netfilter: conntrack: fix dropping packet after l4proto->packet()

We currently use the negative value in the conntrack code to encode
the packet verdict in the error. As NF_DROP is equal to 0, inverting
NF_DROP makes no sense and, as a result, no packets are ever dropped.
Signed-off-by: Christoph Paasch <christoph.paasch@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/netfilter/nf_conntrack_core.c

@@ -726,7 +726,7 @@ nf_conntrack_in(struct net *net, u_int8_t pf, unsigned int hooknum,
 	NF_CT_ASSERT(skb->nfct);
 
 	ret = l4proto->packet(ct, skb, dataoff, ctinfo, pf, hooknum);
-	if (ret < 0) {
+	if (ret <= 0) {
 		/* Invalid: inverse of the return code tells
 		 * the netfilter core what to do */
 		pr_debug("nf_conntrack_in: Can't track with proto module\n");

net/netfilter/nf_conntrack_proto_tcp.c

@@ -859,7 +859,7 @@ static int tcp_packet(struct nf_conn *ct,
 			 */
 			if (nf_ct_kill(ct))
 				return -NF_REPEAT;
-			return -NF_DROP;
+			return NF_DROP;
 		}
 		/* Fall through */
 	case TCP_CONNTRACK_IGNORE:
@@ -892,7 +892,7 @@ static int tcp_packet(struct nf_conn *ct,
 				nf_log_packet(pf, 0, skb, NULL, NULL, NULL,
 					  "nf_ct_tcp: killing out of sync session ");
 			nf_ct_kill(ct);
-			return -NF_DROP;
+			return NF_DROP;
 		}
 		ct->proto.tcp.last_index = index;
 		ct->proto.tcp.last_dir = dir;