| Re: [PATCH] Fix user struct leakage with locked IPC shem segment [message #15014 is a reply to message #15002] |
Mon, 16 July 2007 22:17   |
Andrew Morton
Messages: 127
Registered: December 2005
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Senior Member |
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On Mon, 16 Jul 2007 16:24:12 +0400
Pavel Emelianov <xemul@openvz.org> wrote:
> 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:
I'm curious. For the past few months, people@openvz.org have discovered
(and fixed) an ongoing stream of obscure but serious and quite
long-standing bugs.
How are you discovering these bugs?
> == 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.
That sounds fairly serious - can this lead to memory corruption and crashes?
> 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;
> }
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