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

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Date:   Tue, 22 Jun 2021 17:03:42 +0300
From:   Andrey Konovalov <andreyknvl@...il.com>
To:     yee.lee@...iatek.com
Cc:     wsd_upstream@...iatek.com,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        "open list:KASAN" <kasan-dev@...glegroups.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
        open list <linux-kernel@...r.kernel.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>,
        Marco Elver <elver@...gle.com>
Subject: Re: [PATCH] kasan: unpoison use memset to init unaligned object size

On Mon, Jun 21, 2021 at 6:45 PM <yee.lee@...iatek.com> wrote:
>
> From: Yee Lee <yee.lee@...iatek.com>
>
> 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@...iatek.com>
> ---
>  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
>