bridge: simplify ip_mc_check_igmp() and ipv6_mc_check_mld() internals

With this patch the internal use of the skb_trimmed is reduced to
the ICMPv6/IGMP checksum verification. And for the length checks
the newly introduced helper functions are used instead of calculating
and checking with skb->len directly.

These changes should hopefully make it easier to verify that length
checks are performed properly.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/igmp.c

@@ -1493,22 +1493,22 @@ static int ip_mc_check_igmp_reportv3(struct sk_buff *skb)
 
 	len += sizeof(struct igmpv3_report);
 
-	return pskb_may_pull(skb, len) ? 0 : -EINVAL;
+	return ip_mc_may_pull(skb, len) ? 0 : -EINVAL;
 }
 
 static int ip_mc_check_igmp_query(struct sk_buff *skb)
 {
-	unsigned int len = skb_transport_offset(skb);
-
-	len += sizeof(struct igmphdr);
-	if (skb->len < len)
-		return -EINVAL;
+	unsigned int transport_len = ip_transport_len(skb);
+	unsigned int len;
 
 	/* IGMPv{1,2}? */
-	if (skb->len != len) {
+	if (transport_len != sizeof(struct igmphdr)) {
 		/* or IGMPv3? */
-		len += sizeof(struct igmpv3_query) - sizeof(struct igmphdr);
-		if (skb->len < len || !pskb_may_pull(skb, len))
+		if (transport_len < sizeof(struct igmpv3_query))
+			return -EINVAL;
+
+		len = skb_transport_offset(skb) + sizeof(struct igmpv3_query);
+		if (!ip_mc_may_pull(skb, len))
 			return -EINVAL;
 	}
 
@@ -1544,35 +1544,24 @@ static inline __sum16 ip_mc_validate_checksum(struct sk_buff *skb)
 	return skb_checksum_simple_validate(skb);
 }
 
-static int __ip_mc_check_igmp(struct sk_buff *skb)
-
+static int ip_mc_check_igmp_csum(struct sk_buff *skb)
 {
-	struct sk_buff *skb_chk;
-	unsigned int transport_len;
 	unsigned int len = skb_transport_offset(skb) + sizeof(struct igmphdr);
-	int ret = -EINVAL;
+	unsigned int transport_len = ip_transport_len(skb);
+	struct sk_buff *skb_chk;
 
-	transport_len = ntohs(ip_hdr(skb)->tot_len) - ip_hdrlen(skb);
+	if (!ip_mc_may_pull(skb, len))
+		return -EINVAL;
 
 	skb_chk = skb_checksum_trimmed(skb, transport_len,
 				       ip_mc_validate_checksum);
 	if (!skb_chk)
-		goto err;
+		return -EINVAL;
 
-	if (!pskb_may_pull(skb_chk, len))
-		goto err;
-
-	ret = ip_mc_check_igmp_msg(skb_chk);
-	if (ret)
-		goto err;
-
-	ret = 0;
-
-err:
-	if (skb_chk && skb_chk != skb)
+	if (skb_chk != skb)
 		kfree_skb(skb_chk);
 
-	return ret;
+	return 0;
 }
 
 /**
@@ -1600,7 +1589,11 @@ int ip_mc_check_igmp(struct sk_buff *skb)
 	if (ip_hdr(skb)->protocol != IPPROTO_IGMP)
 		return -ENOMSG;
 
-	return __ip_mc_check_igmp(skb);
+	ret = ip_mc_check_igmp_csum(skb);
+	if (ret < 0)
+		return ret;
+
+	return ip_mc_check_igmp_msg(skb);
 }
 EXPORT_SYMBOL(ip_mc_check_igmp);