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

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

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
>>
>> 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.


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.


>
>>
>>>> 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
>