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

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

On 4/26/24 15:14, Xiongwei Song wrote:
> On Fri, Apr 26, 2024 at 8:18 PM Nicolas Bouchinet
> <nicolas.bouchinet@clip-os.org> wrote:
>> On 4/26/24 11:20, Xiongwei Song wrote:
>>> On Wed, Apr 24, 2024 at 8:48 PM Nicolas Bouchinet
>>> <nicolas.bouchinet@clip-os.org> 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().
>>>>
>>>> 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())
> I understand the call path. I meant here the freepointer is not ZERO
> but an illegal
> value( fp=0x7ee4f480ce0ecd7c),


Yes this is the reason of this patch. The freepointer is an illegal 
value because the memory range where it sits has been overridden by 
zeroes, set_freepointer() is never called and thus the freepointer is 
never properly set.


  The illegal value is obtained after zeroes has been decoded by 
get_freepointer()->freelist_ptr_decode().


>   then check_valid_pointer return 1 with
> it's last line
> and then check_object() printed out the error message. I'm not sure if I
> misunderstood you.


check_valid_pointer() returns 0 since object < base, the object being 
the decoded fp (0x7ee4f480ce0ecd7c < 0xffff.* base addr), hence 
check_object() returns 0, not 1.

This is why the "Object at 0x%p not freed" slab_fix() is called in 
free_debug_processing().


>
> Thank,
> Xiongwei
>