Message-ID: <10c9a07a-1c6d-4ea7-8c1d-03a7dc5b29d8@clip-os.org>
Date: Mon, 29 Apr 2024 18:16:40 +0200
From: Nicolas Bouchinet <nicolas.bouchinet@...p-os.org>
To: Chengming Zhou <chengming.zhou@...ux.dev>,
 Vlastimil Babka <vbabka@...e.cz>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Cc: cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
 iamjoonsoo.kim@....com, akpm@...ux-foundation.org, roman.gushchin@...ux.dev,
 42.hyeyoo@...il.com, Xiongwei Song <xiongwei.song@...driver.com>
Subject: Re: [PATCH] slub: Fixes freepointer encoding for single free

On 4/29/24 16:52, Chengming Zhou wrote:
> On 2024/4/29 22:32, Nicolas Bouchinet wrote:
>> 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@....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 ?
>>
> Ok, it's great. Thanks!

Now I think of it, doesn't it seems a bit odd to only properly 
init_on_free object's freepointer only if it's inside the object ? IMHO 
it is equally necessary to avoid information leaking about the 
freepointer whether it is inside or outside the object.
I think it break the semantic of the commit 0f181f9fbea8bc7ea 
("mm/slub.c: init_on_free=1 should wipe freelist ptr for bulk 
allocations") ?

Thanks.

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