netdev - [PATCH -next 2/3] ipv6: don't increase size when refragmenting forwarded skbs

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Date:	Sat, 11 Apr 2015 00:16:28 +0200
From:	Florian Westphal <fw@...len.de>
To:	<netdev@...r.kernel.org>
Cc:	kaber@...sh.net, Florian Westphal <fw@...len.de>
Subject: [PATCH -next 2/3] ipv6: don't increase size when refragmenting forwarded skbs

since commit 6aafeef03b9d ("netfilter: push reasm skb through instead of
original frag skbs") we will end up sometimes re-fragmenting skbs
that we've reassembled.

In this case, we must not use the device mtu, we might increase the
fragment size, possibly breaking connectivity as later router
might send packet-too-big errors even if sender never sent fragments
exceeding the reported mtu:

mtu 1500 - 1500:1400 - 1400:1280 - 1280
     A         R1         R2        B

- A sends to B, fragment size 1500
- R1 sends pkttoobig error for 1400
- A sends to B, fragment size 1400
- R2 sends pkttoobig error for 1280
- A sends to B, fragment size 1280
- R2 sends pkttoobig error for 1280 again because it sees
fragments of size 1400.

This doesn't occur in practice because we currently use routing
information to fetch the mtu.

Acked-by: Hannes Frederic Sowa <hannes@...essinduktion.org>
Signed-off-by: Florian Westphal <fw@...len.de>
---
 net/ipv6/ip6_output.c | 29 ++++++++++++++++++-----------
 1 file changed, 18 insertions(+), 11 deletions(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 7fde1f2..604c99d 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -564,18 +564,17 @@ int ip6_fragment(struct sock *sk, struct sk_buff *skb,
 	/* We must not fragment if the socket is set to force MTU discovery
 	 * or if the skb it not generated by a local socket.
 	 */
-	if (unlikely(!skb->ignore_df && skb->len > mtu) ||
-		     (IP6CB(skb)->frag_max_size &&
-		      IP6CB(skb)->frag_max_size > mtu)) {
-		if (skb->sk && dst_allfrag(skb_dst(skb)))
-			sk_nocaps_add(skb->sk, NETIF_F_GSO_MASK);
+	if (unlikely(!skb->ignore_df && skb->len > mtu))
+		goto fail_toobig;
 
-		skb->dev = skb_dst(skb)->dev;
-		icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
-		IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
-			      IPSTATS_MIB_FRAGFAILS);
-		kfree_skb(skb);
-		return -EMSGSIZE;
+	if (IP6CB(skb)->frag_max_size) {
+		if (IP6CB(skb)->frag_max_size > mtu)
+			goto fail_toobig;
+
+		/* don't send larger fragments 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) {
@@ -815,6 +814,14 @@ slow_path:
 	consume_skb(skb);
 	return err;
 
+fail_toobig:
+	if (skb->sk && dst_allfrag(skb_dst(skb)))
+		sk_nocaps_add(skb->sk, NETIF_F_GSO_MASK);
+
+	skb->dev = skb_dst(skb)->dev;
+	icmpv6_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
+	err = -EMSGSIZE;
+
 fail:
 	IP6_INC_STATS(net, ip6_dst_idev(skb_dst(skb)),
 		      IPSTATS_MIB_FRAGFAILS);
-- 
2.0.5

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