Re: [PATCH v2 16/16] arm64: Add ARM_SMCCC_ARCH_WORKAROUND_1 BP hardening support

[Hanjun Guo]

On 2018/1/31 23:05, Marc Zyngier wrote:
> On 31/01/18 14:38, Ard Biesheuvel wrote:
>> On 31 January 2018 at 14:35, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote:
>>> On 31 January 2018 at 14:11, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
>>>> On 31/01/18 13:56, Hanjun Guo wrote:
>>>>> Hi Marc,
>>>>>
>>>>> On 2018/1/30 1:45, Marc Zyngier wrote:
>>>>>>  static int enable_psci_bp_hardening(void *data)
>>>>>>  {
>>>>>>      const struct arm64_cpu_capabilities *entry = data;
>>>>>>
>>>>>> -    if (psci_ops.get_version)
>>>>>> +    if (psci_ops.get_version) {
>>>>>> +            if (check_smccc_arch_workaround_1(entry))
>>>>>> +                    return 0;
>>>>>
>>>>> If I'm using the new version SMCCC, the firmware have the choicARM_SMCCC_ARCH_WORKAROUND_1e to decide
>>>>> whether this machine needs the workaround, even if the CPU is vulnerable
>>>>> for CVE-2017-5715, but..
>>>>>
>>>>>> +
>>>>>>              install_bp_hardening_cb(entry,
>>>>>>                                     (bp_hardening_cb_t)psci_ops.get_version,
>>>>>>                                     __psci_hyp_bp_inval_start,
>>>>>>                                     __psci_hyp_bp_inval_end);
>>>>>
>>>>> ..the code above seems will enable get_psci_version() for CPU and will
>>>>> trap to trust firmware even the new version of firmware didn't say
>>>>> we need the workaround, did I understand it correctly?
>>>>
>>>> Well, you only get there if we've established that your CPU is affected
>>>> (it has an entry matching its MIDR with the HARDEN_BRANCH_PREDICTOR
>>>> capability), and that entry points to enable_psci_bp_hardening. It is

I understand, but A53, A57, A72 and etc are always in the list :)

>>>> not the firmware that decides whether we need hardening, but the kernel.
>>>> The firmware merely provides a facility to apply the hardening.
>>>>
>>>>> I'm ask this because some platform will not expose to users to
>>>>> take advantage of CVE-2017-5715, and we can use different firmware
>>>>> to report we need such workaround or not, then use a single kernel
>>>>> image for both vulnerable platforms and no vulnerable ones.
>>>>
>>>> You cannot have your cake and eat it. If you don't want to workaround
>>>> the issue, you can disable the hardening. But asking for the same kernel
>>>> to do both depending on what the firmware reports doesn't make much
>>>> sense to me.
>>>
>>> The SMCCC v1.1. document does appear to imply that systems that
>>> implement SMCCC v1.1 but don't implement ARM_SMCCC_ARCH_WORKAROUND_1
>>> should be assumed to be unaffected.
>>>
>>> """
>>> If the discovery call returns NOT_SUPPORTED:
>>> â SMCCC_ARCH_WORKAROUND_1 must not be invoked on any PE in the system, and
>>> â none of the PEs in the system require firmware mitigation for CVE-2017-5715.
>>> """
>>>
>>> How to deal with conflicting information in this regard (quirk table
>>> vs firmware implementation) is a matter of policy, of course.
> 
> Yup. And the current approach fits the spec, I believe. The

Yes, approach in this patch set fits the spec, it just conflicts with
MIDR based approach.

> PSCI_GET_VERSION band-aid should normally be removed shortly after these
> patches hit mainline.

I'm a big fan of this :)

> 
>>
>> ... and actually, perhaps it makes sense for the
>> SMCCC_ARCH_WORKAROUND_1 check to be completely independent of MIDR
>> based errata matching?
>>
>> I.e., if SMCCC v1.1 and SMCCC_ARCH_WORKAROUND_1 are both implemented,
>> we should probably invoke it even if the MIDR is not known to belong
>> to an affected implementation.
> 
> This would have an impact on big-little systems, for which there is
> often a bunch of unaffected CPUs.

I think it's what we are doing now, SMCCC v1.1 didn't provide the ability
to report per-cpu SMCCC_ARCH_WORKAROUND_1, and it said:
 - The discovery call must return the same result on all PEs in the system.
 - In heterogeneous systems with some PEs that require mitigation and others
   that do not, the firmware must provide a safe implementation of this
   function on all PEs.

So from the spec that it's the firmware to take care of unaffected CPUs,
to the kernel it's the same.

Thanks
Hanjun