Re: [PATCH v2 1/1] kasan: Add memzero init for unaligned size under SLUB debug

[Andrey Konovalov <andreyknvl@gmail.com>]

On Thu, Jun 24, 2021 at 2:26 PM <yee.lee@mediatek.com> wrote:
>
> From: Yee Lee <yee.lee@mediatek.com>
>
> Issue: when SLUB debug is on, hwtag kasan_unpoison() would overwrite
> the redzone of object with unaligned size.
>
> An additional memzero_explicit() path is added to replacing init by
> hwtag instruction for those unaligned size at SLUB debug mode.
>
> Signed-off-by: Yee Lee <yee.lee@mediatek.com>
> ---
>  mm/kasan/kasan.h | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 8f450bc28045..d1054f35838f 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -387,6 +387,12 @@ static inline void kasan_unpoison(const void *addr, size_t size, bool init)
>
>         if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK))
>                 return;
> +#if IS_ENABLED(CONFIG_SLUB_DEBUG)

Is this an issue only with SLUB? SLAB also uses redzones.

> +       if (init && ((unsigned long)size & KASAN_GRANULE_MASK)) {

This needs a comment along the lines of:

/* Explicitly initialize the memory with the precise object size to
avoid overwriting the SLAB redzone. This disables initialization in
the arch code and may thus lead to performance penalty. The penalty is
accepted since SLAB redzones aren't enabled in production builds. */

> +               init = false;
> +               memzero_explicit((void *)addr, size);
> +       }
> +#endif
>         size = round_up(size, KASAN_GRANULE_SIZE);
>
>         hw_set_mem_tag_range((void *)addr, size, tag, init);
> --
> 2.18.0
>