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SubjectRe: [PATCH v2 16/16] arm64: Add ARM_SMCCC_ARCH_WORKAROUND_1 BP hardening support
On 31 January 2018 at 14:11, Marc Zyngier <> 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:
• SMCCC_ARCH_WORKAROUND_1 must not be invoked on any PE in the system, and
• 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.

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