net: bridge: Flush multicast groups when snooping is disabled

When forwarding multicast packets, the bridge takes MDB into account when
IGMP / MLD snooping is enabled. Currently, when snooping is disabled, the
MDB is retained, even though it is not used anymore.

At the same time, during the time that snooping is disabled, the IGMP / MLD
control packets are obviously ignored, and after the snooping is reenabled,
the administrator has to assume it is out of sync. In particular, missed
join and leave messages would lead to traffic being forwarded to wrong
interfaces.

Keeping the MDB entries around thus serves no purpose, and just takes
memory. Note also that disabling per-VLAN snooping does actually flush the
relevant MDB entries.

This patch flushes non-permanent MDB entries as global snooping is
disabled.

Signed-off-by: Petr Machata <[e-mail]>
Reviewed-by: Ido Schimmel <[e-mail]>
Acked-by: Nikolay Aleksandrov <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/bridge/br_multicast.c

···
 	rcu_read_unlock();
 }
 
+static void br_multicast_del_grps(struct net_bridge *br)
+{
+	struct net_bridge_port *port;
+
+	list_for_each_entry(port, &br->port_list, list)
+		__br_multicast_disable_port_ctx(&port->multicast_ctx);
+}
+
 int br_multicast_toggle(struct net_bridge *br, unsigned long val,
 			struct netlink_ext_ack *extack)
 {
···
 	br_opt_toggle(br, BROPT_MULTICAST_ENABLED, !!val);
 	if (!br_opt_get(br, BROPT_MULTICAST_ENABLED)) {
 		change_snoopers = true;
+		br_multicast_del_grps(br);
 		goto unlock;
 	}