Re: [PATCH RFC v9 42/51] KVM: SVM: Support SEV-SNP AP Creation NAE event

[Peter Gonda <pgonda@google.com>]

> +
> +       switch (request) {
> +       case SVM_VMGEXIT_AP_CREATE_ON_INIT:
> +               kick = false;
> +               fallthrough;
> +       case SVM_VMGEXIT_AP_CREATE:
> +               if (!page_address_valid(vcpu, svm->vmcb->control.exit_info_2)) {
> +                       vcpu_unimpl(vcpu, "vmgexit: invalid AP VMSA address [%#llx] from guest\n",
> +                                   svm->vmcb->control.exit_info_2);
> +                       ret = -EINVAL;
> +                       goto out;
> +               }
> +
> +               /*
> +                * Malicious guest can RMPADJUST a large page into VMSA which
> +                * will hit the SNP erratum where the CPU will incorrectly signal
> +                * an RMP violation #PF if a hugepage collides with the RMP entry
> +                * of VMSA page, reject the AP CREATE request if VMSA address from
> +                * guest is 2M aligned.
> +                */
> +               if (IS_ALIGNED(svm->vmcb->control.exit_info_2, PMD_SIZE)) {
> +                       vcpu_unimpl(vcpu,
> +                                   "vmgexit: AP VMSA address [%llx] from guest is unsafe as it is 2M aligned\n",
> +                                   svm->vmcb->control.exit_info_2);

In this case and maybe the above case can we instead return an error
to the guest instead of a #GP?

I think Tom suggested: SW_EXITINFO1[31:0] to 2 (meaning there was an
error with the VMGEXIT request) and SW_EXITINFO2 to 5 (The NAE event
input was not valid (e.g., an invalid SW_EXITINFO1 value for the AP
Jump Table NAE event).

This way a non-malicious but buggy or naive guest gets a little more
information and a chance to retry the request?

> +                       ret = -EINVAL;
> +                       goto out;
> +               }
> +
> +               target_svm->sev_es.snp_vmsa_gpa = svm->vmcb->control.exit_info_2;
> +               break;
> +       case SVM_VMGEXIT_AP_DESTROY:
> +               break;
> +       default:
> +               vcpu_unimpl(vcpu, "vmgexit: invalid AP creation request [%#x] from guest\n",
> +                           request);
> +               ret = -EINVAL;
> +               break;
> +       }
> +