[PATCH] Fix user struct leakage with locked IPC shem segment

[Pavel Emelianov <xemul@openvz.org>]

When user locks an ipc shmem segmant with SHM_LOCK ctl and the
segment is already locked the shmem_lock() function returns 0. 
After this the subsequent code leaks the existing user struct:

== ipc/shm.c: sys_shmctl() ==
     ...
     err = shmem_lock(shp->shm_file, 1, user);
     if (!err) {
          shp->shm_perm.mode |= SHM_LOCKED;
          shp->mlock_user = user;
     }
     ...
==

Other results of this are:
1. the new shp->mlock_user is not get-ed and will point to freed
   memory when the task dies.
2. the RLIMIT_MEMLOCK is screwed on both user structs.

The exploit looks like this:

==
    id = shmget(...);
    setresuid(uid, 0, 0);
    shmctl(id, SHM_LOCK, NULL);
    setresuid(uid + 1, 0, 0);
    shmctl(id, SHM_LOCK, NULL);
==

My solution is to return 0 to the userspace and do not change the
segment's user.

Signed-off-by: Pavel Emelianov <xemul@openvz.org>

---

--- ./ipc/shm.c.shlfix	2007-07-06 10:58:57.000000000 +0400
+++ ./ipc/shm.c	2007-07-16 16:12:34.000000000 +0400
@@ -715,7 +715,7 @@ asmlinkage long sys_shmctl (int shmid, i
 			struct user_struct * user = current->user;
 			if (!is_file_hugepages(shp->shm_file)) {
 				err = shmem_lock(shp->shm_file, 1, user);
-				if (!err) {
+				if (!err && !(shp->shm_perm.mode & SHM_LOCKED)){
 					shp->shm_perm.mode |= SHM_LOCKED;
 					shp->mlock_user = user;
 				}