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

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Date:	Sun, 12 Jan 2014 12:21:14 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andrey Vagin <avagin@...nvz.org>
Cc:	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>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	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, 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>

Thanks Andrey !


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