Re: [PATCH] kasan: unpoison use memset to init unaligned object size

[Andrey Konovalov]

On Mon, Jun 21, 2021 at 6:45 PM <yee.lee@xxxxxxxxxxxx> wrote:
>
> From: Yee Lee <yee.lee@xxxxxxxxxxxx>
>
> This patch adds a memset to initialize object of unaligned size.
> Duing to the MTE granulrity, the integrated initialization using
> hwtag instruction will force clearing out bytes in granular size,
> which may cause undesired effect, such as overwriting to the redzone
> of SLUB debug. In this patch, for the unaligned object size, function
> uses memset to initailize context instead of the hwtag instruction.
>
> Signed-off-by: Yee Lee <yee.lee@xxxxxxxxxxxx>
> ---
>  mm/kasan/kasan.h | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
> index 8f450bc28045..d8faa64614b7 100644
> --- a/mm/kasan/kasan.h
> +++ b/mm/kasan/kasan.h
> @@ -387,8 +387,11 @@ static inline void kasan_unpoison(const void *addr, size_t size, bool init)
>
>         if (WARN_ON((unsigned long)addr & KASAN_GRANULE_MASK))
>                 return;
> +       if (init && ((unsigned long)size & KASAN_GRANULE_MASK)) {
> +               init = false;
> +               memset((void *)addr, 0, size);
> +       }

With this implementation, we loose the benefit of setting tags and
initializing memory with the same instructions.

Perhaps a better implementation would be to call
hw_set_mem_tag_range() with the size rounded down, and then separately
deal with the leftover memory.

>         size = round_up(size, KASAN_GRANULE_SIZE);
> -
>         hw_set_mem_tag_range((void *)addr, size, tag, init);
>  }
>
> --
> 2.18.0
>