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tty: possible deadlock in tty_buffer_flush

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Hi,

I've got the following error report while fuzzing the kernel with syzkaller.

On commit 5ed02dbb497422bf225783f46e6eadd237d23d6b (4.12-rc3).

======================================================
WARNING: possible circular locking dependency detected
4.12.0-rc3+ #369 Not tainted
------------------------------------------------------
kworker/u9:1/31 is trying to acquire lock:
 (&buf->lock){+.+...}, at: [<ffffffff823dd42b>]
tty_buffer_flush+0xbb/0x3b0 drivers/tty/tty_buffer.c:221

but task is already holding lock:
 (&o_tty->termios_rwsem/1){++++..}, at: [<ffffffff823cd5d1>]
isig+0xa1/0x4d0 drivers/tty/n_tty.c:1100

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&o_tty->termios_rwsem/1){++++..}:
       validate_chain kernel/locking/lockdep.c:2281 [inline]
       __lock_acquire+0x220c/0x3690 kernel/locking/lockdep.c:3367
       lock_acquire+0x22d/0x560 kernel/locking/lockdep.c:3855
       down_read+0x96/0x160 kernel/locking/rwsem.c:23
       n_tty_write+0x29a/0xfb0 drivers/tty/n_tty.c:2287
       do_tty_write drivers/tty/tty_io.c:887 [inline]
       tty_write+0x3eb/0x8a0 drivers/tty/tty_io.c:971
       __vfs_write+0x5d5/0x760 fs/read_write.c:508
       vfs_write+0x187/0x500 fs/read_write.c:558
       SYSC_write fs/read_write.c:605 [inline]
       SyS_write+0xfb/0x230 fs/read_write.c:597
       entry_SYSCALL_64_fastpath+0x1f/0xbe

-> #1 (&tty->atomic_write_lock){+.+.+.}:
       validate_chain kernel/locking/lockdep.c:2281 [inline]
       __lock_acquire+0x220c/0x3690 kernel/locking/lockdep.c:3367
       lock_acquire+0x22d/0x560 kernel/locking/lockdep.c:3855
       __mutex_lock_common kernel/locking/mutex.c:756 [inline]
       __mutex_lock+0x18b/0x1900 kernel/locking/mutex.c:893
       mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:908
       tty_port_default_receive_buf+0x109/0x190 drivers/tty/tty_port.c:37
       receive_buf drivers/tty/tty_buffer.c:448 [inline]
       flush_to_ldisc+0x3e3/0x5b0 drivers/tty/tty_buffer.c:497
       process_one_work+0xc03/0x1bd0 kernel/workqueue.c:2097
       worker_thread+0x223/0x1860 kernel/workqueue.c:2231
       kthread+0x35e/0x430 kernel/kthread.c:231
       ret_from_fork+0x2a/0x40 arch/x86/entry/entry_64.S:424

-> #0 (&buf->lock){+.+...}:
       check_prev_add kernel/locking/lockdep.c:1844 [inline]
       check_prevs_add+0xadb/0x1b40 kernel/locking/lockdep.c:1954
       validate_chain kernel/locking/lockdep.c:2281 [inline]
       __lock_acquire+0x220c/0x3690 kernel/locking/lockdep.c:3367
       lock_acquire+0x22d/0x560 kernel/locking/lockdep.c:3855
       __mutex_lock_common kernel/locking/mutex.c:756 [inline]
       __mutex_lock+0x18b/0x1900 kernel/locking/mutex.c:893
       mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:908
       tty_buffer_flush+0xbb/0x3b0 drivers/tty/tty_buffer.c:221
       pty_flush_buffer+0x50/0x160 drivers/tty/pty.c:223
       tty_driver_flush_buffer+0x65/0x80 drivers/tty/tty_ioctl.c:94
       isig+0x16f/0x4d0 drivers/tty/n_tty.c:1111
       n_tty_receive_signal_char+0x22/0xf0 drivers/tty/n_tty.c:1212
       n_tty_receive_char_special+0xc5a/0x2c80 drivers/tty/n_tty.c:1264
       n_tty_receive_buf_fast drivers/tty/n_tty.c:1579 [inline]
       __receive_buf drivers/tty/n_tty.c:1613 [inline]
       n_tty_receive_buf_common+0x1abc/0x2630 drivers/tty/n_tty.c:1711
       n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1746
       tty_ldisc_receive_buf+0x2e9/0x420 drivers/tty/tty_buffer.c:429
       tty_port_default_receive_buf+0x122/0x190 drivers/tty/tty_port.c:38
       receive_buf drivers/tty/tty_buffer.c:448 [inline]
       flush_to_ldisc+0x3e3/0x5b0 drivers/tty/tty_buffer.c:497
       process_one_work+0xc03/0x1bd0 kernel/workqueue.c:2097
       worker_thread+0x223/0x1860 kernel/workqueue.c:2231
       kthread+0x35e/0x430 kernel/kthread.c:231
       ret_from_fork+0x2a/0x40 arch/x86/entry/entry_64.S:424

other info that might help us debug this:

Chain exists of:
  &buf->lock --> &tty->atomic_write_lock --> &o_tty->termios_rwsem/1

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&o_tty->termios_rwsem/1);
                               lock(&tty->atomic_write_lock);
                               lock(&o_tty->termios_rwsem/1);
  lock(&buf->lock);

 *** DEADLOCK ***

6 locks held by kworker/u9:1/31:
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8132fdaf>]
__write_once_size include/linux/compiler.h:283 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8132fdaf>]
atomic64_set arch/x86/include/asm/atomic64_64.h:33 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8132fdaf>]
atomic_long_set include/asm-generic/atomic-long.h:56 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8132fdaf>]
set_work_data kernel/workqueue.c:617 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8132fdaf>]
set_work_pool_and_clear_pending kernel/workqueue.c:644 [inline]
 #0:  ("events_unbound"){.+.+.+}, at: [<ffffffff8132fdaf>]
process_one_work+0xaef/0x1bd0 kernel/workqueue.c:2090
 #1:  ((&buf->work)){+.+...}, at: [<ffffffff8132fe02>]
process_one_work+0xb42/0x1bd0 kernel/workqueue.c:2094
 #2:  (&port->buf.lock/1){+.+...}, at: [<ffffffff823db680>]
flush_to_ldisc+0xb0/0x5b0 drivers/tty/tty_buffer.c:469
 #3:  (&tty->ldisc_sem){++++++}, at: [<ffffffff823d932b>]
tty_ldisc_ref+0x1b/0x80 drivers/tty/tty_ldisc.c:297
 #4:  (&tty->atomic_write_lock){+.+.+.}, at: [<ffffffff823de4c9>]
tty_port_default_receive_buf+0x109/0x190 drivers/tty/tty_port.c:37
 #5:  (&o_tty->termios_rwsem/1){++++..}, at: [<ffffffff823cd5d1>]
isig+0xa1/0x4d0 drivers/tty/n_tty.c:1100

stack backtrace:
CPU: 0 PID: 31 Comm: kworker/u9:1 Not tainted 4.12.0-rc3+ #369
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Workqueue: events_unbound flush_to_ldisc
Call Trace:
 __dump_stack lib/dump_stack.c:16 [inline]
 dump_stack+0x292/0x395 lib/dump_stack.c:52
 print_circular_bug+0x310/0x3c0 kernel/locking/lockdep.c:1218
 check_prev_add kernel/locking/lockdep.c:1844 [inline]
 check_prevs_add+0xadb/0x1b40 kernel/locking/lockdep.c:1954
 validate_chain kernel/locking/lockdep.c:2281 [inline]
 __lock_acquire+0x220c/0x3690 kernel/locking/lockdep.c:3367
 lock_acquire+0x22d/0x560 kernel/locking/lockdep.c:3855
 __mutex_lock_common kernel/locking/mutex.c:756 [inline]
 __mutex_lock+0x18b/0x1900 kernel/locking/mutex.c:893
 mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:908
 tty_buffer_flush+0xbb/0x3b0 drivers/tty/tty_buffer.c:221
 pty_flush_buffer+0x50/0x160 drivers/tty/pty.c:223
 tty_driver_flush_buffer+0x65/0x80 drivers/tty/tty_ioctl.c:94
 isig+0x16f/0x4d0 drivers/tty/n_tty.c:1111
 n_tty_receive_signal_char+0x22/0xf0 drivers/tty/n_tty.c:1212
 n_tty_receive_char_special+0xc5a/0x2c80 drivers/tty/n_tty.c:1264
 n_tty_receive_buf_fast drivers/tty/n_tty.c:1579 [inline]
 __receive_buf drivers/tty/n_tty.c:1613 [inline]
 n_tty_receive_buf_common+0x1abc/0x2630 drivers/tty/n_tty.c:1711
 n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1746
 tty_ldisc_receive_buf+0x2e9/0x420 drivers/tty/tty_buffer.c:429
 tty_port_default_receive_buf+0x122/0x190 drivers/tty/tty_port.c:38
 receive_buf drivers/tty/tty_buffer.c:448 [inline]
 flush_to_ldisc+0x3e3/0x5b0 drivers/tty/tty_buffer.c:497
 process_one_work+0xc03/0x1bd0 kernel/workqueue.c:2097
 worker_thread+0x223/0x1860 kernel/workqueue.c:2231
 kthread+0x35e/0x430 kernel/kthread.c:231
 ret_from_fork+0x2a/0x40 arch/x86/entry/entry_64.S:424

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Thread

tty: possible deadlock in tty_buffer_flush Andrey Konovalov <andreyknvl@google.com> - 2017-05-29 17:20 +0200
  Re: tty: possible deadlock in tty_buffer_flush Bart Van Assche <Bart.VanAssche@sandisk.com> - 2017-06-07 02:50 +0200
    Re: tty: possible deadlock in tty_buffer_flush "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org> - 2017-06-07 11:00 +0200

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