Re: [v2,03/11] arm64: Take into account ID_AA64PFR0_EL1.CSV3

[Jayachandran C]

On Fri, Jan 05, 2018 at 01:12:33PM +0000, Will Deacon wrote:
> For non-KASLR kernels where the KPTI behaviour has not been overridden
> on the command line we can use ID_AA64PFR0_EL1.CSV3 to determine whether
> or not we should unmap the kernel whilst running at EL0.
> 
> Reviewed-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> ---
>  arch/arm64/include/asm/sysreg.h | 1 +
>  arch/arm64/kernel/cpufeature.c  | 8 +++++++-
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> index 08cc88574659..ae519bbd3f9e 100644
> --- a/arch/arm64/include/asm/sysreg.h
> +++ b/arch/arm64/include/asm/sysreg.h
> @@ -437,6 +437,7 @@
>  #define ID_AA64ISAR1_DPB_SHIFT		0
>  
>  /* id_aa64pfr0 */
> +#define ID_AA64PFR0_CSV3_SHIFT		60
>  #define ID_AA64PFR0_SVE_SHIFT		32
>  #define ID_AA64PFR0_GIC_SHIFT		24
>  #define ID_AA64PFR0_ASIMD_SHIFT		20
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 9f0545dfe497..d723fc071f39 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -145,6 +145,7 @@ static const struct arm64_ftr_bits ftr_id_aa64isar1[] = {
>  };
>  
>  static const struct arm64_ftr_bits ftr_id_aa64pfr0[] = {
> +	ARM64_FTR_BITS(FTR_HIDDEN, FTR_NONSTRICT, FTR_LOWER_SAFE, ID_AA64PFR0_CSV3_SHIFT, 4, 0),
>  	ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_SVE_SHIFT, 4, 0),
>  	ARM64_FTR_BITS(FTR_HIDDEN, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_GIC_SHIFT, 4, 0),
>  	S_ARM64_FTR_BITS(FTR_VISIBLE, FTR_STRICT, FTR_LOWER_SAFE, ID_AA64PFR0_ASIMD_SHIFT, 4, ID_AA64PFR0_ASIMD_NI),
> @@ -851,6 +852,8 @@ 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 __unused)
>  {
> +	u64 pfr0 = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
> +
>  	/* Forced on command line? */
>  	if (__kpti_forced) {
>  		pr_info_once("kernel page table isolation forced %s by command line option\n",
> @@ -862,7 +865,9 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
>  	if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
>  		return true;
>  
> -	return false;
> +	/* Defer to CPU feature registers */
> +	return !cpuid_feature_extract_unsigned_field(pfr0,
> +						     ID_AA64PFR0_CSV3_SHIFT);

If I read this correctly, this enables KPTI on all processors without the CSV3
set (which seems to be a future capability).

Turning on KPTI has a small but significant overhead, so I think we should turn
it off on processors that are not vulnerable to CVE-2017-5754. Can we add something
like  this:

--->8
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 19ed09b..202b037 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -862,6 +862,13 @@ static bool unmap_kernel_at_el0(const struct arm64_cpu_capabilities *entry,
                return __kpti_forced > 0;
        }
 
+       /* 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;
+       }
+
        /* Useful for KASLR robustness */
        if (IS_ENABLED(CONFIG_RANDOMIZE_BASE))
                return true;
-- 
JC