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

[Andrew Morton]

On Mon, 16 Jul 2007 16:24:12 +0400
Pavel Emelianov <xemul at 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 at 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 at 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;
>  				}