fs: 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 8a0f2aeaa333f5c4e41b5d366745015fa855232f)
Signed-off-by: Andy Whitcroft <apw@canonical.com>

include/linux/fdtable.h

@@ -81,8 +81,10 @@ static inline struct file *__fcheck_files(struct files_struct *files, unsigned i
 {
 	struct fdtable *fdt = rcu_dereference_raw(files->fdt);
 
-	if (fd < fdt->max_fds)
+	if (fd < fdt->max_fds) {
+		gmb();
 		return rcu_dereference_raw(fdt->fd[fd]);
+	}
 	return NULL;
 }