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

From: Cyrill Gorcunov <[email-masked]>
Date: Mon, 6 Jan 2014 21:21:30 +0400

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?