rtnetlink: bridge: mcast: Relax group address validation in common code

In the upcoming VXLAN MDB implementation, the 0.0.0.0 and :: MDB entries
will act as catchall entries for unregistered IP multicast traffic in a
similar fashion to the 00:00:00:00:00:00 VXLAN FDB entry that is used to
transmit BUM traffic.

In deployments where inter-subnet multicast forwarding is used, not all
the VTEPs in a tenant domain are members in all the broadcast domains.
It is therefore advantageous to transmit BULL (broadcast, unknown
unicast and link-local multicast) and unregistered IP multicast traffic
on different tunnels. If the same tunnel was used, a VTEP only
interested in IP multicast traffic would also pull all the BULL traffic
and drop it as it is not a member in the originating broadcast domain
[1].

Prepare for this change by allowing the 0.0.0.0 group address in the
common rtnetlink MDB code and forbid it in the bridge driver. A similar
change is not needed for IPv6 because the common code only validates
that the group address is not the all-nodes address.

[1] https://datatracker.ietf.org/doc/html/draft-ietf-bess-evpn-irb-mcast#section-2.6

Signed-off-by: Ido Schimmel <[e-mail]>
Reviewed-by: Nikolay Aleksandrov <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/bridge/br_mdb.c

···
 		}
 	}
 
+	if (cfg->entry->addr.proto == htons(ETH_P_IP) &&
+	    ipv4_is_zeronet(cfg->entry->addr.u.ip4)) {
+		NL_SET_ERR_MSG_MOD(extack, "IPv4 entry group address 0.0.0.0 is not allowed");
+		return -EINVAL;
+	}
+
 	if (tb[MDBA_SET_ENTRY_ATTRS])
 		return br_mdb_config_attrs_init(tb[MDBA_SET_ENTRY_ATTRS], cfg,
 						extack);

net/core/rtnetlink.c

···
 	}
 
 	if (entry->addr.proto == htons(ETH_P_IP)) {
-		if (!ipv4_is_multicast(entry->addr.u.ip4)) {
-			NL_SET_ERR_MSG(extack, "IPv4 entry group address is not multicast");
+		if (!ipv4_is_multicast(entry->addr.u.ip4) &&
+		    !ipv4_is_zeronet(entry->addr.u.ip4)) {
+			NL_SET_ERR_MSG(extack, "IPv4 entry group address is not multicast or 0.0.0.0");
 			return -EINVAL;
 		}
 		if (ipv4_is_local_multicast(entry->addr.u.ip4)) {