Re: DDoS attack causing bad effect on conntrack searches

Eric Dumazet
2010-04-22T16:02:08Z

Le jeudi 22 avril 2010 à 08:51 -0700, Paul E. McKenney a écrit :
> On Thu, Apr 22, 2010 at 04:53:49PM +0200, Eric Dumazet wrote:
> > Le jeudi 22 avril 2010 à 16:36 +0200, Eric Dumazet a écrit :
> >
> > > If one hash slot is under attack, then there is a bug somewhere.
> > >
> > > If we cannot avoid this, we can fallback to a secure mode at the second
> > > retry, and take the spinlock.
> > >
> > > Tis way, most of lookups stay lockless (one pass), and some might take
> > > the slot lock to avoid the possibility of a loop.
> > >
> > > I suspect a bug elsewhere, quite frankly !
> > >
> > > We have a chain that have an end pointer that doesnt match the expected
> > > one.
> > >
> >
> > On normal situation, we always finish the lookup :
> >
> > 1) If we found the thing we were looking at.
> >
> > 2) We get the list end (item not found), we then check if it is the
> > expected end.
> >
> > It is _not_ the expected end only if some writer deleted/inserted an
> > element in _this_ chain during our lookup.
>
> So this situation uses SLAB_DESTROY_BY_RCU to quickly recycle deleted
> elements? (Not obvious from the code, but my ignorance of the networking
> code is such that many things in that part of the kernel are not obvious
> to me, I am afraid.)
>

Yes, this uses SLAB_DESTROY_BY_RCU, like tcp/udp lookups.

> Otherwise, of course you would simply allow deleted elements to continue
> pointing where they did previously, so that concurrent readers would not
> miss anything.
>

> Of course, the same potential might arise on insertion, but it is usually
> OK to miss an element that was inserted after you started searching.
>
> > Because our lookup is lockless, we then have to redo it because we might
> > miss the object we are looking for.
>
> Ah... Is there also a resize operation? Herbert did do a resizable
> hash table recently, but I was under the impression that (1) it was in
> some other part of the networking stack and (2) it avoided the need to
> restart readers.
>
> > If we can do the 'retry' a 10 times, it means the attacker was really
> > clever enough to inject new packets (new conntracks) at the right
> > moment, in the right hash chain, and this sounds so higly incredible
> > that I cannot believe it at all :)
>
> Or maybe the DoS attack is injecting so many new conntracks that a large
> fraction of the hash chains are being modified at any given time?
>
> Thanx, Paul

maybe hash table has one slot :)

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