ipv6: don't increase size when refragmenting forwarded ipv6 skbs (485fca66) · Commits · 戴 / test

since commit ("netfilter: push reasm skb through instead of original frag skbs") we will end up sometimes re-fragmenting skbs that we've reassembled. ipv6 defrag preserves the original skbs using the skb frag list, i.e. as long as the skb frag list is preserved there is no problem since we keep original geometry of fragments intact. However, in the rare case where the frag list is munged or skb is linearized, we might send larger fragments than what we originally received. A router in the path might then 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 1 - A sends to B, fragment size 1400 2 - R2 sends pkttoobig error for 1280 3 - A sends to B, fragment size 1280 4 - R2 sends pkttoobig error for 1280 again because it sees fragments of size 1400. make sure ip6_fragment always caps MTU at largest packet size seen when defragmented skb is forwarded. Acked-by:

Hannes Frederic Sowa <hannes@stressinduktion.org> Signed-off-by:

Florian Westphal <fw@strlen.de> Signed-off-by:

David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

+18 −11

Original line number	Diff line number	Diff line
		@@ -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 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) {
		@@ -813,6 +812,14 @@ 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);

Admin message