Re: [PATCH v3] slub: Fixes freepointer encoding for single free

[Chengming Zhou]

On 2024/4/30 19:58, Vlastimil Babka wrote:
> On 4/30/24 1:34 PM, Nicolas Bouchinet wrote:
>> From: Nicolas Bouchinet <nicolas.bouchinet@xxxxxxxxxxx>
>>
>> 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().
>>
>> During bulk free, slab_free_freelist_hook() isn't affected as it always
>> sets it objects freepointer using set_freepointer() to maintain its
>> reconstructed freelist after `init_on_free`.
>>
>> For single free, object's freepointer thus needs to be avoided when
>> stored outside the object if `init_on_free` is set. The freepointer left
>> as is, check_object() may later detect an invalid pointer value due to
>> objects overflow.
>>
>> 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
>>
>> Co-developed-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>
> 
> Chengming Zhou, could you provide your s-o-b please?

Of course.

Signed-off-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>

> 
>> Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@xxxxxxxxxxx>
> 
> Added to slab/for-6.9-rc7/fixes, thanks!

Thanks!