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

[Florian Westphal <fw@strlen.de>]

Jann Horn <jannh@google.com> wrote:
> On Tue, Jul 31, 2018 at 7:54 AM Florian Westphal <fw@strlen.de> wrote:
> >
> > Eric Dumazet <edumazet@google.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  Objection withdrawn.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@strlen.de>