netdev - Re: [PATCH net-next 1/2] ipv4: add forwarding_uses_pmtu knob to protect forward path to use pmtu info

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Date:	Tue, 31 Dec 2013 05:28:40 +0100
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	steffen.klassert@...unet.com
Cc:	netdev@...r.kernel.org, eric.dumazet@...il.com, davem@...emloft.net
Subject: Re: [PATCH net-next 1/2] ipv4: add forwarding_uses_pmtu knob to protect forward path to use pmtu info

Hi Steffen!

On Fri, Dec 20, 2013 at 02:08:22PM +0100, Hannes Frederic Sowa wrote:
> Provide a mode where the forwarding path does not use the protocol path
> MTU to calculate the maximum size for a forwarded packet but instead
> uses the interface or the per-route locked MTU.
> 
> It is easy to inject bogus or malicious path mtu information which
> will cause either unneeded fragmentation-needed icmp errors (in case
> of DF-bit set) or unnecessary fragmentation of packets (by default down
> to min_pmtu). This could be used to either create blackholes on routers
> (if the generated DF-bit gets dropped later on) or to leverage attacks
> on fragmentation.
> 
> Forwarded skbs are marked with IPSKB_FORWARDED in ip_forward. This flag
> was introduced for multicast forwarding, but as it does not conflict with
> our usage in the unicast code path it is perfect for reuse.
> 
> I moved the functions ip_sk_accept_pmtu, ip_sk_use_pmtu and ip_skb_dst_mtu
> along with the new ip_dst_mtu_secure to net/ip.h to fix circular
> dependencies because of IPSKB_FORWARDED.

IIRC you have a (semi-)automatic test suite to test for (p)mtu problems? Would
these checks cover such a change?

Maybe your test suite is publicly available so I could test these changes
myself or otherwise could you give them a testdrive? That would be very
helpful.

Thank you,

  Hannes


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