Fix user struct leakage with locked IPC shem segment

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: NPavel Emelianov <xemul@openvz.org>
Cc: <stable@kernel.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

ipc/shm.c

@@ -716,7 +716,7 @@ asmlinkage long sys_shmctl (int shmid, int cmd, struct shmid_ds __user *buf)
 			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;
 				}