netfilter: conntrack: gre: don't set assured flag for clash entries

Now that conntrack core is allowd to insert clashing entries, make sure
GRE won't set assured flag on NAT_CLASH entries, just like UDP.

Doing so prevents early_drop logic for these entries.

Fixes: d671fd82 ("netfilter: conntrack: allow insertion clash of gre protocol")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_proto_gre.c

@@ -205,6 +205,8 @@ int nf_conntrack_gre_packet(struct nf_conn *ct,
 			    enum ip_conntrack_info ctinfo,
 			    const struct nf_hook_state *state)
 {
+	unsigned long status;
+
 	if (!nf_ct_is_confirmed(ct)) {
 		unsigned int *timeouts = nf_ct_timeout_lookup(ct);
 
@@ -217,11 +219,17 @@ int nf_conntrack_gre_packet(struct nf_conn *ct,
 		ct->proto.gre.timeout = timeouts[GRE_CT_UNREPLIED];
 	}
 
+	status = READ_ONCE(ct->status);
 	/* If we've seen traffic both ways, this is a GRE connection.
 	 * Extend timeout. */
-	if (ct->status & IPS_SEEN_REPLY) {
+	if (status & IPS_SEEN_REPLY) {
 		nf_ct_refresh_acct(ct, ctinfo, skb,
 				   ct->proto.gre.stream_timeout);
+
+		/* never set ASSURED for IPS_NAT_CLASH, they time out soon */
+		if (unlikely((status & IPS_NAT_CLASH)))
+			return NF_ACCEPT;
+
 		/* Also, more likely to be important, and not a probe. */
 		if (!test_and_set_bit(IPS_ASSURED_BIT, &ct->status))
 			nf_conntrack_event_cache(IPCT_ASSURED, ct);