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

[Chengming Zhou]

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@xxxxxxxxxxx>
>>>>>>>
>>>>>>> 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!