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Subject: Re: SLUB: sysfs lets root force slab order below required minimum,
 causing memory corruption
To:     David Rientjes <rientjes@google.com>, Jann Horn <jannh@google.com>
Cc:     Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Matthew Garrett <mjg59@google.com>
References: <CAG48ez31PP--h6_FzVyfJ4H86QYczAFPdxtJHUEEan+7VJETAQ@mail.gmail.com>
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From:   Pekka Enberg <penberg@iki.fi>
Message-ID: <de62c9f9-2274-8a28-22c9-3b704016e7d3@iki.fi>
Date:   Wed, 4 Mar 2020 16:57:33 +0200
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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>