userns: 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 bf5352bb462ac0acf4ebca109e964666f845bd54)
Signed-off-by: Andy Whitcroft <apw@canonical.com>

kernel/user_namespace.c

@@ -502,8 +502,10 @@ static void *m_start(struct seq_file *seq, loff_t *ppos,
 	struct uid_gid_extent *extent = NULL;
 	loff_t pos = *ppos;
 
-	if (pos < map->nr_extents)
+	if (pos < map->nr_extents) {
+		gmb();
 		extent = &map->extent[pos];
+	}
 
 	return extent;
 }