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

[Will Deacon]

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.

Will