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

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Date: Fri, 26 Apr 2024 14:18:56 +0200
From: Nicolas Bouchinet <nicolas.bouchinet@...p-os.org>
To: Xiongwei Song <sxwjean@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, cl@...ux.com,
 penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
 akpm@...ux-foundation.org, vbabka@...e.cz, roman.gushchin@...ux.dev,
 42.hyeyoo@...il.com
Subject: Re: [PATCH] slub: Fixes freepointer encoding for single free

On 4/26/24 11:20, Xiongwei Song wrote:
> On Wed, Apr 24, 2024 at 8:48 PM Nicolas Bouchinet
> <nicolas.bouchinet@...p-os.org> wrote:
>> From: Nicolas Bouchinet <nicolas.bouchinet@....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().
>>
>> 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().
>>
>> 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
> If init_on_free is set,  slab_free_hook() zeros the object first, then
> do_slab_free() calls
> set_freepointer() to set the fp value, so there are 8 bytes non-zero
> at the moment?
> Hence, the issue is not related to init_on_free?
>
> The fp=0x7ee4f480ce0ecd7c here is beyond kernel memory space, is the issue from
> CONFIG_SLAB_FREELIST_HARDENED enabled?

My understanding of the bug is that slab_free_hook() indeed zeroes the 
object and its metadata first, then calls do_slab_free() and directly 
calls __slab_free(), head an tail parameters being set to the object.

If `slub_debug=F` (SLAB_CONSISTENCY_CHECKS) is set, the following call 
path can be executed :

free_to_partial_list() ->

free_debug_processing() ->

free_consistency_checks() ->

check_object() ->

check_valid_pointer(get_freepointer())

When check_valid_pointer() is called, its object parameter is first 
decoded by get_freepointer(), if CONFIG_SLAB_FREELIST_HARDENED is 
enabled, zeroed data is then decoded by freelist_ptr_decode() using the 
(ptr.v ^ s->random ^ swab(ptr_addr)) operation.


Does that makes sense to you or do you think I'm missing something ?


Best regards,

Nicolas

> Xiongwei
>
>> [   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@....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
>> --
>> 2.44.0
>>
>>