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

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Date:	Thu, 22 Apr 2010 16:53:49 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Patrick McHardy <kaber@...sh.net>
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

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.

Because our lookup is lockless, we then have to redo it because we might
miss the object we are looking for.

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 :)



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