Re: [PATCH v6 3/4] mm: kasan: Add free_meta size info in struct kasan_cache

[Andrey Konovalov]

On Tue, Sep 13, 2022 at 8:54 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]
>
> For better information for slab/slub, add free_meta's data size
> into 'struct kasan_cache', so that its users can take right action
> to avoid data conflict.
>
> [1]. https://lore.kernel.org/lkml/YuYm3dWwpZwH58Hu@xsang-OptiPlex-9020/
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> Acked-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> ---
>  include/linux/kasan.h | 2 ++
>  mm/kasan/common.c     | 2 ++
>  2 files changed, 4 insertions(+)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index b092277bf48d..49af9513e8ed 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -100,6 +100,8 @@ static inline bool kasan_has_integrated_init(void)
>  struct kasan_cache {
>         int alloc_meta_offset;
>         int free_meta_offset;
> +       /* size of free_meta data saved in object's data area */
> +       int free_meta_size;
>         bool is_kmalloc;
>  };
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 69f583855c8b..0cb867e92524 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -201,6 +201,8 @@ void __kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
>                         cache->kasan_info.free_meta_offset = KASAN_NO_FREE_META;
>                         *size = ok_size;
>                 }
> +       } else {
> +               cache->kasan_info.free_meta_size = sizeof(struct kasan_free_meta);

Hi Feng,

I just realized that we already have a function that exposes a similar
functionality: kasan_metadata_size. However, this function returns the
size of metadata that is stored in the redzone.

I think, instead of adding free_meta_size, a better approach would be to:

1. Rename kasan_metadata_size to kasan_metadata_size_in_redzone (or
something like that).
2. Add kasan_metadata_size_in_object with appropriate implementation
and use that in your patches.

This allows avoiding exposing KASAN-internal details such as what kind
of fields the kasan_cache struct has to the common code.

Sorry for nor realizing this straight away.

(Note that there's an upcoming patch that fixes a bug in
kasan_metadata_size' implementation [1].)

Thanks!

[1] https://lore.kernel.org/linux-mm/c7b316d30d90e5947eb8280f4dc78856a49298cf.1662411799.git.andreyknvl@xxxxxxxxxx/



>         }
>
>         /* Calculate size with optimal redzone. */
> --
> 2.34.1
>