linux-kernel - Re: [PATCH] netfilter: nf_conntrack: fix RCU race in nf_conntrack_find

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Date:	Wed, 29 Jan 2014 20:21:37 +0100
From:	Pablo Neira Ayuso <pablo@...filter.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Andrey Vagin <avagin@...nvz.org>, netfilter-devel@...r.kernel.org,
	netfilter@...r.kernel.org, coreteam@...filter.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	vvs@...nvz.org, Florian Westphal <fw@...len.de>,
	Patrick McHardy <kaber@...sh.net>,
	Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
	"David S. Miller" <davem@...emloft.net>,
	Cyrill Gorcunov <gorcunov@...nvz.org>
Subject: Re: [PATCH] netfilter: nf_conntrack: fix RCU race in
 nf_conntrack_find_get (v3)

On Sun, Jan 12, 2014 at 12:21:14PM -0800, Eric Dumazet wrote:
> On Sun, 2014-01-12 at 21:50 +0400, Andrey Vagin wrote:
> > Lets look at destroy_conntrack:
> > 
> > hlist_nulls_del_rcu(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode);
> > ...
> > nf_conntrack_free(ct)
> > 	kmem_cache_free(net->ct.nf_conntrack_cachep, ct);
> > 
> > net->ct.nf_conntrack_cachep is created with SLAB_DESTROY_BY_RCU.
> > 
> > The hash is protected by rcu, so readers look up conntracks without
> > locks.
> > A conntrack is removed from the hash, but in this moment a few readers
> > still can use the conntrack. Then this conntrack is released and another
> > thread creates conntrack with the same address and the equal tuple.
> > After this a reader starts to validate the conntrack:
> > * It's not dying, because a new conntrack was created
> > * nf_ct_tuple_equal() returns true.
> ...
> 
> 
> > v2: move nf_ct_is_confirmed into the unlikely() annotation
> > v3: Eric suggested to fix refcnt, so that it becomes zero before adding
> >     in a hash, but we can't find a way how to do that. Another way is to
> >     interpret the confirm bit as part of a search key and check it in
> >     ____nf_conntrack_find() too.
> > 
> > Cc: Eric Dumazet <eric.dumazet@...il.com>
> > Cc: Florian Westphal <fw@...len.de>
> > Cc: Pablo Neira Ayuso <pablo@...filter.org>
> > Cc: Patrick McHardy <kaber@...sh.net>
> > Cc: Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>
> > Cc: "David S. Miller" <davem@...emloft.net>
> > Cc: Cyrill Gorcunov <gorcunov@...nvz.org>
> > Signed-off-by: Andrey Vagin <avagin@...nvz.org>
> > ---
> 
> Acked-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks everyone!
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