Re: DDoS attack causing bad effect on conntrack searches

Changli Gao
2010-06-01T05:48:44Z

On Tue, Jun 1, 2010 at 1:05 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> Le mardi 01 juin 2010 à 08:28 +0800, Changli Gao a écrit :
>> On Tue, Jun 1, 2010 at 5:21 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
>> >
>> > I had a look at current conntrack and found the 'unconfirmed' list was
>> > maybe a candidate for a potential blackhole.
>> >
>> > That is, if a reader happens to hit an entry that is moved from regular
>> > hash table slot 'hash' to unconfirmed list,
>>
>> Sorry, but I can't find where we do this things. unconfirmed list is
>> used to track the unconfirmed cts, whose corresponding skbs are still
>> in path from the first to the last netfilter hooks. As soon as the
>> skbs end their travel in netfilter, the corresponding cts will be
>> confirmed(moving ct from unconfirmed list to regular hash table).
>>
>
> So netfilter is a monolithic thing.
>
> When a packet begins its travel into netfilter, you guarantee that no
> other packet can also begin its travel and find an unconfirmed
> conntrack ?
>
> I wonder why we use atomic ops then to track the confirmed bit :)

seems no need.

>
>
>> unconfirmed list should be small, as networking receiving is in BH.
>
> So according to you, netfilter/ct runs only in input path ?

No. there are another entrances: local out and nf_reinject. If there
isn't any packet queued, as netfilter is in atomic context, the nubmer
of unconfirmed cts should be small( at most, 2 * nr_cpu?).

>
> So I assume a packet is handled by CPU X, creates a new conntrack
> (possibly early droping an old entry that was previously in a standard
> hash chain), inserted in unconfirmed list.
>

Oh, Thanks, I got it.

> _You_ guarantee another CPU
> Y, handling another packet, possibly sent by a hacker reading your
> netdev mails, cannot find the conntrack that was early dropped ?
>
>> How about implementing unconfirmed list as a per cpu variable?
>
> I first implemented such a patch to reduce cache line contention, then I
> asked to myself : What is exactly an unconfirmed conntrack ? Can their
> number be unbounded ? If yes, we have a problem, even on a two cpus
> machine. Using two lists instead of one wont solve the fundamental
> problem.
>
> The real question is, why do we need this unconfirmed 'list' in the
> first place. Is it really a private per cpu thing ?

No, it isn't really private. But most of time, it is accessed locally,
if it is implemented as a per cpu var.

> Can you prove this,
> in respect of lockless lookups, and things like NFQUEUE ?
>
> Each conntrack object has two list anchors. One for IP_CT_DIR_ORIGINAL,
> one for IP_CT_DIR_REPLY.
>
> Unconfirmed list use the first anchor. This means another cpu can
> definitely find an unconfirmed item in a regular hash chain, since we
> dont respect an RCU grace period before re-using an object.
>
> If memory was not a problem, we probably would use a third anchor to
> avoid this, or regular RCU instead of SLAB_DESTROY_BY_RCU variant.
>

thanks for your explaining.

--
Regards,
Changli Gao(xiaosuo@gmail.com)