net: mpls: prevent speculative execution

CVE-2017-5753 (Spectre v1 Intel)

Since the index value in function mpls_route_input_rcu()
seems to be controllable by userspace and later on
conditionally (upon bound check) used to resolve
platform_label, insert an observable speculation
barrier before its usage. This should prevent
observable speculation on that branch and avoid
kernel memory leak.
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@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);
+
+		osb();
 		rt = rcu_dereference(platform_label[index]);
 	}
 	return rt;