Subject: Re: [PATCH] netfilter: nf_conntrack: release conntrack from rcu callback

From: Florian Westphal <[email-masked]>
Date: Mon, 6 Jan 2014 22:23:26 +0100

Cyrill Gorcunov <[email-masked]> wrote:
> On Mon, Jan 06, 2014 at 06:02:35PM +0100, Florian Westphal wrote:
> > 
> > Can you elaborate?
> > Yes, nf_ct_is_dying(ct) might be called for the wrong conntrack.
> > 
> > But, in case we _think_ that its the right one we call
> > nf_ct_tuple_equal() to verify we indeed found the right one:
> > 
> >        h = ____nf_conntrack_find(net, zone, tuple, hash);
> >        if (h) { // might be released right now, but page won't go away (SLAB_BY_RCU)
> >                 ct = nf_ct_tuplehash_to_ctrack(h);
> >                 if (unlikely(nf_ct_is_dying(ct) ||
> >                              !atomic_inc_not_zero(&ct->ct_general.use)))
> > 			// which means we should hit this path (0 ref).
> >                         h = NULL;
> >                 else {
> > 			// otherwise, it cannot go away from under us, since
> > 			// we own a reference now.
> >                         if (unlikely(!nf_ct_tuple_equal(tuple, &h->tuple) ||
> >                                      nf_ct_zone(ct) != zone)) {
> > 			// if we get here, the entry got recycled on other cpu
> > 			// for a different tuple, we can bail out and drop
> > 			// the reference safely and re-try the lookup
> >                                 nf_ct_put(ct);
> >                                 goto begin;
> >                         }
> >                 }
> 
> I think tuple may match if
> 
> task 1                  task 2                  task 3
>                         nf_conntrack_find_get
>                          ____nf_conntrack_find
> destroy_conntrack
>  hlist_nulls_del_rcu
>  nf_conntrack_free
>  kmem_cache_free
>                                                 __nf_conntrack_alloc
>                                                  kmem_cache_alloc
>                          if (nf_ct_is_dying(ct))
> 
> 						data is not yet cleaned
> 
>                                                  memset(&ct->tuplehash[IP_CT_DIR_MAX],
> 
> No? Or there something obvious I'm missing?

IMHO this isn't obvious at all :-)

But, in the example above, the atomic_inc_not_zero() should fail
until after __nf_conntrack_alloc() re-inits the refcount to 1.

The mb there should make sure ____nf_conntrack_find() doesn't
find an outdated tuple before this.