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

[Marc Zyngier]

On 08/01/18 07:24, Jayachandran C wrote:
> 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;
> 

KPTI is also an improvement for KASLR. Why would you deprive a user of
the choice to further secure their system?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...