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

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Date:   Tue, 31 Jul 2018 17:23:56 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Florian Westphal <fw@...len.de>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        eric.dumazet@...il.com, posk@...gle.com, pabeni@...hat.com
Subject: Re: [PATCH net] inet: frag: enforce memory limits earlier

On Tue, Jul 31, 2018 at 7:54 AM Florian Westphal <fw@...len.de> wrote:
>
> 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.

There is only a limited amount of memory available to store fragments.
If you receive lots of fragments that don't form complete packets,
you'll have to drop some packets. I don't see why it matters whether
incoming garbage only prevents the creation of new queues or also the
completion of existing queues.