Re: [PATCH v3 1/4] kasan, arm64: Add KASAN light mode

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Jan 15, 2021 at 12:00:40PM +0000, Vincenzo Frascino wrote:
> Architectures supported by KASAN HW can provide a light mode of
> execution. On an MTE enabled arm64 hw for example this can be identified
> with the asynch mode of execution.
> In this mode, if a tag check fault occurs, the TFSR_EL1 register is
> updated asynchronously. The kernel checks the corresponding bits
> periodically.

What's the expected usage of this relative to prod, given that this has
to be chosen at boot time? When/where is this expected to be used
relative to prod mode?

> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index 18fce223b67b..3a7c5beb7096 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -231,7 +231,7 @@ static inline const void *__tag_set(const void *addr, u8 tag)
>  }
>  
>  #ifdef CONFIG_KASAN_HW_TAGS
> -#define arch_enable_tagging()			mte_enable_kernel()
> +#define arch_enable_tagging(mode)		mte_enable_kernel(mode)

Rather than passing a mode in, I think it'd be better to have:

* arch_enable_tagging_prod()
* arch_enable_tagging_light()

... that we can map in the arch code to separate:

* mte_enable_kernel_sync()
* mte_enable_kernel_async()

... as by construction that avoids calls with an unhandled mode, and we
wouldn't need the mode enum kasan_hw_tags_mode...

> +static inline int hw_init_mode(enum kasan_arg_mode mode)
> +{
> +	switch (mode) {
> +	case KASAN_ARG_MODE_LIGHT:
> +		return KASAN_HW_TAGS_ASYNC;
> +	default:
> +		return KASAN_HW_TAGS_SYNC;
> +	}
> +}

... and we can just have a wrapper like this to call either of the two functions directly, i.e.

static inline void hw_enable_tagging_mode(enum kasan_arg_mode mode)
{
	if (mode == KASAN_ARG_MODE_LIGHT)
		arch_enable_tagging_mode_light();
	else
		arch_enable_tagging_mode_prod();
}

Thanks,
Mark.