[PATCH] tty: fix lock order inversion in tty_buffer_flush()

tty_buffer_flush() calls ld->ops->flush_buffer() while holding
buf->lock. For the N_TTY line discipline, flush_buffer() is
n_tty_flush_buffer(), which acquires termios_rwsem.

However, the documented stable lock order (see tty.h) is:
  termios_rwsem -> buf->lock

Calling flush_buffer() under buf->lock inverts this order and creates
a circular locking dependency detected by lockdep:

  &buf->lock -> console_lock -> &port->mutex -> &tty->termios_rwsem
                                                -> &buf->lock

Fix this by moving the flush_buffer() call outside buf->lock.
The buffer data has already been discarded at this point, so the
ordering change is safe.

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=d84934b4184d7f9a1aed
Signed-off-by: Ömer Mete Kaya <[email protected]>
---
 drivers/tty/tty_buffer.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 96be90db5..bb7b7fbcb 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -244,11 +244,15 @@ void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
 	buf->head->read = buf->head->commit;
 	buf->head->lookahead = buf->head->read;
 
-	if (ld && ld->ops->flush_buffer)
-		ld->ops->flush_buffer(tty);
-
 	atomic_dec(&buf->priority);
 	mutex_unlock(&buf->lock);
+	/*
+	 * Call flush_buffer() outside buf->lock: n_tty_flush_buffer()
+	 * acquires termios_rwsem, but the required lock order is
+	 * termios_rwsem -> buf->lock, not the other way around.
+	 */
+	if (ld && ld->ops->flush_buffer)
+		ld->ops->flush_buffer(tty);
 }
 
 /**
-- 
2.55.0