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

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Message-ID: <20180731155254.o2cnyx7walzu6chj@breakpoint.cc>
Date:   Tue, 31 Jul 2018 17:52:54 +0200
From:   Florian Westphal <fw@...len.de>
To:     Jann Horn <jannh@...gle.com>
Cc:     Florian Westphal <fw@...len.de>,
        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

Jann Horn <jannh@...gle.com> wrote:
> 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.

Agreed.  Objection withdrawn.

Acked-by: Florian Westphal <fw@...len.de>