linux-kernel - Re: [PATCH v4 07/12] arm64: add sysfs vulnerability show for meltdown

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Message-ID: <20190131175418.24b7811c@donnerap.cambridge.arm.com>
Date:   Thu, 31 Jan 2019 17:54:18 +0000
From:   Andre Przywara <andre.przywara@....com>
To:     Jeremy Linton <jeremy.linton@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, stefan.wahren@...e.com,
        mlangsdo@...hat.com, suzuki.poulose@....com, marc.zyngier@....com,
        catalin.marinas@....com, julien.thierry@....com,
        will.deacon@....com, linux-kernel@...r.kernel.org,
        steven.price@....com, ykaukab@...e.de, dave.martin@....com,
        shankerd@...eaurora.org
Subject: Re: [PATCH v4 07/12] arm64: add sysfs vulnerability show for
 meltdown

On Fri, 25 Jan 2019 12:07:06 -0600
Jeremy Linton <jeremy.linton@....com> wrote:

Hi,

> Display the mitigation status if active, otherwise
> assume the cpu is safe unless it doesn't have CSV3
> and isn't in our whitelist.
> 
> Signed-off-by: Jeremy Linton <jeremy.linton@....com>
> ---
>  arch/arm64/kernel/cpufeature.c | 33 +++++++++++++++++++++++++++------
>  1 file changed, 27 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c
> b/arch/arm64/kernel/cpufeature.c
> index a9e18b9cdc1e..624dfe0b5cdd 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -944,6 +944,8 @@ has_useable_cnp(const struct arm64_cpu_capabilities *entry, int scope)
> 	return has_cpuid_feature(entry, scope);
> }
>  
> +/* default value is invalid until unmap_kernel_at_el0() runs */

Shall we somehow enforce this? For instance by making __meltdown_safe
an enum, initialised to UNKNOWN?
Then bail out with a BUG_ON or WARN_ON in the sysfs code?

I just want to avoid to accidentally report "safe" when we actually
aren't.

> +static bool __meltdown_safe = true;
>  static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */ 
>  static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
> @@ -962,6 +964,16 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
> 		{ /* sentinel */ }
>  	};
>  	char const *str = "command line option";
> +	bool meltdown_safe;
> +
> +	meltdown_safe = is_midr_in_range_list(read_cpuid_id(), kpti_safe_list);
> +
> +	/* Defer to CPU feature registers */
> +	if (has_cpuid_feature(entry, scope))
> +		meltdown_safe = true;
> +
> +	if (!meltdown_safe)
> +		__meltdown_safe = false;
>  
>  	/*
>  	 * For reasons that aren't entirely clear, enabling KPTI on Cavium
> @@ -984,12 +996,7 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
> 	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
> 		return kaslr_offset() > 0;
>  
> -	/* Don't force KPTI for CPUs that are not vulnerable */
> -	if (is_midr_in_range_list(read_cpuid_id(), kpti_safe_list))
> -		return false;
> -
> -	/* Defer to CPU feature registers */
> -	return !has_cpuid_feature(entry, scope);
> +	return !meltdown_safe;
>  }
>  
>  static void
> @@ -2055,3 +2062,17 @@ static int __init enable_mrs_emulation(void)
>  }
>  
>  core_initcall(enable_mrs_emulation);
> +
> +#ifdef CONFIG_GENERIC_CPU_VULNERABILITIES
> +ssize_t cpu_show_meltdown(struct device *dev, struct device_attribute *attr,
> +		char *buf)

w/s issue.

Cheers,
Andre.

> +{
> +	if (arm64_kernel_unmapped_at_el0())
> +		return sprintf(buf, "Mitigation: KPTI\n");
> +
> +	if (__meltdown_safe)
> +		return sprintf(buf, "Not affected\n");
> +
> +	return sprintf(buf, "Vulnerable\n");
> +}
> +#endif