net: dsa: Discard frames from unused ports

The Marvell switches under some conditions will pass a frame to the
host with the port being the CPU port. Such frames are invalid, and
should be dropped. Not dropping them can result in a crash when
incrementing the receive statistics for an invalid port.
Reported-by: Chris Healy <cphealy@gmail.com>
Fixes: 91da11f8 ("net: Distributed Switch Architecture protocol support")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/dsa_priv.h

@@ -126,6 +126,7 @@ static inline struct net_device *dsa_master_find_slave(struct net_device *dev,
 	struct dsa_port *cpu_dp = dev->dsa_ptr;
 	struct dsa_switch_tree *dst = cpu_dp->dst;
 	struct dsa_switch *ds;
+	struct dsa_port *slave_port;
 
 	if (device < 0 || device >= DSA_MAX_SWITCHES)
 		return NULL;
@@ -137,7 +138,12 @@ static inline struct net_device *dsa_master_find_slave(struct net_device *dev,
 	if (port < 0 || port >= ds->num_ports)
 		return NULL;
 
-	return ds->ports[port].slave;
+	slave_port = &ds->ports[port];
+
+	if (unlikely(slave_port->type != DSA_PORT_TYPE_USER))
+		return NULL;
+
+	return slave_port->slave;
 }
 
 /* port.c */