Re: [PATCH v10 48/50] KVM: SEV: Provide support for SNP_GUEST_REQUEST NAE event

[Kalra, Ashish]

On 10/18/2023 3:38 PM, Sean Christopherson wrote:
> On Wed, Oct 18, 2023, Ashish Kalra wrote:
> > > static int snp_handle_ext_guest_request(struct vcpu_svm *svm)
> > > {
> > > 	struct kvm_vcpu *vcpu = &svm->vcpu;
> > > 	struct kvm *kvm = vcpu->kvm;
> > > 	struct kvm_sev_info *sev;
> > > 	unsigned long exitcode;
> > > 	u64 data_gpa;
> > >
> > > 	if (!sev_snp_guest(vcpu->kvm)) {
> > > 		ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, SEV_RET_INVALID_GUEST);
> > > 		return 1;
> > > 	}
> > >
> > > 	data_gpa = vcpu->arch.regs[VCPU_REGS_RAX];
> > > 	if (!IS_ALIGNED(data_gpa, PAGE_SIZE)) {
> > > 		ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, SEV_RET_INVALID_ADDRESS);
> > > 		return 1;
> > > 	}
> > >
> > > 	vcpu->run->hypercall.nr		 = KVM_HC_SNP_GET_CERTS;
> > > 	vcpu->run->hypercall.args[0]	 = data_gpa;
> > > 	vcpu->run->hypercall.args[1]	 = vcpu->arch.regs[VCPU_REGS_RBX];
> > > 	vcpu->run->hypercall.flags	 = KVM_EXIT_HYPERCALL_LONG_MODE;
> > > 	vcpu->arch.complete_userspace_io = snp_complete_ext_guest_request;
> > > 	return 0;
> > > }
> >
> > IIRC, the important consideration here is to ensure that getting the
> > attestation report and retrieving the certificates appears atomic to the
> > guest. When SNP live migration is supported we don't want a case where the
> > guest could have migrated between the call to obtain the certificates and
> > obtaining the attestation report, which can potentially cause failure of
> > validation of the attestation report.
>
> Where does "obtaining the attestation report" happen?  I see the guest request
> and the certificate stuff, I don't see anything about attestation reports (though
> I'm not looking very closely).

The guest requests that the firmware construct an attestation report via 
the SNP_GUEST_REQUEST command. The certificates are piggy-backed to the 
guest along with the attestation report (retrieved from the FW via the 
SNP_GUEST_REQUEST command) as part of the SNP Extended Guest Request NAE 
handling.

Thanks,
Ashish