net: l3mdev: Allow the l3mdev to be a loopback

Allow an L3 master device to act as the loopback for that L3 domain.
For IPv4 the device can also have the address 127.0.0.1.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 3e99278..f49b2c5 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -2018,7 +2018,9 @@
 		return ERR_PTR(-EINVAL);
 
 	if (likely(!IN_DEV_ROUTE_LOCALNET(in_dev)))
-		if (ipv4_is_loopback(fl4->saddr) && !(dev_out->flags & IFF_LOOPBACK))
+		if (ipv4_is_loopback(fl4->saddr) &&
+		    !(dev_out->flags & IFF_LOOPBACK) &&
+		    !netif_is_l3_master(dev_out))
 			return ERR_PTR(-EINVAL);
 
 	if (ipv4_is_lbcast(fl4->daddr))
@@ -2302,7 +2304,9 @@
 			else
 				fl4->saddr = fl4->daddr;
 		}
-		dev_out = net->loopback_dev;
+
+		/* L3 master device is the loopback for that domain */
+		dev_out = l3mdev_master_dev_rcu(dev_out) ? : net->loopback_dev;
 		fl4->flowi4_oif = dev_out->ifindex;
 		flags |= RTCF_LOCAL;
 		goto make_route;