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>

1 files changed, 20 insertions, 22 deletions

diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 898e973bd1e8..ed08836558e0 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -27,6 +27,16 @@ int create_user_ns(struct cred *new)
 {
         struct user_namespace *ns, *parent_ns = new->user_ns;
         struct user_struct *root_user;
+        kuid_t owner = make_kuid(new->user_ns, new->euid);
+        kgid_t group = make_kgid(new->user_ns, new->egid);
+
+        /* The creator needs a mapping in the parent user namespace
+         * or else we won't be able to reasonably tell userspace who
+         * created a user_namespace.
+         */
+        if (!kuid_has_mapping(parent_ns, owner) ||
+            !kgid_has_mapping(parent_ns, group))
+                return -EPERM;
 
         ns = kmem_cache_alloc(user_ns_cachep, GFP_KERNEL);
         if (!ns)
@@ -43,7 +53,9 @@ 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;
+        ns->owner = owner;
+        ns->group = group;
+        free_uid(new->user);
         new->user = root_user;
         new->uid = new->euid = new->suid = new->fsuid = 0;
         new->gid = new->egid = new->sgid = new->fsgid = 0;
@@ -63,35 +75,22 @@ int create_user_ns(struct cred *new)
 #endif
         /* tgcred will be cleared in our caller bc CLONE_THREAD won't be set */
 
-        /* Leave the reference to our user_ns with the new cred */
+        /* Leave the new->user_ns reference with the new user namespace. */
+        /* Leave the reference to our user_ns with the new cred. */
         new->user_ns = ns;
 
         return 0;
 }
 
-/*
- * Deferred destructor for a user namespace.  This is required because
- * free_user_ns() may be called with uidhash_lock held, but we need to call
- * back to free_uid() which will want to take the lock again.
- */
-static void free_user_ns_work(struct work_struct *work)
+void free_user_ns(struct kref *kref)
 {
         struct user_namespace *parent, *ns =
-                container_of(work, struct user_namespace, destroyer);
+                container_of(kref, struct user_namespace, kref);
+
         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)
-{
-        struct user_namespace *ns =
-                container_of(kref, struct user_namespace, kref);
-
-        INIT_WORK(&ns->destroyer, free_user_ns_work);
-        schedule_work(&ns->destroyer);
-}
 EXPORT_SYMBOL(free_user_ns);
 
 uid_t user_ns_map_uid(struct user_namespace *to, const struct cred *cred, uid_t uid)
@@ -101,12 +100,11 @@ uid_t user_ns_map_uid(struct user_namespace *to, const struct cred *cred, uid_t
         if (likely(to == cred->user_ns))
                 return uid;
 
-
         /* 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->parent ) {
-                if (cred->user == tmp->creator) {
+                if (uid_eq(cred->user->uid, tmp->owner)) {
                         return (uid_t)0;
                 }
         }
@@ -126,7 +124,7 @@ gid_t user_ns_map_gid(struct user_namespace *to, const struct cred *cred, gid_t
          * or the creator of one of it's parents?
          */
         for ( tmp = to; tmp != &init_user_ns; tmp = tmp->parent ) {
-                if (cred->user == tmp->creator) {
+                if (uid_eq(cred->user->uid, tmp->owner)) {
                         return (gid_t)0;
                 }
         }