======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
kworker/0:2/126 is trying to acquire lock:
ffff88805c28a2e8 (&tty->termios_rwsem){++++}-{3:3}, at: n_tty_flush_buffer+0x2c/0x210 drivers/tty/n_tty.c:370

but task is already holding lock:
ffff8880176280b8 (&buf->lock){+.+.}-{3:3}, at: tty_buffer_flush+0x78/0x3e0 drivers/tty/tty_buffer.c:230

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (&buf->lock){+.+.}-{3:3}:
       __mutex_lock_common kernel/locking/mutex.c:603 [inline]
       __mutex_lock+0x12d/0xae0 kernel/locking/mutex.c:747
       tty_buffer_flush+0x78/0x3e0 drivers/tty/tty_buffer.c:230
       tty_ldisc_flush+0x6a/0xc0 drivers/tty/tty_ldisc.c:389
       __do_SAK+0x25c/0x630 drivers/tty/tty_io.c:3011
       vc_SAK+0x74/0x210 drivers/tty/vt/vt_ioctl.c:995
       process_one_work+0x8ab/0x1160 kernel/workqueue.c:2292
       worker_thread+0xaf5/0x12a0 kernel/workqueue.c:2439
       kthread+0x29d/0x330 kernel/kthread.c:376
       ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

-> #2 (console_lock){+.+.}-{0:0}:
       console_lock+0x160/0x1c0 kernel/printk/printk.c:2629
       uart_configure_port drivers/tty/serial/serial_core.c:2576 [inline]
       uart_add_one_port+0x828/0x1e70 drivers/tty/serial/serial_core.c:3135
       serial8250_register_8250_port+0x1321/0x1b40 drivers/tty/serial/8250/8250_core.c:1092
       serial_pnp_probe+0x56f/0x750 drivers/tty/serial/8250/8250_pnp.c:478
       pnp_device_probe+0x301/0x4b0 drivers/pnp/driver.c:111
       call_driver_probe drivers/base/dd.c:-1 [inline]
       really_probe+0x24a/0xb40 drivers/base/dd.c:638
       __driver_probe_device+0x1f5/0x390 drivers/base/dd.c:804
       driver_probe_device+0x4f/0x420 drivers/base/dd.c:834
       __driver_attach+0x43d/0x6c0 drivers/base/dd.c:1220
       bus_for_each_dev+0x182/0x1f0 drivers/base/bus.c:303
       bus_add_driver+0x324/0x5d0 drivers/base/bus.c:620
       driver_register+0x32d/0x430 drivers/base/driver.c:246
       serial8250_init+0x96/0x161 drivers/tty/serial/8250/8250_core.c:1191
       do_one_initcall+0x257/0x800 init/main.c:1309
       do_initcall_level+0x13d/0x1ed init/main.c:1382
       do_initcalls+0x4b/0x8a init/main.c:1398
       kernel_init_freeable+0x401/0x5ab init/main.c:1637
       kernel_init+0x19/0x1b0 init/main.c:1525
       ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

-> #1 (&port->mutex){+.+.}-{3:3}:
       __mutex_lock_common kernel/locking/mutex.c:603 [inline]
       __mutex_lock+0x12d/0xae0 kernel/locking/mutex.c:747
       uart_set_termios+0x80/0x900 drivers/tty/serial/serial_core.c:1651
       tty_set_termios+0xcad/0x17e0 drivers/tty/tty_ioctl.c:363
       set_termios+0x4fe/0x680 drivers/tty/tty_ioctl.c:537
       tty_mode_ioctl+0x51e/0x6a0 drivers/tty/tty_ioctl.c:-1
       tty_ioctl+0x878/0xba0 drivers/tty/tty_io.c:2785
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:870 [inline]
       __se_sys_ioctl+0xfa/0x170 fs/ioctl.c:856
       do_syscall_x64 arch/x86/entry/common.c:46 [inline]
       do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:76
       entry_SYSCALL_64_after_hwframe+0x68/0xd2

-> #0 (&tty->termios_rwsem){++++}-{3:3}:
       check_prev_add kernel/locking/lockdep.c:3095 [inline]
       check_prevs_add kernel/locking/lockdep.c:3214 [inline]
       validate_chain kernel/locking/lockdep.c:3830 [inline]
       __lock_acquire+0x2d18/0x7c40 kernel/locking/lockdep.c:5063
       lock_acquire+0x1bb/0x4a0 kernel/locking/lockdep.c:5676
       down_write+0x36/0x60 kernel/locking/rwsem.c:1573
       n_tty_flush_buffer+0x2c/0x210 drivers/tty/n_tty.c:370
       tty_buffer_flush+0x31d/0x3e0 drivers/tty/tty_buffer.c:242
       tty_ldisc_flush+0x6a/0xc0 drivers/tty/tty_ldisc.c:389
       __do_SAK+0x25c/0x630 drivers/tty/tty_io.c:3011
       vc_SAK+0x74/0x210 drivers/tty/vt/vt_ioctl.c:995
       process_one_work+0x8ab/0x1160 kernel/workqueue.c:2292
       worker_thread+0xaf5/0x12a0 kernel/workqueue.c:2439
       kthread+0x29d/0x330 kernel/kthread.c:376
       ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

other info that might help us debug this:

Chain exists of:
  &tty->termios_rwsem --> console_lock --> &buf->lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&buf->lock);
                               lock(console_lock);
                               lock(&buf->lock);
  lock(&tty->termios_rwsem);

 *** DEADLOCK ***

5 locks held by kworker/0:2/126:
 #0: ffff888017470938 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work+0x7b4/0x1160 kernel/workqueue.c:2267
 #1: ffffc9000251fd00 ((work_completion)(&vc_cons[currcons].SAK_work)){+.+.}-{0:0}, at: process_one_work+0x7b4/0x1160 kernel/workqueue.c:2267
 #2: ffffffff8cb21e20 (console_lock){+.+.}-{0:0}, at: vc_SAK+0x24/0x210 drivers/tty/vt/vt_ioctl.c:985
 #3: ffff88805c28a098 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref drivers/tty/tty_ldisc.c:264 [inline]
 #3: ffff88805c28a098 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_flush+0x1c/0xc0 drivers/tty/tty_ldisc.c:387
 #4: ffff8880176280b8 (&buf->lock){+.+.}-{3:3}, at: tty_buffer_flush+0x78/0x3e0 drivers/tty/tty_buffer.c:230

stack backtrace:
CPU: 0 PID: 126 Comm: kworker/0:2 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Workqueue: events vc_SAK
Call Trace:
 <TASK>
 dump_stack_lvl+0x188/0x24e lib/dump_stack.c:106
 check_noncircular+0x296/0x330 kernel/locking/lockdep.c:2170
 check_prev_add kernel/locking/lockdep.c:3095 [inline]
 check_prevs_add kernel/locking/lockdep.c:3214 [inline]
 validate_chain kernel/locking/lockdep.c:3830 [inline]
 __lock_acquire+0x2d18/0x7c40 kernel/locking/lockdep.c:5063
 lock_acquire+0x1bb/0x4a0 kernel/locking/lockdep.c:5676
 down_write+0x36/0x60 kernel/locking/rwsem.c:1573
 n_tty_flush_buffer+0x2c/0x210 drivers/tty/n_tty.c:370
 tty_buffer_flush+0x31d/0x3e0 drivers/tty/tty_buffer.c:242
 tty_ldisc_flush+0x6a/0xc0 drivers/tty/tty_ldisc.c:389
 __do_SAK+0x25c/0x630 drivers/tty/tty_io.c:3011
 vc_SAK+0x74/0x210 drivers/tty/vt/vt_ioctl.c:995
 process_one_work+0x8ab/0x1160 kernel/workqueue.c:2292
 worker_thread+0xaf5/0x12a0 kernel/workqueue.c:2439
 kthread+0x29d/0x330 kernel/kthread.c:376
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
 </TASK>
tty tty1: SAK: killed process 4589 (syz.1.122): by fd#3
tty tty1: SAK: killed process 4590 (syz.3.119): by fd#3
tty tty1: SAK: killed process 4591 (syz.0.120): by fd#3