netfilter: nf_conntrack: fix ct refcount leak in l4proto->error()

This patch fixes a refcount leak of ct objects that may occur if
l4proto->error() assigns one conntrack object to one skbuff. In
that case, we have to skip further processing in nf_conntrack_in().

With this patch, we can also fix wrong return values (-NF_ACCEPT)
for special cases in ICMP[v6] that should not bump the invalid/error
statistic counters.
Reported-by: Zoltan Menyhart <Zoltan.Menyhart@bull.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/ipv4/netfilter/nf_conntrack_proto_icmp.c

@@ -160,7 +160,7 @@ icmp_error_message(struct net *net, struct nf_conn *tmpl, struct sk_buff *skb,
 	/* Update skb to refer to this connection */
 	skb->nfct = &nf_ct_tuplehash_to_ctrack(h)->ct_general;
 	skb->nfctinfo = *ctinfo;
-	return -NF_ACCEPT;
+	return NF_ACCEPT;
 }
 
 /* Small and modified version of icmp_rcv */

net/ipv6/netfilter/nf_conntrack_proto_icmpv6.c

@@ -177,7 +177,7 @@ icmpv6_error_message(struct net *net, struct nf_conn *tmpl,
 	/* Update skb to refer to this connection */
 	skb->nfct = &nf_ct_tuplehash_to_ctrack(h)->ct_general;
 	skb->nfctinfo = *ctinfo;
-	return -NF_ACCEPT;
+	return NF_ACCEPT;
 }
 
 static int

net/netfilter/nf_conntrack_core.c

@@ -922,6 +922,9 @@ nf_conntrack_in(struct net *net, u_int8_t pf, unsigned int hooknum,
 			ret = -ret;
 			goto out;
 		}
+		/* ICMP[v6] protocol trackers may assign one conntrack. */
+		if (skb->nfct)
+			goto out;
 	}
 
 	ct = resolve_normal_ct(net, tmpl, skb, dataoff, pf, protonum,