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

[Marc Zyngier]

On 08/01/18 17:40, Jayachandran C wrote:
> On Mon, Jan 08, 2018 at 09:20:09AM +0000, Marc Zyngier wrote:
>> 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?
> 
> The user has a choice with kpti= at the kernel command line, so we are
> not depriving the user of a choice. KASLR is expected to be enabled by
> distributions, and KPTI will be enabled by default as well.
> 
> On systems that are not vulnerable to variant 3, this is an unnecessary
> overhead.

KASLR can be defeated if you don't have KPTI enabled. The original
KAISER paper is quite clear about that.

Thanks,

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