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

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Date:   Thu, 31 Jan 2019 15:48:04 -0600
From:   Jeremy Linton <jeremy.linton@....com>
To:     Julien Thierry <julien.thierry@....com>,
        linux-arm-kernel@...ts.infradead.org
Cc:     catalin.marinas@....com, will.deacon@....com, marc.zyngier@....com,
        suzuki.poulose@....com, dave.martin@....com,
        shankerd@...eaurora.org, linux-kernel@...r.kernel.org,
        ykaukab@...e.de, mlangsdo@...hat.com, steven.price@....com,
        stefan.wahren@...e.com
Subject: Re: [PATCH v4 07/12] arm64: add sysfs vulnerability show for meltdown

Hi,

On 01/31/2019 03:28 AM, Julien Thierry wrote:
> Hi Jeremy,
> 
> On 25/01/2019 18:07, 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@....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 */
>> +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;
> 
> Do we need to check the cpuid registers if the CPU is in the known safe
> list?

I don't believe so. In the previous patch where this was broken out 
these checks were just or'ed together. In this path it just seemed a 
little cleaner than adding the additional check/or'ing the results 
here/whatever as we only want to set it safe (never the other way 
around). AKA, i'm running out of horizontal space, and I want to keep 
the 'defer to registers' comment.


> 
> Otherwise:
> 
> Reviewed-by: Julien Thierry <julien.thierry@....com>
> 
>> +
>> +	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)
>> +{
>> +	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
>>
>