openvswitch: load and reference the NAT helper.

This improves the original commit 17c357e ("openvswitch: load
NAT helper") where it unconditionally tries to load the module for
every flow using NAT, so not efficient when loading multiple flows.
It also doesn't hold any references to the NAT module while the
flow is active.

This change fixes those problems. It will try to load the module
only if it's not present. It grabs a reference to the NAT module
and holds it while the flow is active. Finally, an error message
shows up if either actions above fails.

Fixes: 17c357e ("openvswitch: load NAT helper")
Signed-off-by: Flavio Leitner <fbl@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/openvswitch/conntrack.c

@@ -1307,6 +1307,7 @@ static int ovs_ct_add_helper(struct ovs_conntrack_info *info, const char *name,
 {
 	struct nf_conntrack_helper *helper;
 	struct nf_conn_help *help;
+	int ret = 0;
 
 	helper = nf_conntrack_helper_try_module_get(name, info->family,
 						    key->ip.proto);
@@ -1321,13 +1322,21 @@ static int ovs_ct_add_helper(struct ovs_conntrack_info *info, const char *name,
 		return -ENOMEM;
 	}
 
+#ifdef CONFIG_NF_NAT_NEEDED
+	if (info->nat) {
+		ret = nf_nat_helper_try_module_get(name, info->family,
+						   key->ip.proto);
+		if (ret) {
+			nf_conntrack_helper_put(helper);
+			OVS_NLERR(log, "Failed to load \"%s\" NAT helper, error: %d",
+				  name, ret);
+			return ret;
+		}
+	}
+#endif
 	rcu_assign_pointer(help->helper, helper);
 	info->helper = helper;
-
-	if (info->nat)
-		request_module("ip_nat_%s", name);
-
-	return 0;
+	return ret;
 }
 
 #if IS_ENABLED(CONFIG_NF_NAT)
@@ -1801,8 +1810,13 @@ void ovs_ct_free_action(const struct nlattr *a)
 
 static void __ovs_ct_free_action(struct ovs_conntrack_info *ct_info)
 {
-	if (ct_info->helper)
+	if (ct_info->helper) {
+#ifdef CONFIG_NF_NAT_NEEDED
+		if (ct_info->nat)
+			nf_nat_helper_put(ct_info->helper);
+#endif
 		nf_conntrack_helper_put(ct_info->helper);
+	}
 	if (ct_info->ct) {
 		if (ct_info->timeout[0])
 			nf_ct_destroy_timeout(ct_info->ct);