net: ipv6: mld: implement RFC3810 MLDv2 mode only

RFC3810, 10. Security Considerations says under subsection 10.1.
Query Message:

  A forged Version 1 Query message will put MLDv2 listeners on that
  link in MLDv1 Host Compatibility Mode. This scenario can be avoided
  by providing MLDv2 hosts with a configuration option to ignore
  Version 1 messages completely.

Hence, implement a MLDv2-only mode that will ignore MLDv1 traffic:

  echo 2 > /proc/sys/net/ipv6/conf/ethX/force_mld_version  or
  echo 2 > /proc/sys/net/ipv6/conf/all/force_mld_version

Note that <all> device has a higher precedence as it was previously
also the case in the macro MLD_V1_SEEN() that would "short-circuit"
if condition on <all> case.
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/mcast.c

@@ -1112,11 +1112,34 @@ static bool mld_marksources(struct ifmcaddr6 *pmc, int nsrcs,
 	return true;
 }
 
+static int mld_force_mld_version(const struct inet6_dev *idev)
+{
+	/* Normally, both are 0 here. If enforcement to a particular is
+	 * being used, individual device enforcement will have a lower
+	 * precedence over 'all' device (.../conf/all/force_mld_version).
+	 */
+
+	if (dev_net(idev->dev)->ipv6.devconf_all->force_mld_version != 0)
+		return dev_net(idev->dev)->ipv6.devconf_all->force_mld_version;
+	else
+		return idev->cnf.force_mld_version;
+}
+
+static bool mld_in_v2_mode_only(const struct inet6_dev *idev)
+{
+	return mld_force_mld_version(idev) == 2;
+}
+
+static bool mld_in_v1_mode_only(const struct inet6_dev *idev)
+{
+	return mld_force_mld_version(idev) == 1;
+}
+
 static bool mld_in_v1_mode(const struct inet6_dev *idev)
 {
-	if (dev_net(idev->dev)->ipv6.devconf_all->force_mld_version == 1)
-		return true;
-	if (idev->cnf.force_mld_version == 1)
+	if (mld_in_v2_mode_only(idev))
+		return false;
+	if (mld_in_v1_mode_only(idev))
 		return true;
 	if (idev->mc_v1_seen && time_before(jiffies, idev->mc_v1_seen))
 		return true;
@@ -1223,7 +1246,6 @@ int igmp6_event_query(struct sk_buff *skb)
 		return -EINVAL;
 
 	idev = __in6_dev_get(skb->dev);
-
 	if (idev == NULL)
 		return 0;
 
@@ -1236,6 +1258,10 @@ int igmp6_event_query(struct sk_buff *skb)
 		return -EINVAL;
 
 	if (len == MLD_V1_QUERY_LEN) {
+		/* Ignore v1 queries */
+		if (mld_in_v2_mode_only(idev))
+			return 0;
+
 		/* MLDv1 router present */
 		max_delay = msecs_to_jiffies(ntohs(mld->mld_maxdelay));