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

[Kirill Korotaev]

Andrew Morton wrote:
> 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.

thanks a lot :@)

> How are you discovering these bugs?

Not sure what to answer :) Just trying to do our best.

This bug was thought over by Pavel for about 3 month after a single
uid leak in container was detected by beancounters' kernel memory accounting...

>>== 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?

Yes it can. According to Pavel when the shmem segment is destroyed it
puts the mlock_user pointer, which can already be stalled.

Kirill