Re: [PATCH v6 3/9] arm64, kfence: enable KFENCE for ARM64

[Mark Rutland]

On Thu, Oct 29, 2020 at 02:16:43PM +0100, Marco Elver wrote:
> Add architecture specific implementation details for KFENCE and enable
> KFENCE for the arm64 architecture. In particular, this implements the
> required interface in <asm/kfence.h>.
> 
> KFENCE requires that attributes for pages from its memory pool can
> individually be set. Therefore, force the entire linear map to be mapped
> at page granularity. Doing so may result in extra memory allocated for
> page tables in case rodata=full is not set; however, currently
> CONFIG_RODATA_FULL_DEFAULT_ENABLED=y is the default, and the common case
> is therefore not affected by this change.
> 
> Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Co-developed-by: Alexander Potapenko <glider@xxxxxxxxxx>
> Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
> Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
> ---
> v5:
> * Move generic page allocation code to core.c [suggested by Jann Horn].
> * Remove comment about HAVE_ARCH_KFENCE_STATIC_POOL, since we no longer
>   support static pools.
> * Force page granularity for the linear map [suggested by Mark Rutland].
> ---
>  arch/arm64/Kconfig              |  1 +
>  arch/arm64/include/asm/kfence.h | 19 +++++++++++++++++++
>  arch/arm64/mm/fault.c           |  4 ++++
>  arch/arm64/mm/mmu.c             |  7 ++++++-
>  4 files changed, 30 insertions(+), 1 deletion(-)
>  create mode 100644 arch/arm64/include/asm/kfence.h
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index f858c352f72a..2f8b32dddd8b 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -135,6 +135,7 @@ config ARM64
>  	select HAVE_ARCH_JUMP_LABEL_RELATIVE
>  	select HAVE_ARCH_KASAN if !(ARM64_16K_PAGES && ARM64_VA_BITS_48)
>  	select HAVE_ARCH_KASAN_SW_TAGS if HAVE_ARCH_KASAN
> +	select HAVE_ARCH_KFENCE if (!ARM64_16K_PAGES && !ARM64_64K_PAGES)

Why does this depend on the page size?

If this is functional, but has a larger overhead on 16K or 64K, I'd
suggest removing the dependency, and just updating the Kconfig help text
to explain that.

Otherwise, this patch looks fine to me.

Thanks,
Mark.