Subject: Re: [PATCH] netfilter: nf_conntrack: fix RCU race in nf_conntrack_find_get

From: Andrew Vagin <[email-masked]>
Date: Fri, 10 Jan 2014 01:07:49 +0400

On Thu, Jan 09, 2014 at 09:56:22PM +0100, Florian Westphal wrote:
> Andrew Vagin <[email-masked]> 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?