdx * 2b: 80 3c 02 00 cmpb $0x0,(%rdx,%rax,1) <-- trapping instruction 2f: 0f 85 07 0c 00 00 jne 0xc3c 35: 4c 89 e0 mov %r12,%rax 38: 48 23 43 30 and 0x30(%rbx),%rax 3c: 0f .byte 0xf 3d: 84 .byte 0x84 3e: 9c pushfq