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Subject: Re: [PATCH v3 4/7] arm64: add sysfs vulnerability show for meltdown
To:     Julien Thierry <julien.thierry@arm.com>,
        linux-arm-kernel@lists.infradead.org
Cc:     catalin.marinas@arm.com, will.deacon@arm.com, marc.zyngier@arm.com,
        suzuki.poulose@arm.com, dave.martin@arm.com,
        shankerd@codeaurora.org, linux-kernel@vger.kernel.org,
        ykaukab@suse.de, mlangsdo@redhat.com, steven.price@arm.com,
        stefan.wahren@i2se.com
References: <20190109235544.2992426-1-jeremy.linton@arm.com>
 <20190109235544.2992426-5-jeremy.linton@arm.com>
 <8c8b564a-1d65-bc18-73fb-58b349a47800@arm.com>
From:   Jeremy Linton <jeremy.linton@arm.com>
Message-ID: <7e843245-56c0-d7c1-38ba-27ff231a500a@arm.com>
Date:   Thu, 10 Jan 2019 08:10:43 -0600
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Hi Julien,

On 01/10/2019 03:23 AM, Julien Thierry wrote:
> Hi Jeremy,
> 
> On 09/01/2019 23:55, 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 | 32 +++++++++++++++++++++++++++-----
>>   1 file changed, 27 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index ab784d7a0083..ef7bbc49ef78 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -944,8 +944,12 @@ 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;
>> +
>>   #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
>>   static int __kpti_forced; /* 0: not forced, >0: forced on, <0: forced off */
>> +extern uint arm64_requested_vuln_attrs;
>>   
>>   static bool is_cpu_meltdown_safe(void)
>>   {
>> @@ -972,6 +976,14 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>>   {
>>   	char const *str = "command line option";
>>   
>> +	bool meltdown_safe = is_cpu_meltdown_safe() ||
>> +		has_cpuid_feature(entry, scope);
>> +
>> +	if (!meltdown_safe)
>> +		__meltdown_safe = false;
>> +
>> +	arm64_requested_vuln_attrs |= VULN_MELTDOWN;
>> +
>>   	/*
>>   	 * For reasons that aren't entirely clear, enabling KPTI on Cavium
>>   	 * ThunderX leads to apparent I-cache corruption of kernel text, which
>> @@ -993,11 +1005,7 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>>   	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
>>   		return true;
>>   
>> -	if (is_cpu_meltdown_safe())
>> -		return false;
>> -
>> -	/* Defer to CPU feature registers */
>> -	return !has_cpuid_feature(entry, scope);
>> +	return !meltdown_safe;
>>   }
>>   
>>   static void
>> @@ -2065,3 +2073,17 @@ 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");
> 
> An issue I see is that we don't even bother to check it that CPUs are
> meltdown safe if CONFIG_UNMAP_KERNEL_AT_EL0 is not defined but here
> we'll advertise that the system is meltdown safe.

That check isn't necessary anymore because the sysfs attribute is only 
populated if unmap_kernel_at_el0() runs (assuming I haven't messed 
something up). That was Dave/Will's suggestions in the last thread about 
how to handle this case.


> 
> I think that checking whether we know that CPUs are meltdown safe should
> be separated from whether mitigation is applied.
> 
> Someone who knows thinks their CPUs are in the white list might want to
> compile out code that does the kpti, but it would be good to give them a
> proper diagnostic whether they were wrong or not.
> 
> Cheers,
>