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

From: Florian Westphal <[email-masked]>
Date: Tue, 7 Jan 2014 16:25:20 +0100

Eric Dumazet <[email-masked]> wrote:
> > diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> > index 43549eb..7a34bb2 100644
> > --- a/net/netfilter/nf_conntrack_core.c
> > +++ b/net/netfilter/nf_conntrack_core.c
> > @@ -387,8 +387,12 @@ begin:
> >  			     !atomic_inc_not_zero(&ct->ct_general.use)))
> >  			h = NULL;
> >  		else {
> > +			/* A conntrack can be recreated with the equal tuple,
> > +			 * so we need to check that the conntrack is initialized
> > +			 */
> >  			if (unlikely(!nf_ct_tuple_equal(tuple, &h->tuple) ||
> > -				     nf_ct_zone(ct) != zone)) {
> > +				     nf_ct_zone(ct) != zone) ||
> > +				     !nf_ct_is_confirmed(ct)) {
> >  				nf_ct_put(ct);
> >  				goto begin;
> >  			}
> 
> I do not think this is the right way to fix this problem (if said
> problem is confirmed)
> 
> Remember the rule about SLAB_DESTROY_BY_RCU :
> 
> When a struct is freed, then reused, its important to set the its refcnt
> (from 0 to 1) only when the structure is fully ready for use.
> 
> If a lookup finds a structure which is not yet setup, the
> atomic_inc_not_zero() will fail.

Indeed.  But, the structure itself might be ready (or rather,
can be ready since the allocation side will set the refcount to one
after doing the initial work, such as zapping old ->status flags and
setting tuple information).

The problem is with nat extension area stored in the ct->ext area.
This extension area is preallocated but the snat/dnat action
information is only set up after the ct (or rather, the skb that grabbed
a reference to the nf_conn entry) traverses nat pre/postrouting.

This will also set up a null-binding when no matching SNAT/DNAT/MASQERUADE
rule existed.

The manipulations of the skb->nfct->ext nat area are performed without
a lock.  Concurrent access is supposedly impossible as the conntrack
should not (yet) be in the hash table.

The confirmed bit is set right before we insert the conntrack into
the hash table (after we traversed rules, ct is ready to be
'published').

i.e. when the confirmed bit is NOT set we should not be 'seeing' the nf_conn
struct when we perform the lookup, as it should still be sitting on the
'unconfirmed' list, being invisible to readers.

Does that explanation make sense to you?

Thanks for looking into this.