netdev - Re: [PATCH net] inet: frag: enforce memory limits earlier

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Date:   Tue, 31 Jul 2018 07:54:38 +0200
From:   Florian Westphal <fw@...len.de>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Jann Horn <jannh@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Florian Westphal <fw@...len.de>,
        Peter Oskolkov <posk@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net] inet: frag: enforce memory limits earlier

Eric Dumazet <edumazet@...gle.com> wrote:
> We currently check current frags memory usage only when
> a new frag queue is created. This allows attackers to first
> consume the memory budget (default : 4 MB) creating thousands
> of frag queues, then sending tiny skbs to exceed high_thresh
> limit by 2 to 3 order of magnitude.
> 
> Note that before commit 648700f76b03 ("inet: frags: use rhashtables
> for reassembly units"), work queue could be starved under DOS,
> getting no cpu cycles.
> After commit 648700f76b03, only the per frag queue timer can eventually
> remove an incomplete frag queue and its skbs.

I'm not sure this is a good idea.

This can now prevent "good" queue from completing just because attacker
is sending garbage.