Re: [PATCH v5 03/10] arm64: add sysfs vulnerability show for meltdown

[Andre Przywara <andre.przywara@arm.com>]

Hi,

On 2/26/19 7:05 PM, Jeremy Linton wrote:
> 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@arm.com>
> ---
>   arch/arm64/kernel/cpufeature.c | 47 ++++++++++++++++++++++++++--------
>   1 file changed, 37 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index f6d84e2c92fe..d31bd770acba 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -944,7 +944,7 @@ has_useable_cnp(const struct arm64_cpu_capabilities *entry, int scope)
>   	return has_cpuid_feature(entry, scope);
>   }
>   
> -#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
> +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,
> @@ -963,6 +963,16 @@ 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
> @@ -974,6 +984,11 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>   		__kpti_forced = -1;
>   	}
>   
> +	if (!IS_ENABLED(CONFIG_UNMAP_KERNEL_AT_EL0)) {
> +		pr_info_once("kernel page table isolation disabled by CONFIG\n");
> +		return false;
> +	}
> +
>   	/* Forced? */
>   	if (__kpti_forced) {
>   		pr_info_once("kernel page table isolation forced %s by %s\n",
> @@ -985,14 +1000,10 @@ 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;
>   }
>   
> +#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
>   static void
>   kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
>   {
> @@ -1022,6 +1033,13 @@ kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
>   
>   	return;
>   }
> +#else
> +static void
> +kpti_install_ng_mappings(const struct arm64_cpu_capabilities *__unused)
> +{
> +}
> +#endif	/* CONFIG_UNMAP_KERNEL_AT_EL0 */
> +
>   
>   static int __init parse_kpti(char *str)
>   {
> @@ -1035,7 +1053,6 @@ static int __init parse_kpti(char *str)
>   	return 0;
>   }
>   early_param("kpti", parse_kpti);
> -#endif	/* CONFIG_UNMAP_KERNEL_AT_EL0 */
>   
>   #ifdef CONFIG_ARM64_HW_AFDBM
>   static inline void __cpu_enable_hw_dbm(void)
> @@ -1286,7 +1303,6 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>   		.field_pos = ID_AA64PFR0_EL0_SHIFT,
>   		.min_field_value = ID_AA64PFR0_EL0_32BIT_64BIT,
>   	},
> -#ifdef CONFIG_UNMAP_KERNEL_AT_EL0
>   	{
>   		.desc = "Kernel page table isolation (KPTI)",
>   		.capability = ARM64_UNMAP_KERNEL_AT_EL0,
> @@ -1302,7 +1318,6 @@ static const struct arm64_cpu_capabilities arm64_features[] = {
>   		.matches = unmap_kernel_at_el0,
>   		.cpu_enable = kpti_install_ng_mappings,
>   	},
> -#endif
>   	{
>   		/* FP/SIMD is not implemented */
>   		.capability = ARM64_HAS_NO_FPSIMD,
> @@ -2063,3 +2078,15 @@ static int __init enable_mrs_emulation(void)
>   }
>   
>   core_initcall(enable_mrs_emulation);
> +
> +ssize_t cpu_show_meltdown(struct device *dev, struct device_attribute *attr,
> +		char *buf)
> +{
> +	if (arm64_kernel_unmapped_at_el0())
> +		return sprintf(buf, "Mitigation: KPTI\n");
> +
> +	if (__meltdown_safe)
> +		return sprintf(buf, "Not affected\n");

Shall those two checks be swapped? So it doesn't report about a KPTI 
mitigation if the CPU is safe, but we enable KPTI because of KASLR 
having enabled it? Or is that a different knob?

Cheers,
Andre.

> +
> +	return sprintf(buf, "Vulnerable\n");
> +}
>