Re: [PATCH v7 2/3] mm: kasan: Extend kasan_metadata_size() to also cover in-object size

[Andrey Konovalov]

On Fri, Oct 21, 2022 at 5:24 AM Feng Tang <feng.tang@xxxxxxxxx> wrote:
>
> When kasan is enabled for slab/slub, it may save kasan' free_meta
> data in the former part of slab object data area in slab object's
> free path, which works fine.
>
> There is ongoing effort to extend slub's debug function which will
> redzone the latter part of kmalloc object area, and when both of
> the debug are enabled, there is possible conflict, especially when
> the kmalloc object has small size, as caught by 0Day bot [1].
>
> To solve it, slub code needs to know the in-object kasan's meta
> data size. Currently, there is existing kasan_metadata_size()
> which returns the kasan's metadata size inside slub's metadata
> area, so extend it to also cover the in-object meta size by
> adding a boolean flag 'in_object'.
>
> There is no functional change to existing code logic.
>
> [1]. https://lore.kernel.org/lkml/YuYm3dWwpZwH58Hu@xsang-OptiPlex-9020/
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Suggested-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>
> Cc: Andrey Ryabinin <ryabinin.a.a@xxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> ---
>  include/linux/kasan.h |  5 +++--
>  mm/kasan/generic.c    | 19 +++++++++++++------
>  mm/slub.c             |  4 ++--
>  3 files changed, 18 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index d811b3d7d2a1..96c9d56e5510 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -302,7 +302,7 @@ static inline void kasan_unpoison_task_stack(struct task_struct *task) {}
>
>  #ifdef CONFIG_KASAN_GENERIC
>
> -size_t kasan_metadata_size(struct kmem_cache *cache);
> +size_t kasan_metadata_size(struct kmem_cache *cache, bool in_object);
>  slab_flags_t kasan_never_merge(void);
>  void kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
>                         slab_flags_t *flags);
> @@ -315,7 +315,8 @@ void kasan_record_aux_stack_noalloc(void *ptr);
>  #else /* CONFIG_KASAN_GENERIC */
>
>  /* Tag-based KASAN modes do not use per-object metadata. */
> -static inline size_t kasan_metadata_size(struct kmem_cache *cache)
> +static inline size_t kasan_metadata_size(struct kmem_cache *cache,
> +                                               bool in_object)
>  {
>         return 0;
>  }
> diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
> index d8b5590f9484..b076f597a378 100644
> --- a/mm/kasan/generic.c
> +++ b/mm/kasan/generic.c
> @@ -450,15 +450,22 @@ void kasan_init_object_meta(struct kmem_cache *cache, const void *object)
>                 __memset(alloc_meta, 0, sizeof(*alloc_meta));
>  }
>
> -size_t kasan_metadata_size(struct kmem_cache *cache)
> +size_t kasan_metadata_size(struct kmem_cache *cache, bool in_object)
>  {
> +       struct kasan_cache *info = &cache->kasan_info;
> +
>         if (!kasan_requires_meta())
>                 return 0;
> -       return (cache->kasan_info.alloc_meta_offset ?
> -               sizeof(struct kasan_alloc_meta) : 0) +
> -               ((cache->kasan_info.free_meta_offset &&
> -                 cache->kasan_info.free_meta_offset != KASAN_NO_FREE_META) ?
> -                sizeof(struct kasan_free_meta) : 0);
> +
> +       if (in_object)
> +               return (info->free_meta_offset ?
> +                       0 : sizeof(struct kasan_free_meta));
> +       else
> +               return (info->alloc_meta_offset ?
> +                       sizeof(struct kasan_alloc_meta) : 0) +
> +                       ((info->free_meta_offset &&
> +                       info->free_meta_offset != KASAN_NO_FREE_META) ?
> +                       sizeof(struct kasan_free_meta) : 0);
>  }
>
>  static void __kasan_record_aux_stack(void *addr, bool can_alloc)
> diff --git a/mm/slub.c b/mm/slub.c
> index 17292c2d3eee..adff7553b54e 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -910,7 +910,7 @@ static void print_trailer(struct kmem_cache *s, struct slab *slab, u8 *p)
>         if (slub_debug_orig_size(s))
>                 off += sizeof(unsigned int);
>
> -       off += kasan_metadata_size(s);
> +       off += kasan_metadata_size(s, false);
>
>         if (off != size_from_object(s))
>                 /* Beginning of the filler is the free pointer */
> @@ -1070,7 +1070,7 @@ static int check_pad_bytes(struct kmem_cache *s, struct slab *slab, u8 *p)
>                         off += sizeof(unsigned int);
>         }
>
> -       off += kasan_metadata_size(s);
> +       off += kasan_metadata_size(s, false);
>
>         if (size_from_object(s) == off)
>                 return 1;
> --
> 2.34.1
>

Reviewed-by: Andrey Konovalov <andreyknvl@xxxxxxxxx>

Thanks!