linux-kernel - Re: [PATCH v2 07/11] kasan: docs: update SW

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Message-ID: <YEuEG+K4/V4zn9l4@elver.google.com>
Date:   Fri, 12 Mar 2021 16:09:15 +0100
From:   Marco Elver <elver@...gle.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 07/11] kasan: docs: update SW_TAGS implementation
 details section

On Fri, Mar 12, 2021 at 03:24PM +0100, Andrey Konovalov wrote:
> Update the "Implementation details" section for SW_TAGS KASAN:
> 
> - Clarify the introduction sentence.
> - Punctuation, readability, and other minor clean-ups.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>

Reviewed-by: Marco Elver <elver@...gle.com>

> ---
>  Documentation/dev-tools/kasan.rst | 39 +++++++++++++++----------------
>  1 file changed, 19 insertions(+), 20 deletions(-)
> 
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> index 986410bf269f..5873d80cc1fd 100644
> --- a/Documentation/dev-tools/kasan.rst
> +++ b/Documentation/dev-tools/kasan.rst
> @@ -235,38 +235,37 @@ quarantine (see mm/kasan/quarantine.c for implementation).
>  Software tag-based KASAN
>  ~~~~~~~~~~~~~~~~~~~~~~~~
>  
> -Software tag-based KASAN requires software memory tagging support in the form
> -of HWASan-like compiler instrumentation (see HWASan documentation for details).
> -
> -Software tag-based KASAN is currently only implemented for arm64 architecture.
> +Software tag-based KASAN uses a software memory tagging approach to checking
> +access validity. It is currently only implemented for the arm64 architecture.
>  
>  Software tag-based KASAN uses the Top Byte Ignore (TBI) feature of arm64 CPUs
> -to store a pointer tag in the top byte of kernel pointers. Like generic KASAN
> -it uses shadow memory to store memory tags associated with each 16-byte memory
> -cell (therefore it dedicates 1/16th of the kernel memory for shadow memory).
> +to store a pointer tag in the top byte of kernel pointers. It uses shadow memory
> +to store memory tags associated with each 16-byte memory cell (therefore, it
> +dedicates 1/16th of the kernel memory for shadow memory).
>  
> -On each memory allocation software tag-based KASAN generates a random tag, tags
> -the allocated memory with this tag, and embeds this tag into the returned
> +On each memory allocation, software tag-based KASAN generates a random tag, tags
> +the allocated memory with this tag, and embeds the same tag into the returned
>  pointer.
>  
>  Software tag-based KASAN uses compile-time instrumentation to insert checks
> -before each memory access. These checks make sure that tag of the memory that
> -is being accessed is equal to tag of the pointer that is used to access this
> -memory. In case of a tag mismatch software tag-based KASAN prints a bug report.
> +before each memory access. These checks make sure that the tag of the memory
> +that is being accessed is equal to the tag of the pointer that is used to access
> +this memory. In case of a tag mismatch, software tag-based KASAN prints a bug
> +report.
>  
> -Software tag-based KASAN also has two instrumentation modes (outline, that
> -emits callbacks to check memory accesses; and inline, that performs the shadow
> +Software tag-based KASAN also has two instrumentation modes (outline, which
> +emits callbacks to check memory accesses; and inline, which performs the shadow
>  memory checks inline). With outline instrumentation mode, a bug report is
> -simply printed from the function that performs the access check. With inline
> -instrumentation a brk instruction is emitted by the compiler, and a dedicated
> -brk handler is used to print bug reports.
> +printed from the function that performs the access check. With inline
> +instrumentation, a ``brk`` instruction is emitted by the compiler, and a
> +dedicated ``brk`` handler is used to print bug reports.
>  
>  Software tag-based KASAN uses 0xFF as a match-all pointer tag (accesses through
> -pointers with 0xFF pointer tag aren't checked). The value 0xFE is currently
> +pointers with the 0xFF pointer tag are not checked). The value 0xFE is currently
>  reserved to tag freed memory regions.
>  
> -Software tag-based KASAN currently only supports tagging of
> -kmem_cache_alloc/kmalloc and page_alloc memory.
> +Software tag-based KASAN currently only supports tagging of slab and page_alloc
> +memory.
>  
>  Hardware tag-based KASAN
>  ~~~~~~~~~~~~~~~~~~~~~~~~
> -- 
> 2.31.0.rc2.261.g7f71774620-goog
>