[PATCH 16/18] net: mpls: prevent bounds-check bypass via speculative execution

[Dan Williams]

Static analysis reports that 'index' may be a user controlled value that
is used as a data dependency reading 'rt' from the 'platform_label'
array.  In order to avoid potential leaks of kernel memory values, block
speculative execution of the instruction stream that could issue further
reads based on an invalid 'rt' value.

Based on an original patch by Elena Reshetova.

Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Cc: netdev@xxxxxxxxxxxxxxx
Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 net/mpls/af_mpls.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/net/mpls/af_mpls.c b/net/mpls/af_mpls.c
index 8ca9915befc8..ebcf0e246cfe 100644
--- a/net/mpls/af_mpls.c
+++ b/net/mpls/af_mpls.c
@@ -8,6 +8,7 @@
 #include <linux/ipv6.h>
 #include <linux/mpls.h>
 #include <linux/netconf.h>
+#include <linux/compiler.h>
 #include <linux/vmalloc.h>
 #include <linux/percpu.h>
 #include <net/ip.h>
@@ -77,12 +78,13 @@ static void rtmsg_lfib(int event, u32 label, struct mpls_route *rt,
 static struct mpls_route *mpls_route_input_rcu(struct net *net, unsigned index)
 {
 	struct mpls_route *rt = NULL;
+	struct mpls_route __rcu **platform_label =
+		rcu_dereference(net->mpls.platform_label);
+	struct mpls_route __rcu **rtp;
 
-	if (index < net->mpls.platform_labels) {
-		struct mpls_route __rcu **platform_label =
-			rcu_dereference(net->mpls.platform_label);
-		rt = rcu_dereference(platform_label[index]);
-	}
+	if ((rtp = nospec_array_ptr(platform_label, index,
+					net->mpls.platform_labels)))
+		rt = rcu_dereference(*rtp);
 	return rt;
 }