netfilter: restart search if moved to other chain

commit 95a8d19f upstream.

In case nf_conntrack_tuple_taken did not find a conflicting entry
check that all entries in this hash slot were tested and restart
in case an entry was moved to another chain.
Reported-by: Eric Dumazet <edumazet@google.com>
Fixes: ea781f19 ("netfilter: nf_conntrack: use SLAB_DESTROY_BY_RCU and get rid of call_rcu()")
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
[bwh: Backported to 3.16:
 - Adjust context
 - Use NF_CT_STAT_INC(), not the _ATOMIC variant, since we disable BHs]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/netfilter/nf_conntrack_core.c

@@ -726,6 +726,7 @@ nf_conntrack_tuple_taken(const struct nf_conntrack_tuple *tuple,
 	 * least once for the stats anyway.
 	 */
 	rcu_read_lock_bh();
+ begin:
 	hlist_nulls_for_each_entry_rcu(h, n, &net->ct.hash[hash], hnnode) {
 		ct = nf_ct_tuplehash_to_ctrack(h);
 		if (ct != ignored_conntrack &&
@@ -737,6 +738,12 @@ nf_conntrack_tuple_taken(const struct nf_conntrack_tuple *tuple,
 		}
 		NF_CT_STAT_INC(net, searched);
 	}
+
+	if (get_nulls_value(n) != hash) {
+		NF_CT_STAT_INC(net, search_restart);
+		goto begin;
+	}
+
 	rcu_read_unlock_bh();
 
 	return 0;