userns: Add an explicit reference to the parent user namespace

I am about to remove the struct user_namespace reference from struct user_struct.
So keep an explicit track of the parent user namespace.

Take advantage of this new reference and replace instances of user_ns->creator->user_ns
with user_ns->parent.
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

include/linux/user_namespace.h

@@ -12,6 +12,7 @@
 struct user_namespace {
 	struct kref		kref;
 	struct hlist_head	uidhash_table[UIDHASH_SZ];
+	struct user_namespace	*parent;
 	struct user_struct	*creator;
 	struct work_struct	destroyer;
 };

kernel/user_namespace.c

@@ -45,6 +45,7 @@ int create_user_ns(struct cred *new)
 	}
 
 	/* set the new root user in the credentials under preparation */
+	ns->parent = parent_ns;
 	ns->creator = new->user;
 	new->user = root_user;
 	new->uid = new->euid = new->suid = new->fsuid = 0;
@@ -60,8 +61,6 @@ int create_user_ns(struct cred *new)
 	/* Leave the reference to our user_ns with the new cred */
 	new->user_ns = ns;
 
-	put_user_ns(parent_ns);
-
 	return 0;
 }
 
@@ -72,10 +71,12 @@ int create_user_ns(struct cred *new)
  */
 static void free_user_ns_work(struct work_struct *work)
 {
-	struct user_namespace *ns =
+	struct user_namespace *parent, *ns =
 		container_of(work, struct user_namespace, destroyer);
+	parent = ns->parent;
 	free_uid(ns->creator);
 	kmem_cache_free(user_ns_cachep, ns);
+	put_user_ns(parent);
 }
 
 void free_user_ns(struct kref *kref)
@@ -99,8 +100,7 @@ uid_t user_ns_map_uid(struct user_namespace *to, const struct cred *cred, uid_t
 	/* Is cred->user the creator of the target user_ns
 	 * or the creator of one of it's parents?
 	 */
-	for ( tmp = to; tmp != &init_user_ns;
-	      tmp = tmp->creator->user_ns ) {
+	for ( tmp = to; tmp != &init_user_ns; tmp = tmp->parent ) {
 		if (cred->user == tmp->creator) {
 			return (uid_t)0;
 		}
@@ -120,8 +120,7 @@ gid_t user_ns_map_gid(struct user_namespace *to, const struct cred *cred, gid_t
 	/* Is cred->user the creator of the target user_ns
 	 * or the creator of one of it's parents?
 	 */
-	for ( tmp = to; tmp != &init_user_ns;
-	      tmp = tmp->creator->user_ns ) {
+	for ( tmp = to; tmp != &init_user_ns; tmp = tmp->parent ) {
 		if (cred->user == tmp->creator) {
 			return (gid_t)0;
 		}

security/commoncap.c

@@ -92,7 +92,7 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
 		 *If you have a capability in a parent user ns, then you have
 		 * it over all children user namespaces as well.
 		 */
-		targ_ns = targ_ns->creator->user_ns;
+		targ_ns = targ_ns->parent;
 	}
 
 	/* We never get here */