xfrm: choose protocol family by skb protocol

We need to choose the protocol family by skb->protocol. Otherwise we
call the wrong xfrm{4,6}_local_error handler in case an ipv6 sockets is
used in ipv4 mode, in which case we should call down to xfrm4_local_error
(ip6 sockets are a superset of ip4 ones).

We are called before before ip_output functions, so skb->protocol is
not reset.

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

include/net/xfrm.h

@@ -1728,9 +1728,9 @@ static inline int xfrm_skb_dst_mtu(struct sk_buff *skb)
 {
 	struct sock *sk = skb->sk;
 
-	if (sk && sk->sk_family == AF_INET6)
+	if (sk && skb->protocol == htons(ETH_P_IPV6))
 		return ip6_skb_dst_mtu(skb);
-	else if (sk && sk->sk_family == AF_INET)
+	else if (sk && skb->protocol == htons(ETH_P_IP))
 		return ip_skb_dst_mtu(skb);
 	return dst_mtu(skb_dst(skb));
 }

net/xfrm/xfrm_output.c

@@ -216,9 +216,17 @@ int xfrm_inner_extract_output(struct xfrm_state *x, struct sk_buff *skb)
 
 void xfrm_local_error(struct sk_buff *skb, int mtu)
 {
+	unsigned int proto;
 	struct xfrm_state_afinfo *afinfo;
 
-	afinfo = xfrm_state_get_afinfo(skb->sk->sk_family);
+	if (skb->protocol == htons(ETH_P_IP))
+		proto = AF_INET;
+	else if (skb->protocol == htons(ETH_P_IPV6))
+		proto = AF_INET6;
+	else
+		return;
+
+	afinfo = xfrm_state_get_afinfo(proto);
 	if (!afinfo)
 		return;