netdev - Re: [PATCH v3 -next] ipv6: don't increase size when refragmenting forwarded ipv6 skbs

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Message-Id: <20150525.172246.451341177696280298.davem@davemloft.net>
Date:	Mon, 25 May 2015 17:22:46 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	fw@...len.de
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH v3 -next] ipv6: don't increase size when refragmenting
 forwarded ipv6 skbs

From: Florian Westphal <fw@...len.de>
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 <hannes@...essinduktion.org>
> Signed-off-by: Florian Westphal <fw@...len.de>

Applied to net-next, thanks.
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