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

[Ard Biesheuvel]

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
>> 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.

... 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.