xfrm: introduce helper for safe determination of mtu

skb->sk socket can be of AF_INET or AF_INET6 address family. Thus we
always have to make sure we a referring to the correct interpretation
of skb->sk.

We only depend on header defines to query the mtu, so we don't introduce
a new dependency to ipv6 by this change.

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

include/net/route.h

@@ -317,4 +317,12 @@ static inline int ip4_dst_hoplimit(const struct dst_entry *dst)
 	return hoplimit;
 }
 
+static inline int ip_skb_dst_mtu(struct sk_buff *skb)
+{
+	struct inet_sock *inet = skb->sk ? inet_sk(skb->sk) : NULL;
+
+	return (inet && inet->pmtudisc == IP_PMTUDISC_PROBE) ?
+	       skb_dst(skb)->dev->mtu : dst_mtu(skb_dst(skb));
+}
+
 #endif	/* _ROUTE_H */

include/net/xfrm.h

@@ -20,6 +20,7 @@
 #include <net/route.h>
 #include <net/ipv6.h>
 #include <net/ip6_fib.h>
+#include <net/ip6_route.h>
 #include <net/flow.h>
 
 #include <linux/interrupt.h>
@@ -1723,4 +1724,15 @@ static inline int xfrm_mark_put(struct sk_buff *skb, const struct xfrm_mark *m)
 	return ret;
 }
 
+static inline int xfrm_skb_dst_mtu(struct sk_buff *skb)
+{
+	struct sock *sk = skb->sk;
+
+	if (sk && sk->sk_family == AF_INET6)
+		return ip6_skb_dst_mtu(skb);
+	else if (sk && sk->sk_family == AF_INET)
+		return ip_skb_dst_mtu(skb);
+	return dst_mtu(skb_dst(skb));
+}
+
 #endif	/* _NET_XFRM_H */

net/ipv4/ip_output.c

@@ -211,14 +211,6 @@ static inline int ip_finish_output2(struct sk_buff *skb)
 	return -EINVAL;
 }
 
-static inline int ip_skb_dst_mtu(struct sk_buff *skb)
-{
-	struct inet_sock *inet = skb->sk ? inet_sk(skb->sk) : NULL;
-
-	return (inet && inet->pmtudisc == IP_PMTUDISC_PROBE) ?
-	       skb_dst(skb)->dev->mtu : dst_mtu(skb_dst(skb));
-}
-
 static int ip_finish_output(struct sk_buff *skb)
 {
 #if defined(CONFIG_NETFILTER) && defined(CONFIG_XFRM)

net/ipv4/xfrm4_output.c

@@ -21,7 +21,6 @@
 static int xfrm4_tunnel_check_size(struct sk_buff *skb)
 {
 	int mtu, ret = 0;
-	struct dst_entry *dst;
 
 	if (IPCB(skb)->flags & IPSKB_XFRM_TUNNEL_SIZE)
 		goto out;
@@ -29,8 +28,7 @@ static int xfrm4_tunnel_check_size(struct sk_buff *skb)
 	if (!(ip_hdr(skb)->frag_off & htons(IP_DF)) || skb->local_df)
 		goto out;
 
-	dst = skb_dst(skb);
-	mtu = dst_mtu(dst);
+	mtu = xfrm_skb_dst_mtu(skb);
 	if (skb->len > mtu) {
 		if (skb->sk)
 			xfrm_local_error(skb, mtu);

net/ipv6/xfrm6_output.c

@@ -138,7 +138,10 @@ static int __xfrm6_output(struct sk_buff *skb)
 {
 	struct dst_entry *dst = skb_dst(skb);
 	struct xfrm_state *x = dst->xfrm;
-	int mtu = ip6_skb_dst_mtu(skb);
+	int mtu = xfrm_skb_dst_mtu(skb);
+
+	if (mtu < IPV6_MIN_MTU)
+		mtu = IPV6_MIN_MTU;
 
 	if (skb->len > mtu && xfrm6_local_dontfrag(skb)) {
 		xfrm6_local_rxpmtu(skb, mtu);