linux-kernel - Re: [PATCH 3/4] arm64/kasan: don't allocate extra shadow memory

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Message-ID: <20170601163442.GC17711@leverpostej>
Date:   Thu, 1 Jun 2017 17:34:43 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        <linux-kernel@...r.kernel.org>, <kasan-dev@...glegroups.com>,
        <linux-mm@...ck.org>, Alexander Potapenko <glider@...gle.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH 3/4] arm64/kasan: don't allocate extra shadow memory

On Thu, Jun 01, 2017 at 07:23:37PM +0300, Andrey Ryabinin wrote:
> We used to read several bytes of the shadow memory in advance.
> Therefore additional shadow memory mapped to prevent crash if
> speculative load would happen near the end of the mapped shadow memory.
>
> Now we don't have such speculative loads, so we no longer need to map
> additional shadow memory.

I see that patch 1 fixed up the Linux helpers for outline
instrumentation.

Just to check, is it also true that the inline instrumentation never
performs unaligned accesses to the shadow memory?

If so, this looks good to me; it also avoids a potential fencepost issue
when memory exists right at the end of the linear map. Assuming that
holds:

Acked-by: Mark Rutland <mark.rutland@....com>

Thanks,
Mark.

>
> Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: linux-arm-kernel@...ts.infradead.org
> ---
>  arch/arm64/mm/kasan_init.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
>
> diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
> index 687a358a3733..81f03959a4ab 100644
> --- a/arch/arm64/mm/kasan_init.c
> +++ b/arch/arm64/mm/kasan_init.c
> @@ -191,14 +191,8 @@ void __init kasan_init(void)
>               if (start >= end)
>                       break;
>
> -             /*
> -              * end + 1 here is intentional. We check several shadow bytes in
> -              * advance to slightly speed up fastpath. In some rare cases
> -              * we could cross boundary of mapped shadow, so we just map
> -              * some more here.
> -              */
>               vmemmap_populate((unsigned long)kasan_mem_to_shadow(start),
> -                             (unsigned long)kasan_mem_to_shadow(end) + 1,
> +                             (unsigned long)kasan_mem_to_shadow(end),
>                               pfn_to_nid(virt_to_pfn(start)));
>       }
>
> --
> 2.13.0
>
>
> _______________________________________________
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> linux-arm-kernel@...ts.infradead.org
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