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

[Vlastimil Babka <vbabka@suse.cz>]

On 8/19/24 04:43, Peng Fan wrote:
> Hi Nicolas,
>> Subject: [PATCH v3] slub: Fixes freepointer encoding for single free
>> 
> 
> With slub_debug=FUZ init_on_free=1 loglevel=7, I see error
> In 6.6 kernel. Should this patch be backported to 6.6?

But it was a fix for commit 284f17ac13fe that's from 6.8 and wasn't
backported to 6.6. Are you seeing the same error happening for the same
reason, or is it a different one?

> I also had a hack together with your patch applied to 6.6
> diff --git a/mm/slub.c b/mm/slub.c
> index 96406f9813e8..ff8cdc737722 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1209,7 +1209,8 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
>                         if (s->object_size > orig_size  &&
>                                 !check_bytes_and_report(s, slab, object,
>                                         "kmalloc Redzone", p + orig_size,
> -                                       val, s->object_size - orig_size)) {
> +                                       slab_want_init_on_free(s) ? 0 : val,
> +                                       s->object_size - orig_size)) {
>                                 return 0;
>                         }
>                 }
> 
> Thanks,
> Peng.
> 
>> 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().
>> 
>> 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@linux.dev>
>> Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
>> ---
>> Changes since v2:
>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2F
>> lore.kernel.org%2Fall%2FZjCxZfD1d36zfq-
>> R%40archlinux%2F&data=05%7C02%7Cpeng.fan%40nxp.com%7C38af
>> dff178a0422aeddc08dc690996f5%7C686ea1d3bc2b4c6fa92cd99c5c3
>> 01635%7C0%7C0%7C638500737122575573%7CUnknown%7CTWFpb
>> GZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLC
>> JXVCI6Mn0%3D%7C0%7C%7C%7C&sdata=Q4Vs38E%2F7HSIETGC2hYD
>> 07gN6U8hkVZR764Yn4TFWlw%3D&reserved=0
>> 
>> * Reword commit message in order to clarify the patch approach as
>> suggested by Vlastimil Babka
>> 
>> Changes since v1:
>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2F
>> lore.kernel.org%2Fall%2FZij_fGjRS_rK-
>> 65r%40archlinux%2F&data=05%7C02%7Cpeng.fan%40nxp.com%7C38
>> afdff178a0422aeddc08dc690996f5%7C686ea1d3bc2b4c6fa92cd99c5c
>> 301635%7C0%7C0%7C638500737122594077%7CUnknown%7CTWFp
>> bGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiL
>> CJXVCI6Mn0%3D%7C0%7C%7C%7C&sdata=xWgnIygCHwUPjnFENutIai
>> cMTk0HQTne8hQfOqIzbA0%3D&reserved=0
>> 
>> * Jump above out of object freepointer if init_on_free is set instead of
>> initializing it with set_freepointer() as suggested by Vlastimil Babka.
>> 
>> * Adapt maybe_wipe_obj_freeptr() to avoid wiping out of object on
>> alloc freepointer as suggested by Chengming Zhou.
>> 
>> * Reword commit message.
>> ---
>>  mm/slub.c | 11 ++++++++---
>>  1 file changed, 8 insertions(+), 3 deletions(-)
>> 
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 3aa12b9b323d..173c340ec1d3 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -2102,15 +2102,20 @@ bool slab_free_hook(struct kmem_cache
>> *s, void *x, bool init)
>>  	 *
>>  	 * The initialization memset's clear the object and the
>> metadata,
>>  	 * but don't touch the SLAB redzone.
>> +	 *
>> +	 * The object's freepointer is also avoided if stored outside the
>> +	 * object.
>>  	 */
>>  	if (unlikely(init)) {
>>  		int rsize;
>> +		unsigned int inuse;
>> 
>> +		inuse = get_info_end(s);
>>  		if (!kasan_has_integrated_init())
>>  			memset(kasan_reset_tag(x), 0, s->object_size);
>>  		rsize = (s->flags & SLAB_RED_ZONE) ? s->red_left_pad :
>> 0;
>> -		memset((char *)kasan_reset_tag(x) + s->inuse, 0,
>> -		       s->size - s->inuse - rsize);
>> +		memset((char *)kasan_reset_tag(x) + inuse, 0,
>> +			s->size - inuse - rsize);
>>  	}
>>  	/* KASAN might put x into memory quarantine, delaying its
>> reuse. */
>>  	return !kasan_slab_free(s, x, init);
>> @@ -3789,7 +3794,7 @@ static void *__slab_alloc_node(struct
>> kmem_cache *s,  static __always_inline void
>> maybe_wipe_obj_freeptr(struct kmem_cache *s,
>>  						   void *obj)
>>  {
>> -	if (unlikely(slab_want_init_on_free(s)) && obj)
>> +	if (unlikely(slab_want_init_on_free(s)) && obj &&
>> +!freeptr_outside_object(s))
>>  		memset((void *)((char *)kasan_reset_tag(obj) + s-
>> >offset),
>>  			0, sizeof(void *));
>>  }
>> --
>> 2.44.0
>> 
>