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

[Pavel Emelianov]

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@xxxxxxxxxx>

---

--- ./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;
				}

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