[PATCH 4.1 92/95] tty: Fix tty_send_xchar() lock order inversion

[Greg Kroah-Hartman]

4.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>

commit ee0c1a65cf95230d5eb3d9de94fd2ead9a428c67 upstream.

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@xxxxxxxxxx>
Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/tty/tty_io.c    |    4 ++++
 drivers/tty/tty_ioctl.c |    4 ----
 2 files changed, 4 insertions(+), 4 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1287,18 +1287,22 @@ int tty_send_xchar(struct tty_struct *tt
 	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;
 }
--- a/drivers/tty/tty_ioctl.c
+++ b/drivers/tty/tty_ioctl.c
@@ -1144,16 +1144,12 @@ int n_tty_ioctl_helper(struct tty_struct
 			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;


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