Re: [PATCH] slub: Fixes freepointer encoding for single free

[Nicolas Bouchinet <nicolas.bouchinet@clip-os.org>]

On 4/29/24 15:35, Chengming Zhou wrote:
> On 2024/4/29 20:59, Nicolas Bouchinet wrote:
>> On 4/29/24 11:09, Nicolas Bouchinet wrote:
>>> Hi Vlastimil,
>>>
>>> thanks for your review and your proposal.
>>>
>>> On 4/29/24 10:52, Vlastimil Babka wrote:
>>>> On 4/25/24 5:14 PM, Chengming Zhou wrote:
>>>>> On 2024/4/25 23:02, Nicolas Bouchinet wrote:
>>>> Thanks for finding the bug and the fix!
>>>>
>>>>>> Hy,
>>>>>>
>>>>>> First of all, thanks a lot for your time.
>>>>>>
>>>>>> On 4/25/24 10:36, Chengming Zhou wrote:
>>>>>>> On 2024/4/24 20:47, Nicolas Bouchinet wrote:
>>>>>>>> From: Nicolas Bouchinet<nicolas.bouchinet@ssi.gouv.fr>
>>>>>>>>
>>>>>>>> Commit 284f17ac13fe ("mm/slub: handle bulk and single object freeing
>>>>>>>> separately") splits single and bulk object freeing in two functions
>>>>>>>> slab_free() and slab_free_bulk() which leads slab_free() to call
>>>>>>>> slab_free_hook() directly instead of slab_free_freelist_hook().
>>>>>>> Right.
>>>>>>> y not suitable for a stable-candidate fix we need
>>>>>>>> If `init_on_free` is set, slab_free_hook() zeroes the object.
>>>>>>>> Afterward, if `slub_debug=F` and `CONFIG_SLAB_FREELIST_HARDENED` are
>>>>>>>> set, the do_slab_free() slowpath executes freelist consistency
>>>>>>>> checks and try to decode a zeroed freepointer which leads to a
>>>>>>>> "Freepointer corrupt" detection in check_object().
>>>>>>> IIUC, the "freepointer" can be checked on the free path only when
>>>>>>> it's outside the object memory. Here slab_free_hook() zeroed the
>>>>>>> freepointer and caused the problem.
>>>>>>>
>>>>>>> But why we should zero the memory outside the object_size? It seems
>>>>>>> more reasonable to only zero the object_size when init_on_free is set?
>>>>>> The original purpose was to avoid leaking information through the object and its metadata / tracking information as described in init_on_free initial Commit 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options").
>>>>>>
>>>>>> I have to admit I didn't read the entire lore about the original patchset yet, though it could be interesting to know a bit more the threat models, specifically regarding the object metadata init.
>>>>> Thank you for the reference! I also don't get why it needs to zero
>>>>> the metadata and tracking information.
>>>> Hmm taking a step back, it seems really suboptimal to initialize the
>>>> outside-object freepointer as part of init_on_free:
>>>>
>>>> - the freeing itself will always set it one way or another, in this case
>>>> free_to_partial_list() will do set_freepointer() after free_debug_processing()
>>>>
>>>> - we lose the ability to detect if the allocated slab object's user wrote to
>>>> it, which is a buffer overflow
> Ah, right, this ability seems important for debugging overflow problem.
>
>>>> So the best option to me would be to adjust the init in slab_free_hook() to
>>>> avoid the outside-object freepointer similarly to how it avoids the red zone.
> Agree.
>
>>>> We'll still not have the buffer overflow detection ability for bulk free
>>>> where slab_free_freelist_hook() will set the free pointer before we reach
>>>> the checks, but changing that is most likely not worth the trouble, and
>>>> especially not suitable for a stable-candidate fix we need here.
>>> It seems like a good alternative to me, I'll push a V2 patch with those changes.
>>>
>>> I help maintaining the Linux-Hardened patchset in which we have a slab object canary feature that helps detecting overflows. It is located just after the object freepointer.
>>
>> I've tried a patch where the freepointer is avoided but it results in the same bug. It seems that the commit 0f181f9fbea8bc7ea ("mm/slub.c: init_on_free=1 should wipe freelist ptr for bulk allocations") inits the freepointer on allocation if init_on_free is set in order to return a clean initialized object to the caller.
>>
> Good catch! You may need to change maybe_wipe_obj_freeptr() too,
> I haven't tested this, not sure whether it works for you. :)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 3e33ff900d35..3f250a167cb5 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3796,7 +3796,8 @@ static void *__slab_alloc_node(struct kmem_cache *s,
>   static __always_inline void maybe_wipe_obj_freeptr(struct kmem_cache *s,
>                                                     void *obj)
>   {
> -       if (unlikely(slab_want_init_on_free(s)) && obj)
> +       if (unlikely(slab_want_init_on_free(s)) && obj &&
> +           !freeptr_outside_object(s))
>                  memset((void *)((char *)kasan_reset_tag(obj) + s->offset),
>                          0, sizeof(void *));
>   }
>
> Thanks!

Indeed since check_object() avoids objects for which freepointer is in 
the object and since val is equal to SLUB_RED_ACTIVE in our specific 
case it should work. Do you want me to add you as Co-authored ?

Best regards,

Nicolas