netdev - Re: [PATCH 7/9] rhashtable: Per bucket locks & deferred expansion/shrinking

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Date:	Sat, 17 Jan 2015 09:51:46 +0000
From:	Patrick McHardy <kaber@...sh.net>
To:	Herbert Xu <herbert@...dor.apana.org.au>
CC:	Thomas Graf <tgraf@...g.ch>,
	David Laight <David.Laight@...LAB.COM>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"john.r.fastabend@...el.com" <john.r.fastabend@...el.com>,
	"josh@...htriplett.org" <josh@...htriplett.org>,
	"netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>
Subject: Re: [PATCH 7/9] rhashtable: Per bucket locks & deferred expansion/shrinking

Am 17. Januar 2015 09:32:28 GMT+00:00, schrieb Herbert Xu <herbert@...dor.apana.org.au>:
>On Sat, Jan 17, 2015 at 08:06:21AM +0000, Patrick McHardy wrote:
>> 
>> Resizing might also fail because of memory allocation problems, but
>> I'd argue that its better to continue with a non-optimal sized table
>> and retry later than to completely fail, at least unless the API
>> user has explicitly requested this behaviour.
>> 
>> As for the element counter, yeah, it should prevent overflow. In that
>> case I agree that failing insertion is the easiest solution.
>
>Well you have to consider the security aspect.  These days root-
>only is no longer an acceptable excuse given things like namespaces.
>
>If you don't fail the insertions while the expansion is ongoing,
>and assuming a dump can postpone expansions, then you can essentially
>insert entries into the hash table at will which is an easy DoS
>attack.

I agree, however at least in the case of nftables you can easily do the same thing by adding millions of rules.

It doesn't make things worse.

>Note that you don't have to fail the insertion right away.  I
>think waiting until you reach max * 2 would be fine.



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