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

[Ard Biesheuvel]

On 22 January 2018 at 11:41, Will Deacon <will.deacon@xxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
>> ---
>>  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@xxxxxxx>
>
> Suzuki has a series reworking much of the cpufeatures code so that we can
> do this properly for 4.17.
>

If we start adding opt outs here, we should at least include A53, and
probably replace

>>       if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
>>               return true;

with

>>       if (IS_ENABLED(CONFIG_RANDOMIZE_BASE) && kaslr_offset() > 0)
>>               return true;

so that adding 'nokaslr' to the command line also disables KPTI.