userns: Simplify the user_namespace by making userns->creator a kuid.

- Transform userns->creator from a user_struct reference to a simple
  kuid_t, kgid_t pair.

  In cap_capable this allows the check to see if we are the creator of
  a namespace to become the classic suser style euid permission check.

  This allows us to remove the need for a struct cred in the mapping
  functions and still be able to dispaly the user namespace creators
  uid and gid as 0.

- Remove the now unnecessary delayed_work in free_user_ns.

  All that is left for free_user_ns to do is to call kmem_cache_free
  and put_user_ns.  Those functions can be called in any context
  so call them directly from free_user_ns removing the need for delayed work.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

security/commoncap.c

@@ -76,8 +76,9 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
 		int cap, int audit)
 {
 	for (;;) {
-		/* The creator of the user namespace has all caps. */
-		if (targ_ns != &init_user_ns && targ_ns->creator == cred->user)
+		/* The owner of the user namespace has all caps. */
+		if (targ_ns != &init_user_ns && uid_eq(targ_ns->owner,
+						       make_kuid(cred->user_ns, cred->euid)))
 			return 0;
 
 		/* Do we have the necessary capabilities? */