Fix user namespace exiting OOPs

It turned out, that the user namespace is released during the do_exit() in
exit_task_namespaces(), but the struct user_struct is released only during the
put_task_struct(), i.e.  MUCH later.

On debug kernels with poisoned slabs this will cause the oops in
uid_hash_remove() because the head of the chain, which resides inside the
struct user_namespace, will be already freed and poisoned.

Since the uid hash itself is required only when someone can search it, i.e.
when the namespace is alive, we can safely unhash all the user_struct-s from
it during the namespace exiting.  The subsequent free_uid() will complete the
user_struct destruction.

For example simple program

   #include <sched.h>

   char stack[2 * 1024 * 1024];

   int f(void *foo)
   {
   	return 0;
   }

   int main(void)
   {
   	clone(f, stack + 1 * 1024 * 1024, 0x10000000, 0);
   	return 0;
   }

run on kernel with CONFIG_USER_NS turned on will oops the
kernel immediately.

This was spotted during OpenVZ kernel testing.
Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Alexey Dobriyan <adobriyan@openvz.org>
Acked-by: "Serge E. Hallyn" <serue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -1472,6 +1472,7 @@ static inline struct user_struct *get_uid(struct user_struct *u)
 }
 extern void free_uid(struct user_struct *);
 extern void switch_uid(struct user_struct *);
+extern void release_uids(struct user_namespace *ns);
 
 #include <asm/current.h>
 

kernel/user.c

@@ -62,7 +62,7 @@ static inline void uid_hash_insert(struct user_struct *up, struct hlist_head *ha
 
 static inline void uid_hash_remove(struct user_struct *up)
 {
-	hlist_del(&up->uidhash_node);
+	hlist_del_init(&up->uidhash_node);
 }
 
 static inline struct user_struct *uid_hash_find(uid_t uid, struct hlist_head *hashent)
@@ -199,6 +199,30 @@ void switch_uid(struct user_struct *new_user)
 	suid_keys(current);
 }
 
+void release_uids(struct user_namespace *ns)
+{
+	int i;
+	unsigned long flags;
+	struct hlist_head *head;
+	struct hlist_node *nd;
+
+	spin_lock_irqsave(&uidhash_lock, flags);
+	/*
+	 * collapse the chains so that the user_struct-s will
+	 * be still alive, but not in hashes. subsequent free_uid()
+	 * will free them.
+	 */
+	for (i = 0; i < UIDHASH_SZ; i++) {
+		head = ns->uidhash_table + i;
+		while (!hlist_empty(head)) {
+			nd = head->first;
+			hlist_del_init(nd);
+		}
+	}
+	spin_unlock_irqrestore(&uidhash_lock, flags);
+
+	free_uid(ns->root_user);
+}
 
 static int __init uid_cache_init(void)
 {

kernel/user_namespace.c

@@ -81,7 +81,7 @@ void free_user_ns(struct kref *kref)
 	struct user_namespace *ns;
 
 	ns = container_of(kref, struct user_namespace, kref);
-	free_uid(ns->root_user);
+	release_uids(ns);
 	kfree(ns);
 }