Re: [PATCH 35/35] kasan: add documentation for hardware tag-based mode

[Marco Elver <elver@google.com>]

On Fri, Aug 14, 2020 at 07:27PM +0200, Andrey Konovalov wrote:
> Add documentation for hardware tag-based KASAN mode and also add some
> clarifications for software tag-based mode.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
>  Documentation/dev-tools/kasan.rst | 73 +++++++++++++++++++++----------
>  1 file changed, 51 insertions(+), 22 deletions(-)
> 
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> index a3030fc6afe5..aeed89d6eaf5 100644
[...]  
> -Tag-based KASAN uses the Top Byte Ignore (TBI) feature of modern 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).
> +Software tag-based KASAN uses the Top Byte Ignore (TBI) feature of modern 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).

It might be helpful to be more specific vs. saying "modern arm64 CPUs".
Does the "modern" qualifier suggest not all arm64 CPUs support the
feature?  (HW tag-based KASAN below is specific, and mentions ARMv8.5.)

> +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
> +pointer.
>  
> -On each memory allocation tag-based KASAN generates a random tag, tags the
> -allocated memory with this tag, and embeds this 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 tag-based KASAN prints a bug report.
> +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
> @@ -215,9 +222,31 @@ 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.
>  
> -A potential expansion of this mode is a hardware tag-based mode, which would
> -use hardware memory tagging support instead of compiler instrumentation and
> -manual shadow memory manipulation.
> +Software tag-based KASAN uses 0xFF as a match-all pointer tag (accesses aren't
> +checked).
> +
> +Software tag-based KASAN currently only supports tagging of slab memory.
> +
> +Hardware tag-based KASAN
> +~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +Hardware tag-based KASAN is similar to the software mode in concept, but uses
> +hardware memory tagging support instead of compiler instrumentation and
> +shadow memory.
> +
> +Hardware tag-based KASAN is based on both arm64 Memory Tagging Extension (MTE)
> +introduced in ARMv8.5 Instruction Set Architecture, and Top Byte Ignore (TBI).

Is there anything inherently tying tag-based KASAN to arm64? I guess if
some other architecture supports MTE, they just have to touch arch/,
right?

You could reword to say that "Hardware tag-based KASAN is currently only
supported on the ARM64 architecture.

On the ARM64 architecture, tag-based KASAN is based on both ..."

Thanks,
-- Marco

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