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

[Vlastimil Babka <vbabka@suse.cz>]

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.
>>>
>>>> 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.

>> 
>> 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