net: bridge: multicast: fix igmp/mld port context null pointer dereferences

With the recent change to use bridge/port multicast context pointers
instead of bridge/port I missed to convert two locations which pass the
port pointer as-is, but with the new model we need to verify the port
context is non-NULL first and retrieve the port from it. The first
location is when doing querier selection when a query is received, the
second location is when leaving a group. The port context will be null
if the packets originated from the bridge device (i.e. from the host).
The fix is simple just check if the port context exists and retrieve
the port pointer from it.

Fixes: adc47037a7d5 ("net: bridge: multicast: use multicast contexts instead of bridge or port")
Signed-off-by: Nikolay Aleksandrov <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/bridge/br_multicast.c

···
 #endif
 
 static bool br_ip4_multicast_select_querier(struct net_bridge_mcast *brmctx,
-					    struct net_bridge_port *port,
+					    struct net_bridge_mcast_port *pmctx,
 					    __be32 saddr)
 {
+	struct net_bridge_port *port = pmctx ? pmctx->port : NULL;
+
 	if (!timer_pending(&brmctx->ip4_own_query.timer) &&
 	    !timer_pending(&brmctx->ip4_other_query.timer))
 		goto update;
···
 
 #if IS_ENABLED(CONFIG_IPV6)
 static bool br_ip6_multicast_select_querier(struct net_bridge_mcast *brmctx,
-					    struct net_bridge_port *port,
+					    struct net_bridge_mcast_port *pmctx,
 					    struct in6_addr *saddr)
 {
+	struct net_bridge_port *port = pmctx ? pmctx->port : NULL;
+
 	if (!timer_pending(&brmctx->ip6_own_query.timer) &&
 	    !timer_pending(&brmctx->ip6_other_query.timer))
 		goto update;
···
 				struct br_ip *saddr,
 				unsigned long max_delay)
 {
-	if (!br_ip4_multicast_select_querier(brmctx, pmctx->port, saddr->src.ip4))
+	if (!br_ip4_multicast_select_querier(brmctx, pmctx, saddr->src.ip4))
 		return;
 
 	br_multicast_update_query_timer(brmctx, query, max_delay);
···
 				struct br_ip *saddr,
 				unsigned long max_delay)
 {
-	if (!br_ip6_multicast_select_querier(brmctx, pmctx->port, &saddr->src.ip6))
+	if (!br_ip6_multicast_select_querier(brmctx, pmctx, &saddr->src.ip6))
 		return;
 
 	br_multicast_update_query_timer(brmctx, query, max_delay);
···
 		mod_timer(&own_query->timer, time);
 
 		for (p = mlock_dereference(mp->ports, brmctx->br);
-		     p != NULL;
+		     p != NULL && pmctx != NULL;
 		     p = mlock_dereference(p->next, brmctx->br)) {
 			if (!br_port_group_equal(p, pmctx->port, src))
 				continue;