net: vrf: Do not allow looback to be moved to a VRF

Moving the loopback into a VRF breaks networking for the default VRF.
Since the VRF device is the loopback for VRF domains, there is no
reason to move the loopback. Given the repercussions, block attempts
to set lo into a VRF.
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/vrf.c

@@ -877,6 +877,12 @@ static int do_vrf_add_slave(struct net_device *dev, struct net_device *port_dev)
 {
 	int ret;
 
+	/* do not allow loopback device to be enslaved to a VRF.
+	 * The vrf device acts as the loopback for the vrf.
+	 */
+	if (port_dev == dev_net(dev)->loopback_dev)
+		return -EOPNOTSUPP;
+
 	port_dev->priv_flags |= IFF_L3MDEV_SLAVE;
 	ret = netdev_master_upper_dev_link(port_dev, dev, NULL, NULL);
 	if (ret < 0)