linux-kernel - Re: [PATCH v2 2/4] arm64: mte: Add asynchronous mode support

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Date:   Wed, 13 Jan 2021 17:22:28 +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, 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>
Subject: Re: [PATCH v2 2/4] arm64: mte: Add asynchronous mode support

On Thu, Jan 07, 2021 at 05:29:06PM +0000, Vincenzo Frascino wrote:
> MTE provides an asynchronous mode for detecting tag exceptions. In
> particular instead of triggering a fault the arm64 core updates a
> register which is checked by the kernel at the first entry after the tag
> exception has occurred.

Just rephrase the "tag exception" here as there's no exception taken.
Also we don't check this only when the kernel is first entered after a
tag check fault, as per patch 3.

> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -153,8 +153,35 @@ void mte_init_tags(u64 max_tag)
>  
>  void mte_enable_kernel(enum kasan_arg_mode mode)
>  {
> -	/* Enable MTE Sync Mode for EL1. */
> -	sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_SYNC);
> +	const char *m;
> +
> +	/* Preset parameter values based on the mode. */
> +	switch (mode) {
> +	case KASAN_ARG_MODE_OFF:
> +		return;
> +	case KASAN_ARG_MODE_LIGHT:
> +		/* Enable MTE Async Mode for EL1. */
> +		sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_ASYNC);
> +		m = "asynchronous";
> +		break;
> +	case KASAN_ARG_MODE_DEFAULT:
> +	case KASAN_ARG_MODE_PROD:
> +	case KASAN_ARG_MODE_FULL:
> +		/* Enable MTE Sync Mode for EL1. */
> +		sysreg_clear_set(sctlr_el1, SCTLR_ELx_TCF_MASK, SCTLR_ELx_TCF_SYNC);
> +		m = "synchronous";
> +		break;
> +	default:
> +		/*
> +		 * kasan mode should be always set hence we should
> +		 * not reach this condition.
> +		 */
> +		WARN_ON_ONCE(1);
> +		return;
> +	}

I guess the switch statement here will be re-written as we want kasan to
drive the actual sync/async modes as it sees fit rather than MTE
guessing what PROD/FULL/LIGHT means.

-- 
Catalin