linux-kernel - Re: [PATCH 1/6] arm64: kpti: move check for non-vulnerable CPUs to a function

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Message-ID: <672f0f2f-5ebc-ea3d-d3c0-4a2dd253ae09@arm.com>
Date:   Wed, 12 Dec 2018 08:36:19 -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, mark.rutland@....com,
        linux-kernel@...r.kernel.org, ykaukab@...e.de
Subject: Re: [PATCH 1/6] arm64: kpti: move check for non-vulnerable CPUs to a
 function

Hi Julien,

Thanks for looking at this,

On 12/13/2018 03:13 AM, Julien Thierry wrote:
> Hi,
> 
> On 06/12/2018 23:44, Jeremy Linton wrote:
>> From: Mian Yousaf Kaukab <ykaukab@...e.de>
>>
>> Add is_meltdown_safe() which is a whitelist of known safe cores.
>>
>> Signed-off-by: Mian Yousaf Kaukab <ykaukab@...e.de>
>> [Moved location of function]
>> Signed-off-by: Jeremy Linton <jeremy.linton@....com>
>> ---
>>   arch/arm64/kernel/cpufeature.c | 16 ++++++++++++----
>>   1 file changed, 12 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index aec5ecb85737..242898395f68 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -908,8 +908,7 @@ has_useable_cnp(const struct arm64_cpu_capabilities *entry, int scope)
>>   #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
>>   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,
>> -				int scope)
>> +static bool is_cpu_meltdown_safe(void)
>>   {
>>   	/* List of CPUs that are not vulnerable and don't need KPTI */
>>   	static const struct midr_range kpti_safe_list[] = {
>> @@ -917,6 +916,16 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>>   		MIDR_ALL_VERSIONS(MIDR_BRCM_VULCAN),
>>   		{ /* sentinel */ }
>>   	};
>> +	/* Don't force KPTI for CPUs that are not vulnerable */
> 
> This is really a nit, but that comment would make more sense where
> is_cpu_meltdown_safe() is called since unmap_kernel_at_el0 is the one
> deciding whether to apply KPTI, is_cpu_meltdown_safe() just states
> whether the core is safe of not.

That is a good point, thanks.


> 
> Otherwise:
> 
> Reviewed-by: Julien Thierry <julien.thierry@....com>
> 
> Cheers,
> 
> Julien
> 
>> +	if (is_midr_in_range_list(read_cpuid_id(), kpti_safe_list))
>> +		return true;
>> +
>> +	return false;
>> +}
>> +
>> +static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>> +				int scope)
>> +{
>>   	char const *str = "command line option";
>>   
>>   	/*
>> @@ -940,8 +949,7 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>>   	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
>>   		return true;
>>   
>> -	/* Don't force KPTI for CPUs that are not vulnerable */
>> -	if (is_midr_in_range_list(read_cpuid_id(), kpti_safe_list))
>> +	if (is_cpu_meltdown_safe())
>>   		return false;
>>   
>>   	/* Defer to CPU feature registers */
>>
>