mpls, nospec: Sanitize array index in mpls_label_ok()

commit 3968523f upstream.

mpls_label_ok() validates that the 'platform_label' array index from a
userspace netlink message payload is valid. Under speculation the
mpls_label_ok() result may not resolve in the CPU pipeline until after
the index is used to access an array element. Sanitize the index to zero
to prevent userspace-controlled arbitrary out-of-bounds speculation, a
precursor for a speculative execution side channel vulnerability.

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 4.4:
 - mpls_label_ok() doesn't take an extack parameter
 - Drop change in mpls_getroute()]
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/mpls/af_mpls.c

@@ -7,6 +7,7 @@
 #include <linux/if_arp.h>
 #include <linux/ipv6.h>
 #include <linux/mpls.h>
+#include <linux/nospec.h>
 #include <linux/vmalloc.h>
 #include <net/ip.h>
 #include <net/dst.h>
@@ -756,17 +757,20 @@ static int mpls_nh_build_multi(struct mpls_route_config *cfg,
 	return err;
 }
 
-static bool mpls_label_ok(struct net *net, unsigned int index)
+static bool mpls_label_ok(struct net *net, unsigned int *index)
 {
+	bool is_ok = true;
+
 	/* Reserved labels may not be set */
-	if (index < MPLS_LABEL_FIRST_UNRESERVED)
-		return false;
+	if (*index < MPLS_LABEL_FIRST_UNRESERVED)
+		is_ok = false;
 
 	/* The full 20 bit range may not be supported. */
-	if (index >= net->mpls.platform_labels)
-		return false;
+	if (is_ok && *index >= net->mpls.platform_labels)
+		is_ok = false;
 
-	return true;
+	*index = array_index_nospec(*index, net->mpls.platform_labels);
+	return is_ok;
 }
 
 static int mpls_route_add(struct mpls_route_config *cfg)
@@ -787,7 +791,7 @@ static int mpls_route_add(struct mpls_route_config *cfg)
 		index = find_free_label(net);
 	}
 
-	if (!mpls_label_ok(net, index))
+	if (!mpls_label_ok(net, &index))
 		goto errout;
 
 	/* Append makes no sense with mpls */
@@ -848,7 +852,7 @@ static int mpls_route_del(struct mpls_route_config *cfg)
 
 	index = cfg->rc_label;
 
-	if (!mpls_label_ok(net, index))
+	if (!mpls_label_ok(net, &index))
 		goto errout;
 
 	mpls_route_update(net, index, NULL, &cfg->rc_nlinfo);
@@ -1283,7 +1287,7 @@ static int rtm_to_route_config(struct sk_buff *skb,  struct nlmsghdr *nlh,
 				goto errout;
 
 			if (!mpls_label_ok(cfg->rc_nlinfo.nl_net,
-					   cfg->rc_label))
+					   &cfg->rc_label))
 				goto errout;
 			break;
 		}