Linux Ethernet Bridging - Mar 2014 - [Bridge] [PATCH 1/2] bridge: multicast: add sanity check for general query destination

General IGMP and MLD queries are supposed to have the multicast
link-local all-nodes address as their destination according to RFC2236
section 9, RFC3376 section 4.1.12/9.1, RFC2710 section 8 and RFC3810
section 5.1.15.

Without this check, such malformed IGMP/MLD queries can result in a
denial of service: The queries are ignored by most IGMP/MLD listeners
therefore they will not respond with an IGMP/MLD report. However,
without this patch these malformed MLD queries would enable the
snooping part in the bridge code, potentially shutting down the
according ports towards these hosts for multicast traffic as the
bridge did not learn about these listeners.

Reported-by: Jan Stancek <jstancek at redhat.com>
Signed-off-by: Linus L?ssing <linus.luessing at web.de>
---
 net/bridge/br_multicast.c |   19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index fb0e36f..c77f073 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -1181,6 +1181,14 @@ static int br_ip4_multicast_query(struct net_bridge *br,
 			    IGMPV3_MRC(ih3->code) * (HZ / IGMP_TIMER_SCALE) : 1;
 	}
 
+	/* RFC2236+RFC3376 (IGMPv2+IGMPv3) require the multicast link layer
+	 * all-systems destination addresses (224.0.0.1) for general queries
+	 */
+	if (!group && iph->daddr != htonl(INADDR_ALLHOSTS_GROUP)) {
+		err = -EINVAL;
+		goto out;
+	}
+
 	br_multicast_query_received(br, port, &br->ip4_querier,
!!iph->saddr,
 				    max_delay);
 
@@ -1228,6 +1236,7 @@ static int br_ip6_multicast_query(struct net_bridge *br,
 	unsigned long max_delay;
 	unsigned long now = jiffies;
 	const struct in6_addr *group = NULL;
+	bool is_general_query;
 	int err = 0;
 
 	spin_lock(&br->multicast_lock);
@@ -1262,6 +1271,16 @@ static int br_ip6_multicast_query(struct net_bridge *br,
 		max_delay = max(msecs_to_jiffies(mldv2_mrc(mld2q)), 1UL);
 	}
 
+	is_general_query = group && ipv6_addr_any(group) ? true : false;
+
+	/* RFC2710+RFC3810 (MLDv1+MLDv2) require the multicast link layer
+	 * all-nodes destination address (ff02::1) for general queries
+	 */
+	if (is_general_query &&
!ipv6_addr_is_ll_all_nodes(&ip6h->daddr)) {
+		err = -EINVAL;
+		goto out;
+	}
+
 	br_multicast_query_received(br, port, &br->ip6_querier,
 				    !ipv6_addr_any(&ip6h->saddr), max_delay);
 
-- 
1.7.10.4

Without this check someone could easily create a denial of service
by injecting multicast-specific queries to enable the bridge
snooping part if no real querier issuing periodic general queries
is present on the link which would result in the bridge wrongly
shutting down ports for multicast traffic as the bridge did not learn
about these listeners.

With this patch the snooping code is enabled upon receiving valid,
general queries only.

Signed-off-by: Linus L?ssing <linus.luessing at web.de>
---
 net/bridge/br_multicast.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index c77f073..5dd4fec 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -1127,9 +1127,10 @@ static void br_multicast_query_received(struct net_bridge
*br,
 					struct net_bridge_port *port,
 					struct bridge_mcast_querier *querier,
 					int saddr,
+					bool is_general_query,
 					unsigned long max_delay)
 {
-	if (saddr)
+	if (saddr && is_general_query)
 		br_multicast_update_querier_timer(br, querier, max_delay);
 	else if (timer_pending(&querier->timer))
 		return;
@@ -1190,7 +1191,7 @@ static int br_ip4_multicast_query(struct net_bridge *br,
 	}
 
 	br_multicast_query_received(br, port, &br->ip4_querier,
!!iph->saddr,
-				    max_delay);
+				    !group, max_delay);
 
 	if (!group)
 		goto out;
@@ -1282,7 +1283,8 @@ static int br_ip6_multicast_query(struct net_bridge *br,
 	}
 
 	br_multicast_query_received(br, port, &br->ip6_querier,
-				    !ipv6_addr_any(&ip6h->saddr), max_delay);
+				    !ipv6_addr_any(&ip6h->saddr),
+				    is_general_query, max_delay);
 
 	if (!group)
 		goto out;
-- 
1.7.10.4

On Sun, Mar 9, 2014 at 8:39 PM, Linus L?ssing <linus.luessing at web.de>
wrote:
> +       is_general_query = group && ipv6_addr_any(group) ? true :
false;
> +
Just "is_general_query = group && ipv6_addr_any(group);"
should be okay.