netdev - Re: [PATCH v3 23/30] ipv6: defrag: drop non-last frags smaller than min mtu

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Date:   Fri, 11 Jan 2019 15:21:25 +0100
From:   Michal Kubecek <mkubecek@...e.cz>
To:     netdev <netdev@...r.kernel.org>
Cc:     Timothy Winters <twinters@....unh.edu>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Florian Westphal <fw@...len.de>,
        Tom Herbert <tom@...bertland.com>,
        David Miller <davem@...emloft.net>,
        Peter Oskolkov <posk@...gle.com>
Subject: Re: [PATCH v3 23/30] ipv6: defrag: drop non-last frags smaller than min mtu

On Friday, 11 January 2019 14:26 Timothy Winters wrote:
> Hi Eric,
> 
> So I understand correctly the attack that you are trying to prevent is
> many small fragments from different IPs?
> 
> The 6MAN working group has had some discussion about this topic, if
> you want read some IPv6 networking prospectives.
> 
> https://mailarchive.ietf.org/arch/browse/ipv6/?gbt=1&index=zsYUwQTt0mA
> dXdk9MN4dP-uiiag

I haven't read all mails in that discussion but most seem to be missing 
the point. The problem is not memory consumption, we have a (runtime 
configurable) limit for that. The problem is that by sending many small 
(8 bytes) fragments of a large (up to 64 KB) packet but never finishing 
it, an attacker can force receiving host into using quite a lot of CPU 
time just by looking up the fragment queue of the partially reassembled 
packet.

Currently, IPv6 reassembly uses a simple linear list which is fine with 
1280 byte long fragments (up to ~50 of them) but not with 8 byte long 
ones (there could be as many as ~8000). The IPv4 reassembly code (where 
we cannot assume minimal size of non-last fragment) switched to rbtree 
(with logarithmic lookup time) exactly for this reason.

> What about lowering the value of accepted fragments?  to something
> like 1280/2?

That would be probably sufficient to mitigate the DoS attacks.

Michal Kubecek