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

[Pekka Enberg <penberg@iki.fi>]

On 3/4/20 3:26 AM, 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);
> 

Reviewed-by: Pekka Enberg <penberg@iki.fi>