net/ipv4: Allow send to local broadcast from a socket bound to a VRF

Message sends to the local broadcast address (255.255.255.255) require
uc_index or sk_bound_dev_if to be set to an egress device. However,
responses or only received if the socket is bound to the device. This
is overly constraining for processes running in an L3 domain. This
patch allows a socket bound to the VRF device to send to the local
broadcast address by using IP_UNICAST_IF to set the egress interface
with packet receipt handled by the VRF binding.

Similar to IP_MULTICAST_IF, relax the constraint on setting
IP_UNICAST_IF if a socket is bound to an L3 master device. In this
case allow uc_index to be set to an enslaved if sk_bound_dev_if is
an L3 master device and is the master device for the ifindex.

In udp and raw sendmsg, allow uc_index to override the oif if
uc_index master device is oif (ie., the oif is an L3 master and the
index is an L3 slave).

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 14 insertions, 1 deletions

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 853321555a4e..3f018f34cf56 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -977,8 +977,21 @@ int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 		if (!saddr)
 			saddr = inet->mc_addr;
 		connected = 0;
-	} else if (!ipc.oif)
+	} else if (!ipc.oif) {
 		ipc.oif = inet->uc_index;
+	} else if (ipv4_is_lbcast(daddr) && inet->uc_index) {
+		/* oif is set, packet is to local broadcast and
+		 * and uc_index is set. oif is most likely set
+		 * by sk_bound_dev_if. If uc_index != oif check if the
+		 * oif is an L3 master and uc_index is an L3 slave.
+		 * If so, we want to allow the send using the uc_index.
+		 */
+		if (ipc.oif != inet->uc_index &&
+		    ipc.oif == l3mdev_master_ifindex_by_index(sock_net(sk),
+							      inet->uc_index)) {
+			ipc.oif = inet->uc_index;
+		}
+	}
 
 	if (connected)
 		rt = (struct rtable *)sk_dst_check(sk, 0);