virtio: console: Don't access vqs if device was unplugged

If a virtio-console device gets unplugged while a port is open, a
subsequent close() call on the port accesses vqs to free up buffers.
This can lead to a crash.

The buffers are already freed up as a result of the call to
unplug_ports() from virtcons_remove().  The fix is to simply not access
vq information if port->portdev is NULL.
Reported-by: Njuzhang <juzhang@redhat.com>
CC: stable@kernel.org
Signed-off-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/char/virtio_console.c

@@ -388,6 +388,10 @@ static void discard_port_data(struct port *port)
 	unsigned int len;
 	int ret;
 
+	if (!port->portdev) {
+		/* Device has been unplugged.  vqs are already gone. */
+		return;
+	}
 	vq = port->in_vq;
 	if (port->inbuf)
 		buf = port->inbuf;
@@ -470,6 +474,10 @@ static void reclaim_consumed_buffers(struct port *port)
 	void *buf;
 	unsigned int len;
 
+	if (!port->portdev) {
+		/* Device has been unplugged.  vqs are already gone. */
+		return;
+	}
 	while ((buf = virtqueue_get_buf(port->out_vq, &len))) {
 		kfree(buf);
 		port->outvq_full = false;