Re: [PATCH mm] slub, kasan: improve interaction of KASAN and slub_debug poisoning

[Feng Tang <feng.tang@intel.com>]

Hi Andrey,

On Thu, Nov 23, 2023 at 07:12:02AM +0800, andrey.konovalov@linux.dev wrote:
> From: Andrey Konovalov <andreyknvl@google.com>
> 
> When both KASAN and slub_debug are enabled, when a free object is being
> prepared in setup_object, slub_debug poisons the object data before KASAN
> initializes its per-object metadata.
> 
> Right now, in setup_object, KASAN only initializes the alloc metadata,
> which is always stored outside of the object. slub_debug is aware of
> this and it skips poisoning and checking that memory area.
> 
> However, with the following patch in this series, KASAN also starts
> initializing its free medata in setup_object. As this metadata might be
> stored within the object, this initialization might overwrite the
> slub_debug poisoning. This leads to slub_debug reports.
> 
> Thus, skip checking slub_debug poisoning of the object data area that
> overlaps with the in-object KASAN free metadata.
> 
> Also make slub_debug poisoning of tail kmalloc redzones more precise when
> KASAN is enabled: slub_debug can still poison and check the tail kmalloc
> allocation area that comes after the KASAN free metadata.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> 
> ---
> 
> Andrew, please put this patch right before "kasan: use stack_depot_put
> for Generic mode".
> ---
>  mm/slub.c | 41 ++++++++++++++++++++++++++---------------
>  1 file changed, 26 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 63d281dfacdb..782bd8a6bd34 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -870,20 +870,20 @@ static inline void set_orig_size(struct kmem_cache *s,
>  				void *object, unsigned int orig_size)
>  {
>  	void *p = kasan_reset_tag(object);
> +	unsigned int kasan_meta_size;
>  
>  	if (!slub_debug_orig_size(s))
>  		return;
>  
> -#ifdef CONFIG_KASAN_GENERIC
>  	/*
> -	 * KASAN could save its free meta data in object's data area at
> -	 * offset 0, if the size is larger than 'orig_size', it will
> -	 * overlap the data redzone in [orig_size+1, object_size], and
> -	 * the check should be skipped.
> +	 * KASAN can save its free meta data inside of the object at offset 0.
> +	 * If this meta data size is larger than 'orig_size', it will overlap
> +	 * the data redzone in [orig_size+1, object_size]. Thus, we adjust
> +	 * 'orig_size' to be as at least as big as KASAN's meta data.
>  	 */
> -	if (kasan_metadata_size(s, true) > orig_size)
> -		orig_size = s->object_size;
> -#endif
> +	kasan_meta_size = kasan_metadata_size(s, true);
> +	if (kasan_meta_size > orig_size)
> +		orig_size = kasan_meta_size;

'orig_size' is to save the orignal request size for kmalloc object,
and its main purpose is to detect the memory wastage of kmalloc
objects, see commit 6edf2576a6cc "mm/slub: enable debugging memory
wasting of kmalloc"

Setting "orig_size = s->object_size" was to skip the wastage check
and the redzone sanity check for this 'wasted space'.

So it's better not to set 'kasan_meta_size' to orig_size.

And from the below code, IIUC, the orig_size is not used in fixing
the boot problem found by Hyeonggon?

Thanks,
Feng

>  
>  	p += get_info_end(s);
>  	p += sizeof(struct track) * 2;
> @@ -1192,7 +1192,7 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
>  {
>  	u8 *p = object;
>  	u8 *endobject = object + s->object_size;
> -	unsigned int orig_size;
> +	unsigned int orig_size, kasan_meta_size;
>  
>  	if (s->flags & SLAB_RED_ZONE) {
>  		if (!check_bytes_and_report(s, slab, object, "Left Redzone",
> @@ -1222,12 +1222,23 @@ static int check_object(struct kmem_cache *s, struct slab *slab,
>  	}
>  
>  	if (s->flags & SLAB_POISON) {
> -		if (val != SLUB_RED_ACTIVE && (s->flags & __OBJECT_POISON) &&
> -			(!check_bytes_and_report(s, slab, p, "Poison", p,
> -					POISON_FREE, s->object_size - 1) ||
> -			 !check_bytes_and_report(s, slab, p, "End Poison",
> -				p + s->object_size - 1, POISON_END, 1)))
> -			return 0;
> +		if (val != SLUB_RED_ACTIVE && (s->flags & __OBJECT_POISON)) {
> +			/*
> +			 * KASAN can save its free meta data inside of the
> +			 * object at offset 0. Thus, skip checking the part of
> +			 * the redzone that overlaps with the meta data.
> +			 */
> +			kasan_meta_size = kasan_metadata_size(s, true);
> +			if (kasan_meta_size < s->object_size - 1 &&
> +			    !check_bytes_and_report(s, slab, p, "Poison",
> +					p + kasan_meta_size, POISON_FREE,
> +					s->object_size - kasan_meta_size - 1))
> +				return 0;
> +			if (kasan_meta_size < s->object_size &&
> +			    !check_bytes_and_report(s, slab, p, "End Poison",
> +					p + s->object_size - 1, POISON_END, 1))
> +				return 0;
> +		}
>  		/*
>  		 * check_pad_bytes cleans up on its own.
>  		 */
> -- 
> 2.25.1
>