Re: [PATCH 02/11] kasan: clarify HW_TAGS impact on TBI

[Marco Elver <elver@google.com>]

On Tue, Jan 05, 2021 at 07:27PM +0100, Andrey Konovalov wrote:
> 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/Iba2a6697e3c6304cb53f89ec61dedc77fa29e3ae

Reviewed-by: Marco Elver <elver@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 kinds 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 around 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 below) 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 supports
>  boot parameters that allow to disable KASAN competely or otherwise control
>  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 KASAN
> +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=off is provided, or when the hardware doesn't
> +support MTE (but supports TBI).
> +
>  What memory accesses are sanitised by KASAN?
>  --------------------------------------------
>  
> -- 
> 2.29.2.729.g45daf8777d-goog
>