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

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

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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.

The patch could also be optimized a bit by restricting set_freepointer() 
call to the `CONFIG_SLAB_FREELIST_HARDENED` option value.

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

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