netdev - Re: DDoS attack causing bad effect on conntrack searches

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Date:	Fri, 23 Apr 2010 12:56:57 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Changli Gao <xiaosuo@...il.com>, hawk@...x.dk,
	Linux Kernel Network Hackers <netdev@...r.kernel.org>,
	netfilter-devel@...r.kernel.org,
	Paul E McKenney <paulmck@...ux.vnet.ibm.com>
Subject: Re: DDoS attack causing bad effect on conntrack searches

Eric Dumazet wrote:
> Le jeudi 22 avril 2010 à 15:17 +0200, Patrick McHardy a écrit :
>> Changli Gao wrote:
>>>> struct nf_conntrack_tuple_hash *
>>>> __nf_conntrack_find(struct net *net, const struct nf_conntrack_tuple *tuple)
>>>> ...
>>> We should add a retry limit there.
>> We can't do that since that would allow false negatives.
> 
> 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.

That sounds like a good idea. But lets what for Jesper's test results
before we start fixing this problem :)
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