[PATCH v2 2/2] KVM: SEV: Add certificate support for SNP_EXTENDED_GUEST_REQUEST events

[Michael Roth <michael.roth@amd.com>]

Currently KVM implements a stub version of SNP Extended Guest Requests
that always supplies NULL certificate data alongside the attestation
report. Make use of the newly-defined KVM_EXIT_COCO_REQ_CERTS event to
provide a way for userspace to optionally supply this certificate data.

Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 arch/x86/kvm/svm/sev.c         | 44 +++++++++++++++++++++++++++++-----
 include/uapi/linux/sev-guest.h |  8 +++++++
 2 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 72674b8825c4..4827a8ed4d16 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4077,6 +4077,30 @@ static int snp_handle_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t resp_
 	return ret;
 }
 
+static int snp_complete_req_certs(struct kvm_vcpu *vcpu)
+{
+	struct vcpu_svm *svm = to_svm(vcpu);
+	struct vmcb_control_area *control = &svm->vmcb->control;
+
+	if (vcpu->run->coco.ret) {
+		if (vcpu->run->coco.ret == ENOSPC) {
+			vcpu->arch.regs[VCPU_REGS_RBX] = vcpu->run->coco.req_certs.npages;
+			ghcb_set_sw_exit_info_2(svm->sev_es.ghcb,
+						SNP_GUEST_ERR(SNP_GUEST_VMM_ERR_INVALID_LEN, 0));
+		} else if (vcpu->run->coco.ret == EAGAIN) {
+			ghcb_set_sw_exit_info_2(svm->sev_es.ghcb,
+						SNP_GUEST_ERR(SNP_GUEST_VMM_ERR_BUSY, 0));
+		} else {
+			ghcb_set_sw_exit_info_2(svm->sev_es.ghcb,
+						SNP_GUEST_ERR(SNP_GUEST_VMM_ERR_GENERIC, 0));
+		}
+
+		return 1; /* resume guest */
+	}
+
+	return snp_handle_guest_req(svm, control->exit_info_1, control->exit_info_2);
+}
+
 static int snp_handle_ext_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t resp_gpa)
 {
 	struct kvm *kvm = svm->vcpu.kvm;
@@ -4092,12 +4116,10 @@ static int snp_handle_ext_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t r
 	/*
 	 * As per GHCB spec, requests of type MSG_REPORT_REQ also allow for
 	 * additional certificate data to be provided alongside the attestation
-	 * report via the guest-provided data pages indicated by RAX/RBX. The
-	 * certificate data is optional and requires additional KVM enablement
-	 * to provide an interface for userspace to provide it, but KVM still
-	 * needs to be able to handle extended guest requests either way. So
-	 * provide a stub implementation that will always return an empty
-	 * certificate table in the guest-provided data pages.
+	 * report via the guest-provided data pages indicated by RAX/RBX. If
+	 * userspace enables KVM_EXIT_COCO_REQ_CERTS, then exit to userspace
+	 * to fetch the certificate data. Otherwise, return an empty certificate
+	 * table in the guest-provided data pages.
 	 */
 	if (msg_type == SNP_MSG_REPORT_REQ) {
 		struct kvm_vcpu *vcpu = &svm->vcpu;
@@ -4113,6 +4135,16 @@ static int snp_handle_ext_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t r
 		if (!PAGE_ALIGNED(data_gpa))
 			goto request_invalid;
 
+		if ((vcpu->kvm->arch.coco_exit_enabled & BIT_ULL(KVM_EXIT_COCO_REQ_CERTS))) {
+			vcpu->run->exit_reason = KVM_EXIT_COCO;
+			vcpu->run->coco.nr = KVM_EXIT_COCO_REQ_CERTS;
+			vcpu->run->coco.req_certs.gfn = gpa_to_gfn(data_gpa);
+			vcpu->run->coco.req_certs.npages = data_npages;
+			vcpu->arch.complete_userspace_io = snp_complete_req_certs;
+			vcpu->run->coco.ret = 0;
+			return 0; /* fetch certs from userspace */
+		}
+
 		/*
 		 * As per GHCB spec (see "SNP Extended Guest Request"), the
 		 * certificate table is terminated by 24-bytes of zeroes.
diff --git a/include/uapi/linux/sev-guest.h b/include/uapi/linux/sev-guest.h
index fcdfea767fca..4c4ed8bc71d7 100644
--- a/include/uapi/linux/sev-guest.h
+++ b/include/uapi/linux/sev-guest.h
@@ -95,5 +95,13 @@ struct snp_ext_report_req {
 
 #define SNP_GUEST_VMM_ERR_INVALID_LEN	1
 #define SNP_GUEST_VMM_ERR_BUSY		2
+/*
+ * The GHCB spec essentially states that all non-zero error codes other than
+ * those explicitly defined above should be treated as an error by the guest.
+ * Define a generic error to cover that case, and choose a value that is not
+ * likely to overlap with new explicit error codes should more be added to
+ * the GHCB spec later.
+ */
+#define SNP_GUEST_VMM_ERR_GENERIC       ((u32)~0U)
 
 #endif /* __UAPI_LINUX_SEV_GUEST_H_ */
-- 
2.25.1