[PATCH] tty: Fix tty_send_xchar() lock order inversion

[Peter Hurley <peter@hurleysoftware.com>]

The correct lock order is atomic_write_lock => termios_rwsem, as
established by tty_write() => n_tty_write().

Fixes: c274f6ef1c666 ("tty: Hold termios_rwsem for tcflow(TCIxxx)")
Reported-and-Tested-by: Dmitry Vyukov <dvyukov@google.com>
Cc: <stable@vger.kernel.org> # v3.18+
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
---
 drivers/tty/tty_io.c    | 4 ++++
 drivers/tty/tty_ioctl.c | 4 ----
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 2f8c21e..346a1a5 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1283,18 +1283,22 @@ int tty_send_xchar(struct tty_struct *tty, char ch)
 	int	was_stopped = tty->stopped;
 
 	if (tty->ops->send_xchar) {
+		down_read(&tty->termios_rwsem);
 		tty->ops->send_xchar(tty, ch);
+		up_read(&tty->termios_rwsem);
 		return 0;
 	}
 
 	if (tty_write_lock(tty, 0) < 0)
 		return -ERESTARTSYS;
 
+	down_read(&tty->termios_rwsem);
 	if (was_stopped)
 		start_tty(tty);
 	tty->ops->write(tty, &ch, 1);
 	if (was_stopped)
 		stop_tty(tty);
+	up_read(&tty->termios_rwsem);
 	tty_write_unlock(tty);
 	return 0;
 }
diff --git a/drivers/tty/tty_ioctl.c b/drivers/tty/tty_ioctl.c
index b8c5c12..0ea3513 100644
--- a/drivers/tty/tty_ioctl.c
+++ b/drivers/tty/tty_ioctl.c
@@ -1140,16 +1140,12 @@ int n_tty_ioctl_helper(struct tty_struct *tty, struct file *file,
 			spin_unlock_irq(&tty->flow_lock);
 			break;
 		case TCIOFF:
-			down_read(&tty->termios_rwsem);
 			if (STOP_CHAR(tty) != __DISABLED_CHAR)
 				retval = tty_send_xchar(tty, STOP_CHAR(tty));
-			up_read(&tty->termios_rwsem);
 			break;
 		case TCION:
-			down_read(&tty->termios_rwsem);
 			if (START_CHAR(tty) != __DISABLED_CHAR)
 				retval = tty_send_xchar(tty, START_CHAR(tty));
-			up_read(&tty->termios_rwsem);
 			break;
 		default:
 			return -EINVAL;
-- 
2.6.3