debugfs: more tightly restrict default mount mode

Since the debugfs is mostly only used by root, make the default mount
mode 0700. Most system owners do not need a more permissive value,
but they can choose to weaken the restrictions via their fstab.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/filesystems/debugfs.txt

@@ -15,8 +15,8 @@ Debugfs is typically mounted with a command like:
     mount -t debugfs none /sys/kernel/debug
 
 (Or an equivalent /etc/fstab line).
-The debugfs root directory is accessible by anyone by default. To
-restrict access to the tree the "uid", "gid" and "mode" mount
+The debugfs root directory is accessible only to the root user by
+default. To change access to the tree the "uid", "gid" and "mode" mount
 options can be used.
 
 Note that the debugfs API is exported GPL-only to modules.

fs/debugfs/inode.c

@@ -28,7 +28,7 @@
 #include <linux/magic.h>
 #include <linux/slab.h>
 
-#define DEBUGFS_DEFAULT_MODE	0755
+#define DEBUGFS_DEFAULT_MODE	0700
 
 static struct vfsmount *debugfs_mount;
 static int debugfs_mount_count;