Re: [PATCH 1/2] kasan, mm: fix conflicts with init_on_alloc/free

[Vlastimil Babka]

On 1/13/21 5:03 PM, Andrey Konovalov wrote:
> A few places where SLUB accesses object's data or metadata were missed in
> a previous patch. This leads to false positives with hardware tag-based
> KASAN when bulk allocations are used with init_on_alloc/free.
> 
> Fix the false-positives by resetting pointer tags during these accesses.
> 
> Link: https://linux-review.googlesource.com/id/I50dd32838a666e173fe06c3c5c766f2c36aae901
> Fixes: aa1ef4d7b3f67 ("kasan, mm: reset tags when accessing metadata")
> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  mm/slub.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index dc5b42e700b8..75fb097d990d 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2791,7 +2791,8 @@ static __always_inline void maybe_wipe_obj_freeptr(struct kmem_cache *s,
>  						   void *obj)
>  {
>  	if (unlikely(slab_want_init_on_free(s)) && obj)
> -		memset((void *)((char *)obj + s->offset), 0, sizeof(void *));
> +		memset((void *)((char *)kasan_reset_tag(obj) + s->offset),
> +			0, sizeof(void *));
>  }
>  
>  /*
> @@ -2883,7 +2884,7 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s,
>  		stat(s, ALLOC_FASTPATH);
>  	}
>  
> -	maybe_wipe_obj_freeptr(s, kasan_reset_tag(object));
> +	maybe_wipe_obj_freeptr(s, object);

And in that case the reset was unnecessary, right. (commit log only mentions
adding missing resets).

>  	if (unlikely(slab_want_init_on_alloc(gfpflags, s)) && object)
>  		memset(kasan_reset_tag(object), 0, s->object_size);
> @@ -3329,7 +3330,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size,
>  		int j;
>  
>  		for (j = 0; j < i; j++)
> -			memset(p[j], 0, s->object_size);
> +			memset(kasan_reset_tag(p[j]), 0, s->object_size);
>  	}
>  
>  	/* memcg and kmem_cache debug support */
>