linux-kernel - [PATCH v1 5/5] KVM: SEV: Add certificate support for SNP_EXTENDED_GUEST

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Date: Fri, 21 Jun 2024 08:40:41 -0500
From: Michael Roth <michael.roth@....com>
To: <kvm@...r.kernel.org>
CC: <linux-coco@...ts.linux.dev>, <linux-kernel@...r.kernel.org>,
	<x86@...nel.org>, <pbonzini@...hat.com>, <seanjc@...gle.com>,
	<jroedel@...e.de>, <thomas.lendacky@....com>, <pgonda@...gle.com>,
	<ashish.kalra@....com>, <bp@...en8.de>, <pankaj.gupta@....com>,
	<liam.merwick@...cle.com>
Subject: [PATCH v1 5/5] KVM: SEV: Add certificate support for SNP_EXTENDED_GUEST_REQUEST events

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.

This implements the actual handling for KVM_EXIT_COCO_REQ_CERTS, so
allow it to be enabled via KVM_CAP_EXIT_COCO.

Signed-off-by: Michael Roth <michael.roth@....com>
---
 arch/x86/kvm/svm/sev.c | 41 +++++++++++++++++++++++++++++++++++------
 arch/x86/kvm/x86.c     |  2 +-
 2 files changed, 36 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index b5dcf36b50f5..8af56a4544d1 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -4006,6 +4006,27 @@ 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.req_certs.ret) {
+		if (vcpu->run->coco.req_certs.ret == KVM_EXIT_COCO_REQ_CERTS_ERR_INVALID_LEN) {
+			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 {
+			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);
+}
+
 /*
  * As per GHCB spec (see "SNP Extended Guest Request"), the certificate table
  * is terminated by 24-bytes of zeroes.
@@ -4027,12 +4048,10 @@ 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;
@@ -4048,6 +4067,16 @@ static int snp_handle_ext_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t r
 		if (!PAGE_ALIGNED(data_gpa))
 			goto abort_request;
 
+		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->run->coco.req_certs.ret = 0;
+			vcpu->arch.complete_userspace_io = snp_complete_req_certs;
+			return 0; /* fetch certs from userspace */
+		}
+
 		if (data_npages &&
 		    kvm_write_guest(kvm, data_gpa, empty_certs_table,
 				    sizeof(empty_certs_table)))
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 94c3a82b02c7..1a0087af1714 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -125,7 +125,7 @@ static u64 __read_mostly cr4_reserved_bits = CR4_RESERVED_BITS;
 #define KVM_X2APIC_API_VALID_FLAGS (KVM_X2APIC_API_USE_32BIT_IDS | \
                                     KVM_X2APIC_API_DISABLE_BROADCAST_QUIRK)
 
-#define KVM_EXIT_COCO_VALID_MASK 0
+#define KVM_EXIT_COCO_VALID_MASK BIT_ULL(KVM_EXIT_COCO_REQ_CERTS)
 
 static void update_cr8_intercept(struct kvm_vcpu *vcpu);
 static void process_nmi(struct kvm_vcpu *vcpu);
-- 
2.25.1