[PATCH v3 2/2] arm64: Turn on KPTI only on CPUs that need it

Jon Masters
2018-01-22T18:59:17Z

On 01/22/2018 06:41 AM, Will Deacon wrote:
> On Fri, Jan 19, 2018 at 04:22:48AM -0800, Jayachandran C wrote:
>> Whitelist Broadcom Vulcan/Cavium ThunderX2 processors in
>> unmap_kernel_at_el0(). These CPUs are not vulnerable to
>> CVE-2017-5754 and do not need KPTI when KASLR is off.
>>
>> Signed-off-by: Jayachandran C <jnair@caviumnetworks.com>
>> ---
>> arch/arm64/kernel/cpufeature.c | 7 +++++++
>> 1 file changed, 7 insertions(+)
>>
>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
>> index 647d44b..fb698ca 100644
>> --- a/arch/arm64/kernel/cpufeature.c
>> +++ b/arch/arm64/kernel/cpufeature.c
>> @@ -866,6 +866,13 @@ 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 */
>> + switch (read_cpuid_id() & MIDR_CPU_MODEL_MASK) {
>> + case MIDR_CAVIUM_THUNDERX2:
>> + case MIDR_BRCM_VULCAN:
>> + return false;
>> + }
>> +
>> /* Defer to CPU feature registers */
>> return !cpuid_feature_extract_unsigned_field(pfr0,
>> ID_AA64PFR0_CSV3_SHIFT);
>
> We'll need to re-jig this to work properly with big/little because this is
> only called once, but that's ok for now:
>
> Acked-by: Will Deacon <will.deacon@arm.com>
>
> Suzuki has a series reworking much of the cpufeatures code so that we can
> do this properly for 4.17.

Thanks, much appreciated.

Jon.