======================================================
WARNING: possible circular locking dependency detected
5.6.0-rc3-next-20200228-syzkaller #0 Not tainted
------------------------------------------------------
syz-executor.0/20675 is trying to acquire lock:
ffff8880a44202a0 (&tty->termios_rwsem){++++}, at: n_tty_receive_buf_common+0x87/0x2ac0 drivers/tty/n_tty.c:1705

but task is already holding lock:
ffffffff89f414c0 (sel_lock){+.+.}, at: paste_selection+0x155/0x490 drivers/tty/vt/selection.c:379

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (sel_lock){+.+.}:
       __mutex_lock_common kernel/locking/mutex.c:956 [inline]
       __mutex_lock+0x156/0x13c0 kernel/locking/mutex.c:1103
       set_selection_kernel+0x38a/0x13c0 drivers/tty/vt/selection.c:222
       set_selection_user+0x94/0xd7 drivers/tty/vt/selection.c:186
       tioclinux+0x115/0x480 drivers/tty/vt/vt.c:3051
       vt_ioctl+0x18a6/0x2450 drivers/tty/vt/vt_ioctl.c:379
       tty_ioctl+0xedd/0x1440 drivers/tty/tty_io.c:2660
       vfs_ioctl fs/ioctl.c:47 [inline]
       ksys_ioctl+0x11a/0x180 fs/ioctl.c:763
       __do_sys_ioctl fs/ioctl.c:772 [inline]
       __se_sys_ioctl fs/ioctl.c:770 [inline]
       __x64_sys_ioctl+0x6f/0xb0 fs/ioctl.c:770
       do_syscall_64+0xf6/0x790 arch/x86/entry/common.c:295
       entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #1 (console_lock){+.+.}:
       console_lock+0x44/0x80 kernel/printk/printk.c:2290
       con_flush_chars drivers/tty/vt/vt.c:3224 [inline]
       con_flush_chars+0x35/0x90 drivers/tty/vt/vt.c:3216
       n_tty_write+0xd49/0xf90 drivers/tty/n_tty.c:2350
       do_tty_write drivers/tty/tty_io.c:962 [inline]
       tty_write+0x48f/0x7f0 drivers/tty/tty_io.c:1046
       __vfs_write+0x76/0x100 fs/read_write.c:494
       vfs_write+0x262/0x5c0 fs/read_write.c:558
       ksys_write+0x127/0x250 fs/read_write.c:611
       do_syscall_64+0xf6/0x790 arch/x86/entry/common.c:295
       entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #0 (&tty->termios_rwsem){++++}:
       check_prev_add kernel/locking/lockdep.c:2481 [inline]
       check_prevs_add kernel/locking/lockdep.c:2586 [inline]
       validate_chain kernel/locking/lockdep.c:3203 [inline]
       __lock_acquire+0x24b3/0x5270 kernel/locking/lockdep.c:4190
       lock_acquire+0x197/0x420 kernel/locking/lockdep.c:4720
       down_read+0x96/0x430 kernel/locking/rwsem.c:1492
       n_tty_receive_buf_common+0x87/0x2ac0 drivers/tty/n_tty.c:1705
       tty_ldisc_receive_buf+0xa9/0x190 drivers/tty/tty_buffer.c:461
       paste_selection+0x1e5/0x490 drivers/tty/vt/selection.c:394
       tioclinux+0x12c/0x480 drivers/tty/vt/vt.c:3056
       vt_ioctl+0x18a6/0x2450 drivers/tty/vt/vt_ioctl.c:379
       tty_ioctl+0xedd/0x1440 drivers/tty/tty_io.c:2660
       vfs_ioctl fs/ioctl.c:47 [inline]
       ksys_ioctl+0x11a/0x180 fs/ioctl.c:763
       __do_sys_ioctl fs/ioctl.c:772 [inline]
       __se_sys_ioctl fs/ioctl.c:770 [inline]
       __x64_sys_ioctl+0x6f/0xb0 fs/ioctl.c:770
       do_syscall_64+0xf6/0x790 arch/x86/entry/common.c:295
       entry_SYSCALL_64_after_hwframe+0x49/0xbe

other info that might help us debug this:

Chain exists of:
  &tty->termios_rwsem --> console_lock --> sel_lock

 Possible unsafe locking scenario:

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

 *** DEADLOCK ***

3 locks held by syz-executor.0/20675:
 #0: ffff8880a4420090 (&tty->ldisc_sem){++++}, at: tty_ldisc_ref_wait+0x22/0x80 drivers/tty/tty_ldisc.c:267
 #1: ffff8880aa1be0a8 (&buf->lock){+.+.}, at: paste_selection+0x122/0x490 drivers/tty/vt/selection.c:376
 #2: ffffffff89f414c0 (sel_lock){+.+.}, at: paste_selection+0x155/0x490 drivers/tty/vt/selection.c:379

stack backtrace:
CPU: 1 PID: 20675 Comm: syz-executor.0 Not tainted 5.6.0-rc3-next-20200228-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x188/0x20d lib/dump_stack.c:118
 check_noncircular+0x32e/0x3e0 kernel/locking/lockdep.c:1812
 check_prev_add kernel/locking/lockdep.c:2481 [inline]
 check_prevs_add kernel/locking/lockdep.c:2586 [inline]
 validate_chain kernel/locking/lockdep.c:3203 [inline]
 __lock_acquire+0x24b3/0x5270 kernel/locking/lockdep.c:4190
 lock_acquire+0x197/0x420 kernel/locking/lockdep.c:4720
 down_read+0x96/0x430 kernel/locking/rwsem.c:1492
 n_tty_receive_buf_common+0x87/0x2ac0 drivers/tty/n_tty.c:1705
 tty_ldisc_receive_buf+0xa9/0x190 drivers/tty/tty_buffer.c:461
 paste_selection+0x1e5/0x490 drivers/tty/vt/selection.c:394
 tioclinux+0x12c/0x480 drivers/tty/vt/vt.c:3056
 vt_ioctl+0x18a6/0x2450 drivers/tty/vt/vt_ioctl.c:379
 tty_ioctl+0xedd/0x1440 drivers/tty/tty_io.c:2660
 vfs_ioctl fs/ioctl.c:47 [inline]
 ksys_ioctl+0x11a/0x180 fs/ioctl.c:763
 __do_sys_ioctl fs/ioctl.c:772 [inline]
 __se_sys_ioctl fs/ioctl.c:770 [inline]
 __x64_sys_ioctl+0x6f/0xb0 fs/ioctl.c:770
 do_syscall_64+0xf6/0x790 arch/x86/entry/common.c:295
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45c849
Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fb4dee4fc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007fb4dee506d4 RCX: 000000000045c849
RDX: 0000000020000040 RSI: 000000000000541c RDI: 0000000000000003
RBP: 000000000076bfa0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 0000000000000570 R14: 00000000004c802f R15: 000000000076bfac