linux-kernel - Re: [PATCH v15 5/8] arm64: mte: Enable TCO in functions that can read beyond buffer limits

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Date:   Fri, 12 Mar 2021 15:13:00 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Will Deacon <will@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Branislav Rankov <Branislav.Rankov@....com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>
Subject: Re: [PATCH v15 5/8] arm64: mte: Enable TCO in functions that can
 read beyond buffer limits

On Fri, Mar 12, 2021 at 02:22:07PM +0000, Vincenzo Frascino wrote:
> diff --git a/arch/arm64/include/asm/mte.h b/arch/arm64/include/asm/mte.h
> index 9b557a457f24..8603c6636a7d 100644
> --- a/arch/arm64/include/asm/mte.h
> +++ b/arch/arm64/include/asm/mte.h
> @@ -90,5 +90,20 @@ static inline void mte_assign_mem_tag_range(void *addr, size_t size)
>  
>  #endif /* CONFIG_ARM64_MTE */
>  
> +#ifdef CONFIG_KASAN_HW_TAGS
> +/* Whether the MTE asynchronous mode is enabled. */
> +DECLARE_STATIC_KEY_FALSE(mte_async_mode);
> +
> +static inline bool system_uses_mte_async_mode(void)
> +{
> +	return static_branch_unlikely(&mte_async_mode);
> +}
> +#else
> +static inline bool system_uses_mte_async_mode(void)
> +{
> +	return false;
> +}
> +#endif /* CONFIG_KASAN_HW_TAGS */

You can write this with fewer lines:

DECLARE_STATIC_KEY_FALSE(mte_async_mode);

static inline bool system_uses_mte_async_mode(void)
{
	return IS_ENABLED(CONFIG_KASAN_HW_TAGS) &&
		static_branch_unlikely(&mte_async_mode);
}

The compiler will ensure that mte_async_mode is not referred when
!CONFIG_KASAN_HW_TAGS and therefore doesn't need to be defined.

> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index fa755cf94e01..9362928ba0d5 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -26,6 +26,10 @@ u64 gcr_kernel_excl __ro_after_init;
>  
>  static bool report_fault_once = true;
>  
> +/* Whether the MTE asynchronous mode is enabled. */
> +DEFINE_STATIC_KEY_FALSE(mte_async_mode);
> +EXPORT_SYMBOL_GPL(mte_async_mode);

Maybe keep these bracketed by #ifdef CONFIG_KASAN_HW_TAGS. I think the
mte_enable_kernel_*() aren't needed either if KASAN_HW is disabled (you
can do it with an additional patch).

With these, you can add:

Reviewed-by: Catalin Marinas <catalin.marinas@....com>