Re: SLUB: sysfs lets root force slab order below required minimum, causing memory corruption

[Kees Cook <keescook@chromium.org>]

On Tue, Mar 03, 2020 at 05:26:14PM -0800, David Rientjes wrote:
> On Wed, 4 Mar 2020, Jann Horn wrote:
> 
> > Hi!
> > 
> > FYI, I noticed that if you do something like the following as root,
> > the system blows up pretty quickly with error messages about stuff
> > like corrupt freelist pointers because SLUB actually allows root to
> > force a page order that is smaller than what is required to store a
> > single object:
> > 
> >     echo 0 > /sys/kernel/slab/task_struct/order
> > 
> > The other SLUB debugging options, like red_zone, also look kind of
> > suspicious with regards to races (either racing with other writes to
> > the SLUB debugging options, or with object allocations).
> > 
> 
> Thanks for the report, Jann.  To address the most immediate issue, 
> allowing a smaller order than allowed, I think we'd need something like 
> this.
> 
> I can propose it as a formal patch if nobody has any alternate 
> suggestions?
> ---
>  mm/slub.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3598,7 +3598,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
>  	 */
>  	size = ALIGN(size, s->align);
>  	s->size = size;
> -	if (forced_order >= 0)
> +	if (forced_order >= slab_order(size, 1, MAX_ORDER, 1))
>  		order = forced_order;
>  	else
>  		order = calculate_order(size);

Seems reasonable!

For the race concerns, should this logic just make sure the resulting
order can never shrink? Or does it need much stronger atomicity?

-- 
Kees Cook