tty: Fix tty_send_xchar() lock order inversion

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 4 insertions, 4 deletions

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 0c41dbcb90b8..bcc8e1e8bb72 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1282,18 +1282,22 @@ 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 9c5aebfe7053..1445dd39aa62 100644
--- a/drivers/tty/tty_ioctl.c
+++ b/drivers/tty/tty_ioctl.c
@@ -1147,16 +1147,12 @@ 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;