From: Florian Westphal <[email protected]>
Date: Fri, 22 May 2015 00:44:16 +0200
> 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.
>
> 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 <[email protected]>
> Signed-off-by: Florian Westphal <[email protected]>
Applied to net-next, thanks.
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