userns: Only allow the creator of the userns unprivileged mappings

If you did not create the user namespace and are allowed
to write to uid_map or gid_map you should already have the necessary
privilege in the parent user namespace to establish any mapping
you want so this will not affect userspace in practice.

Limiting unprivileged uid mapping establishment to the creator of the
user namespace makes it easier to verify all credentials obtained with
the uid mapping can be obtained without the uid mapping without
privilege.

Limiting unprivileged gid mapping establishment (which is temporarily
absent) to the creator of the user namespace also ensures that the
combination of uid and gid can already be obtained without privilege.

This is part of the fix for CVE-2014-8989.

Cc: [e-mail]
Reviewed-by: Andy Lutomirski <[e-mail]>
Signed-off-by: "Eric W. Biederman" <[e-mail]>

kernel/user_namespace.c

···
 				struct user_namespace *ns, int cap_setid,
 				struct uid_gid_map *new_map)
 {
+	const struct cred *cred = file->f_cred;
 	/* Don't allow mappings that would allow anything that wouldn't
 	 * be allowed without the establishment of unprivileged mappings.
 	 */
-	if ((new_map->nr_extents == 1) && (new_map->extent[0].count == 1)) {
+	if ((new_map->nr_extents == 1) && (new_map->extent[0].count == 1) &&
+	    uid_eq(ns->owner, cred->euid)) {
 		u32 id = new_map->extent[0].lower_first;
 		if (cap_setid == CAP_SETUID) {
 			kuid_t uid = make_kuid(ns->parent, id);
-			if (uid_eq(uid, file->f_cred->euid))
+			if (uid_eq(uid, cred->euid))
 				return true;
 		}
 	}