bridge: Sanitize IFLA_EXT_MASK for AF_BRIDGE:RTM_GETLINK

Only search for IFLA_EXT_MASK if the message actually carries a
ifinfomsg header and validate minimal length requirements for
IFLA_EXT_MASK.

Fixes: 6cbdceeb ("bridge: Dump vlan information from a bridge port")
Cc: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -2685,13 +2685,20 @@ static int rtnl_bridge_getlink(struct sk_buff *skb, struct netlink_callback *cb)
 	int idx = 0;
 	u32 portid = NETLINK_CB(cb->skb).portid;
 	u32 seq = cb->nlh->nlmsg_seq;
-	struct nlattr *extfilt;
 	u32 filter_mask = 0;
 
+	if (nlmsg_len(cb->nlh) > sizeof(struct ifinfomsg)) {
+		struct nlattr *extfilt;
+
 		extfilt = nlmsg_find_attr(cb->nlh, sizeof(struct ifinfomsg),
 					  IFLA_EXT_MASK);
-	if (extfilt)
+		if (extfilt) {
+			if (nla_len(extfilt) < sizeof(filter_mask))
+				return -EINVAL;
+
 			filter_mask = nla_get_u32(extfilt);
+		}
+	}
 
 	rcu_read_lock();
 	for_each_netdev_rcu(net, dev) {