[kernel-hardening] Re: [PATCH] shm: handle separate PID namespaces case

[Oleg Nesterov <oleg@redhat.com>]

On 07/04, Vasiliy Kulikov wrote:
>
> On Mon, Jul 04, 2011 at 17:05 +0200, Oleg Nesterov wrote:
> > On 07/04, Vasiliy Kulikov wrote:
> > >
> > > @@ -239,7 +239,23 @@ static int shm_try_destroy_current(int id, void *p, void *data)
> > >  	if (IS_ERR(shp))
> > >  		return 0;
> > >
> > > -	if (shp->shm_cprid != task_tgid_vnr(current)) {
> > > +	if (shp->shm_creator != current) {
> > > +		shm_unlock(shp);
> > > +		return 0;
> >
> > I know absolutely nothing about ipc/, so probably I am wrong. But do
> > we really need shm_lock()
>
> It is needed to protect against parallel reads.  To read one may just
> hold shm_lock, but to write both shm_lock and rw_mutex are needed.

Hmm. Still can't understand... Once again, it seems to me we can
check shp->shm_creator != current and return lockless. Or do you
think without shm_lock() we can see the false positive?

If shp->shm_creator was current, it was set by use, we can't miss
this shp. Of course, if we are going to shm_destroy() then we need
shm_lock().

Thanks,

Oleg.