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

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

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
>
> 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.
>
> 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.

>
>>> The patch could also be optimized a bit by restricting set_freepointer() call to the `CONFIG_SLAB_FREELIST_HARDENED` option value.
>>>
>> Yeah. Maybe memcg_alloc_abort_single() needs this too.
>>
>> Thanks.
>>
>>> Thanks again, Nicolas
>>>
>>>> Thanks.
>>>>
>>>>> Object's freepointer thus needs to be properly set using
>>>>> set_freepointer() after init_on_free.
>>>>>
>>>>> To reproduce, set `slub_debug=FU init_on_free=1 log_level=7` on the
>>>>> command line of a kernel build with `CONFIG_SLAB_FREELIST_HARDENED=y`.
>>>>>
>>>>> dmesg sample log:
>>>>> [   10.708715] =============================================================================
>>>>> [   10.710323] BUG kmalloc-rnd-05-32 (Tainted: G    B           T ): Freepointer corrupt
>>>>> [   10.712695] -----------------------------------------------------------------------------
>>>>> [   10.712695]
>>>>> [   10.712695] Slab 0xffffd8bdc400d580 objects=32 used=4 fp=0xffff9d9a80356f80 flags=0x200000000000a00(workingset|slab|node=0|zone=2)
>>>>> [   10.716698] Object 0xffff9d9a80356600 @offset=1536 fp=0x7ee4f480ce0ecd7c
>>>>> [   10.716698]
>>>>> [   10.716698] Bytes b4 ffff9d9a803565f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>>>>> [   10.720703] Object   ffff9d9a80356600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>>>>> [   10.720703] Object   ffff9d9a80356610: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>>>>> [   10.724696] Padding  ffff9d9a8035666c: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>>>>> [   10.724696] Padding  ffff9d9a8035667c: 00 00 00 00                                      ....
>>>>> [   10.724696] FIX kmalloc-rnd-05-32: Object at 0xffff9d9a80356600 not freed
>>>>>
>>>>> Signed-off-by: Nicolas Bouchinet<nicolas.bouchinet@ssi.gouv.fr>
>>>>> ---
>>>>>    mm/slub.c | 8 +++++++-
>>>>>    1 file changed, 7 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/mm/slub.c b/mm/slub.c
>>>>> index 3aa12b9b323d9..71dbff9ad8f17 100644
>>>>> --- a/mm/slub.c
>>>>> +++ b/mm/slub.c
>>>>> @@ -4342,10 +4342,16 @@ static __fastpath_inline
>>>>>    void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
>>>>>               unsigned long addr)
>>>>>    {
>>>>> +    bool init = false;
>>>>> +
>>>>>        memcg_slab_free_hook(s, slab, &object, 1);
>>>>> +    init = slab_want_init_on_free(s);
>>>>>    -    if (likely(slab_free_hook(s, object, slab_want_init_on_free(s))))
>>>>> +    if (likely(slab_free_hook(s, object, init))) {
>>>>> +        if (init)
>>>>> +            set_freepointer(s, object, NULL);
>>>>>            do_slab_free(s, slab, object, object, 1, addr);
>>>>> +    }
>>>>>    }
>>>>>      static __fastpath_inline
Thanks again for your review,

Nicolas