net: mpls: prevent speculative execution

CVE-2017-5753
CVE-2017-5715

Real commit text tbd
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
(cherry picked from commit f84272a4586cddd342fa5570d4aafc223345a844)
Signed-off-by: Andy Whitcroft <apw@canonical.com>

net/mpls/af_mpls.c

@@ -43,6 +43,8 @@ static struct mpls_route *mpls_route_input_rcu(struct net *net, unsigned index)
 	if (index < net->mpls.platform_labels) {
 		struct mpls_route __rcu **platform_label =
 			rcu_dereference(net->mpls.platform_label);
+
+		gmb();
 		rt = rcu_dereference(platform_label[index]);
 	}
 	return rt;