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

From: Florian Westphal <[email-masked]>
Date: Mon, 6 Jan 2014 18:02:35 +0100

Andrey Vagin <[email-masked]> wrote:
> Lets look at destroy_conntrack:
> 
> hlist_nulls_del_rcu(&ct->tuplehash[IP_CT_DIR_ORIGINAL].hnnode);
> ...
> nf_conntrack_free(ct)
> 	kmem_cache_free(net->ct.nf_conntrack_cachep, ct);
> 
> The hash is protected by rcu, so readers look up conntracks without
> locks.
> A conntrack is removed from the hash, but in this moment a few readers
> still can use the conntrack, so if we call kmem_cache_free now, all
> readers will read released object.
> 
> Bellow you can find more tricky race condition of three tasks.
> 
> 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
> 						 memset(&ct->tuplehash[IP_CT_DIR_MAX],
> 			 if (nf_ct_is_dying(ct))
> 
> In this case the task 2 will not understand, that it uses a wrong
> conntrack.

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;
                        }
                }