net: Implmement RFC 6936 (zero RX csums for UDP/IPv6)

RFC 6936 relaxes the requirement of RFC 2460 that UDP/IPv6 packets which
are received with a zero UDP checksum value must be dropped. RFC 6936
allows zero checksums to support tunnels over UDP.

When sk_no_check is set we allow on a socket we allow a zero IPv6
UDP checksum. This is for both sending zero checksum and accepting
a zero checksum on receive.
Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_checksum.c

@@ -75,16 +75,12 @@ int udp6_csum_init(struct sk_buff *skb, struct udphdr *uh, int proto)
 			return err;
 	}
 
-	if (uh->check == 0) {
-		/* RFC 2460 section 8.1 says that we SHOULD log
-		   this error. Well, it is reasonable.
+	/* To support RFC 6936 (allow zero checksum in UDP/IPV6 for tunnels)
+	 * we accept a checksum of zero here. When we find the socket
+	 * for the UDP packet we'll check if that socket allows zero checksum
+	 * for IPv6 (set by socket option).
 	 */
-		LIMIT_NETDEBUG(KERN_INFO "IPv6: udp checksum is 0 for [%pI6c]:%u->[%pI6c]:%u\n",
-			       &ipv6_hdr(skb)->saddr, ntohs(uh->source),
-			       &ipv6_hdr(skb)->daddr, ntohs(uh->dest));
-		return 1;
-	}
-
-	return skb_checksum_init(skb, IPPROTO_UDP, ip6_compute_pseudo);
+	return skb_checksum_init_zero_check(skb, proto, uh->check,
+					   ip6_compute_pseudo);
 }
 EXPORT_SYMBOL(udp6_csum_init);

net/ipv6/udp.c

@@ -760,6 +760,17 @@ static void flush_stack(struct sock **stack, unsigned int count,
 	if (unlikely(skb1))
 		kfree_skb(skb1);
 }
+
+static void udp6_csum_zero_error(struct sk_buff *skb)
+{
+	/* RFC 2460 section 8.1 says that we SHOULD log
+	 * this error. Well, it is reasonable.
+	 */
+	LIMIT_NETDEBUG(KERN_INFO "IPv6: udp checksum is 0 for [%pI6c]:%u->[%pI6c]:%u\n",
+		       &ipv6_hdr(skb)->saddr, ntohs(udp_hdr(skb)->source),
+		       &ipv6_hdr(skb)->daddr, ntohs(udp_hdr(skb)->dest));
+}
+
 /*
  * Note: called only from the BH handler context,
  * so we don't need to lock the hashes.
@@ -779,7 +790,12 @@ static int __udp6_lib_mcast_deliver(struct net *net, struct sk_buff *skb,
 	dif = inet6_iif(skb);
 	sk = udp_v6_mcast_next(net, sk, uh->dest, daddr, uh->source, saddr, dif);
 	while (sk) {
+		/* If zero checksum and sk_no_check is not on for
+		 * the socket then skip it.
+		 */
+		if (uh->check || sk->sk_no_check)
 			stack[count++] = sk;
+
 		sk = udp_v6_mcast_next(net, sk_nulls_next(sk), uh->dest, daddr,
 				       uh->source, saddr, dif);
 		if (unlikely(count == ARRAY_SIZE(stack))) {
@@ -867,6 +883,11 @@ int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 	if (sk != NULL) {
 		int ret;
 
+		if (!uh->check && !sk->sk_no_check) {
+			udp6_csum_zero_error(skb);
+			goto csum_error;
+		}
+
 		ret = udpv6_queue_rcv_skb(sk, skb);
 		sock_put(sk);
 
@@ -879,6 +900,11 @@ int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 		return 0;
 	}
 
+	if (!uh->check) {
+		udp6_csum_zero_error(skb);
+		goto csum_error;
+	}
+
 	if (!xfrm6_policy_check(NULL, XFRM_POLICY_IN, skb))
 		goto discard;
 
@@ -1006,7 +1032,10 @@ static int udp_v6_push_pending_frames(struct sock *sk)
 
 	if (is_udplite)
 		csum = udplite_csum_outgoing(sk, skb);
-	else if (skb->ip_summed == CHECKSUM_PARTIAL) { /* UDP hardware csum */
+	else if (sk->sk_no_check == UDP_CSUM_NOXMIT) {   /* UDP csum disabled */
+		skb->ip_summed = CHECKSUM_NONE;
+		goto send;
+	} else if (skb->ip_summed == CHECKSUM_PARTIAL) { /* UDP hardware csum */
 		udp6_hwcsum_outgoing(sk, skb, &fl6->saddr, &fl6->daddr,
 				     up->len);
 		goto send;