serial_core: Fix race in uart_handle_dcd_change

If a serial driver is called post hangup with a second DCD event then we
will attempt to get the ldisc of NULL. Check we have a tty before trying to
do anything with it.

This is still only safe within the uart layer if the caller holds the
relevant uart locks. We could do with a version where the tty is passed for
more general use.

Signed-off-by: Alan Cox <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/serial_core.c

···
 {
 	struct uart_state *state = uport->state;
 	struct tty_port *port = &state->port;
-	struct tty_ldisc *ld = tty_ldisc_ref(port->tty);
+	struct tty_ldisc *ld = NULL;
 	struct pps_event_time ts;
+	struct tty_struct *tty = port->tty;
 
+	if (tty)
+	        ld = tty_ldisc_ref(tty);
 	if (ld && ld->ops->dcd_change)
 		pps_get_ts(&ts);
 
···
 	if (port->flags & ASYNC_CHECK_CD) {
 		if (status)
 			wake_up_interruptible(&port->open_wait);
-		else if (port->tty)
-			tty_hangup(port->tty);
+		else if (tty)
+			tty_hangup(tty);
 	}
 
 	if (ld && ld->ops->dcd_change)
-		ld->ops->dcd_change(port->tty, status, &ts);
+		ld->ops->dcd_change(tty, status, &ts);
 	if (ld)
 		tty_ldisc_deref(ld);
 }