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

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Date:	Fri, 10 Jan 2014 01:07:49 +0400
From:	Andrew Vagin <avagin@...allels.com>
To:	Florian Westphal <fw@...len.de>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	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>,
	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

On Thu, Jan 09, 2014 at 09:56:22PM +0100, Florian Westphal wrote:
> Andrew Vagin <avagin@...allels.com> wrote:
> > Can we allocate conntrack with zero ct_general.use and increment it at
> > the first time before inserting the conntrack into the hash table?
> > When conntrack is allocated it is attached exclusively to one skb.
> > It must be destroyed with skb, if it has not been confirmed, so we
> > don't need refcnt on this stage.
> > 
> > I found only one place, where a reference counter of unconfirmed
> > conntract can incremented. It's ctnetlink_dump_table().
> 
> What about skb_clone, etc?  They will also increment the refcnt
> if a conntrack entry is attached to the skb.

We can not attach an unconfirmed conntrack to a few skb, because
nf_nat_setup_info can be executed concurrently for the same conntrack.

How do we avoid this race condition for cloned skb-s?
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