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

[Andrew Morton]

On Tue, 17 Jul 2007 13:07:55 +0400 Kirill Korotaev <dev at sw.ru> wrote:

> 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.

hm, OK, I was visualising some mysterious Russian bugfinding machine or
something.

Don't stop ;)

> 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.

OK, thanks, I'll feed a copy in stable at kernel.org's direction.