rdisc: reject invalid prefix lengths from router advertisements

Later in NMDevice's rdisc_config_changed(), we already reject
routes with plen==0. Just do it earlier.

We would however not reject bogus routes with a prefix larger then 128.
That would later lead to an error when trying to add such a route to the
kernel.

src/rdisc/nm-lndp-rdisc.c

@@ -158,6 +158,8 @@ receive_ra (struct ndp *ndp, struct ndp_msg *msg, gpointer user_data)
 		/* Device route */
 
 		r_plen = ndp_msg_opt_prefix_len (msg, offset);
+		if (r_plen == 0 || r_plen > 128)
+			continue;
 		nm_utils_ip6_address_clear_host_address (&r_network, ndp_msg_opt_prefix (msg, offset), r_plen);
 
 		if (ndp_msg_opt_prefix_flag_on_link (msg, offset)) {
@@ -197,6 +199,9 @@ receive_ra (struct ndp *ndp, struct ndp_msg *msg, gpointer user_data)
 		    .preference = translate_preference (ndp_msg_opt_route_preference (msg, offset)),
 		};
 
+		if (route.plen == 0 || route.plen > 128)
+			continue;
+
 		/* Routers through this particular gateway */
 		nm_utils_ip6_address_clear_host_address (&route.network, ndp_msg_opt_route_prefix (msg, offset), route.plen);
 		if (nm_rdisc_add_route (rdisc, &route))

src/rdisc/nm-rdisc.c

@@ -212,6 +212,9 @@ nm_rdisc_add_route (NMRDisc *rdisc, const NMRDiscRoute *new)
 {
 	int i, insert_idx = -1;
 
+	if (new->plen == 0 || new->plen > 128)
+		return FALSE;
+
 	for (i = 0; i < rdisc->routes->len; i++) {
 		NMRDiscRoute *item = &g_array_index (rdisc->routes, NMRDiscRoute, i);