net: dsa: flood multicast to CPU when slave has IFF_PROMISC

Certain DSA switches can eliminate flooding to the CPU when none of the
ports have the IFF_ALLMULTI or IFF_PROMISC flags set. This is done by
synthesizing a call to dsa_port_bridge_flags() for the CPU port, a call
which normally comes from the bridge driver via switchdev.

The bridge port flags and IFF_PROMISC|IFF_ALLMULTI have slightly
different semantics, and due to inattention/lack of proper testing, the
IFF_PROMISC flag allows unknown unicast to be flooded to the CPU, but
not unknown multicast.

This must be fixed by setting both BR_FLOOD (unicast) and BR_MCAST_FLOOD
in the synthesized dsa_port_bridge_flags() call, since IFF_PROMISC means
that packets should not be filtered regardless of their MAC DA.

Fixes: 7569459a ("net: dsa: manage flooding on the CPU ports")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/slave.c

@@ -285,7 +285,7 @@ static void dsa_port_manage_cpu_flood(struct dsa_port *dp)
 		if (other_dp->slave->flags & IFF_ALLMULTI)
 			flags.val |= BR_MCAST_FLOOD;
 		if (other_dp->slave->flags & IFF_PROMISC)
-			flags.val |= BR_FLOOD;
+			flags.val |= BR_FLOOD | BR_MCAST_FLOOD;
 	}
 
 	err = dsa_port_pre_bridge_flags(dp, flags, NULL);