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From:   Alexander Potapenko <glider@google.com>
Date:   Tue, 12 Jan 2021 08:40:41 +0100
Message-ID: <CAG_fn=WOSsVPOKVS8GF8h-hHavgcqMEdbjy4Puy=bajTmW7sbg@mail.gmail.com>
Subject: Re: [PATCH 02/11] kasan: clarify HW_TAGS impact on TBI
To:     Andrey Konovalov <andreyknvl@google.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Marco Elver <elver@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Will Deacon <will.deacon@arm.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Evgenii Stepanov <eugenis@google.com>,
        Branislav Rankov <Branislav.Rankov@arm.com>,
        Kevin Brodsky <kevin.brodsky@arm.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Jan 5, 2021 at 7:28 PM Andrey Konovalov <andreyknvl@google.com> wro=
te:
>
> Mention in the documentation that enabling CONFIG_KASAN_HW_TAGS
> always results in in-kernel TBI (Top Byte Ignore) being enabled.
>
> Also do a few minor documentation cleanups.
>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> Link: https://linux-review.googlesource.com/id/Iba2a6697e3c6304cb53f89ec6=
1dedc77fa29e3ae
Reviewed-by: Alexander Potapenko <glider@google.com>

> ---
>  Documentation/dev-tools/kasan.rst | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/=
kasan.rst
> index 0fc3fb1860c4..26c99852a852 100644
> --- a/Documentation/dev-tools/kasan.rst
> +++ b/Documentation/dev-tools/kasan.rst
> @@ -147,15 +147,14 @@ negative values to distinguish between different ki=
nds of inaccessible memory
>  like redzones or freed memory (see mm/kasan/kasan.h).
>
>  In the report above the arrows point to the shadow byte 03, which means =
that
> -the accessed address is partially accessible.
> -
> -For tag-based KASAN this last report section shows the memory tags aroun=
d the
> -accessed address (see `Implementation details`_ section).
> +the accessed address is partially accessible. For tag-based KASAN modes =
this
> +last report section shows the memory tags around the accessed address
> +(see the `Implementation details`_ section).
>
>  Boot parameters
>  ~~~~~~~~~~~~~~~
>
> -Hardware tag-based KASAN mode (see the section about different mode belo=
w) is
> +Hardware tag-based KASAN mode (see the section about various modes below=
) is
>  intended for use in production as a security mitigation. Therefore it su=
pports
>  boot parameters that allow to disable KASAN competely or otherwise contr=
ol
>  particular KASAN features.
> @@ -305,6 +304,13 @@ reserved to tag freed memory regions.
>  Hardware tag-based KASAN currently only supports tagging of
>  kmem_cache_alloc/kmalloc and page_alloc memory.
>
> +If the hardware doesn't support MTE (pre ARMv8.5), hardware tag-based KA=
SAN
> +won't be enabled. In this case all boot parameters are ignored.
> +
> +Note, that enabling CONFIG_KASAN_HW_TAGS always results in in-kernel TBI=
 being
> +enabled. Even when kasan.mode=3Doff is provided, or when the hardware do=
esn't
> +support MTE (but supports TBI).
> +
>  What memory accesses are sanitised by KASAN?
>  --------------------------------------------
>
> --
> 2.29.2.729.g45daf8777d-goog
>


--=20
Alexander Potapenko
Software Engineer

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