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

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Message-ID: <4C04E3E2.7020209@trash.net>
Date:	Tue, 01 Jun 2010 12:41:38 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Changli Gao <xiaosuo@...il.com>, hawk@...x.dk,
	Jesper Dangaard Brouer <hawk@...u.dk>,
	paulmck@...ux.vnet.ibm.com,
	Linux Kernel Network Hackers <netdev@...r.kernel.org>,
	Netfilter Developers <netfilter-devel@...r.kernel.org>
Subject: Re: DDoS attack causing bad effect on conntrack searches

Eric Dumazet wrote:
> Le mardi 01 juin 2010 à 12:18 +0200, Patrick McHardy a écrit :
> 
>> If a new conntrack is created in PRE_ROUTING or LOCAL_OUT, it will be
>> added to the unconfirmed list and moved to the hash as soon as the
>> packet passes POST_ROUTING. This means the number of unconfirmed entries
>> created by the network is bound by the number of CPUs due to BH
>> processing. The number created by locally generated packets is unbound
>> in case of preemptible kernels however.
>>
> 
> OK, we should have a percpu list then.

Yes, that makes sense.

> BTW, I notice nf_conntrack_untracked is incorrectly annotated
> '__read_mostly'.
> 
> It can be written very often :(
> 
> Should'nt we special case it and let be really const ?

That would need quite a bit of special-casing to avoid touching
the reference counts. So far this is completely hidden, so I'd
say it just shouldn't be marked __read_mostly. Alternatively we
can make "untracked" a nfctinfo state.
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