[PATCH] xfrm:ignore big packets when tunnel mode

[mtk81216]

In tunnel mode, when inner interface is ipv4,outer interface is ipv6, flags
of tunnel mode's xfrm state is af-unspec, if a larger packet who is bigger
than mtu goes through tunnel interface, it enters ip6_fragment, goes to
fail_toobig, and ICMPV6(ICMPV6_PKT_TOOBIG) will be sent. It is unnecessary
to do so. Ip6_fragment will fragment such packet with outer interface's mtu
minus tunnelled esp header,it wonot be too big.

The same things happen, when a larger fragmented packet whose frag_max_size
is larger than mtu.

When a larger fragmented packet is forwarded, it also meets the same 
scenary.

This patch has handled three above scenaries, if it is tunnel mode,just 
ignore skb_len or frag_max_size, keep going.

Signed-off-by: mtk81216 <lina.wang@xxxxxxxxxxxx>
---
 net/ipv6/ip6_output.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index c78e67d7747f..0e1e6fcd7a5d 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -402,12 +402,14 @@ static inline int ip6_forward_finish(struct net *net, struct sock *sk,
 
 static bool ip6_pkt_too_big(const struct sk_buff *skb, unsigned int mtu)
 {
+	struct rt6_info *rt = (struct rt6_info *)skb_dst(skb);
 	if (skb->len <= mtu)
 		return false;
 
 	/* ipv6 conntrack defrag sets max_frag_size + ignore_df */
 	if (IP6CB(skb)->frag_max_size && IP6CB(skb)->frag_max_size > mtu)
-		return true;
+		if (rt && !(rt->dst.flags & DST_XFRM_TUNNEL))
+			return true;
 
 	if (skb->ignore_df)
 		return false;
@@ -787,16 +789,19 @@ int ip6_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 	 * or if the skb it not generated by a local socket.
 	 */
 	if (unlikely(!skb->ignore_df && skb->len > mtu))
-		goto fail_toobig;
-
-	if (IP6CB(skb)->frag_max_size) {
-		if (IP6CB(skb)->frag_max_size > mtu)
+		if (rt && (rt->dst.flags & DST_XFRM_TUNNEL))
 			goto fail_toobig;
 
-		/* don't send fragments larger than what we received */
-		mtu = IP6CB(skb)->frag_max_size;
-		if (mtu < IPV6_MIN_MTU)
-			mtu = IPV6_MIN_MTU;
+	if (IP6CB(skb)->frag_max_size) {
+		if (IP6CB(skb)->frag_max_size > mtu) {
+			if (rt && !(rt->dst.flags & DST_XFRM_TUNNEL))
+				goto fail_toobig;
+		} else {
+			/* don't send fragments larger than what we received */
+			mtu = IP6CB(skb)->frag_max_size;
+			if (mtu < IPV6_MIN_MTU)
+				mtu = IPV6_MIN_MTU;
+		}
 	}
 
 	if (np && np->frag_size < mtu) {
-- 
2.18.0