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From: Michael Roth <michael.roth@amd.com>
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Subject: [PATCH v14 22/22] KVM: SEV: Provide support for SNP_EXTENDED_GUEST_REQUEST NAE event
Date: Sun, 21 Apr 2024 13:01:22 -0500
Message-ID: <20240421180122.1650812-23-michael.roth@amd.com>
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Version 2 of GHCB specification added support for the SNP Extended Guest
Request Message NAE event. This event serves a nearly identical purpose
to the previously-added SNP_GUEST_REQUEST event, but allows for
additional certificate data to be supplied via an additional
guest-supplied buffer to be used mainly for verifying the signature of
an attestation report as returned by firmware.

This certificate data is supplied by userspace, so unlike with
SNP_GUEST_REQUEST events, SNP_EXTENDED_GUEST_REQUEST events are first
forwarded to userspace via a KVM_EXIT_VMGEXIT exit type, and then the
firmware request is made only afterward.

Implement handling for these events.

Since there is a potential for race conditions where the
userspace-supplied certificate data may be out-of-sync relative to the
reported TCB or VLEK that firmware will use when signing attestation
reports, make use of the synchronization mechanisms wired up to the
SNP_{PAUSE,RESUME}_ATTESTATION SEV device ioctls such that the guest
will be told to retry the request while attestation has been paused due
to an update being underway on the system.

Signed-off-by: Michael Roth <michael.roth@amd.com>
---
 Documentation/virt/kvm/api.rst | 26 +++++++++++
 arch/x86/include/asm/sev.h     |  6 +++
 arch/x86/kvm/svm/sev.c         | 82 ++++++++++++++++++++++++++++++++++
 arch/x86/kvm/svm/svm.h         |  3 ++
 arch/x86/virt/svm/sev.c        | 37 +++++++++++++++
 include/uapi/linux/kvm.h       |  6 +++
 6 files changed, 160 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 85099198a10f..6cf186ed8f66 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7066,6 +7066,7 @@ values in kvm_run even if the corresponding bit in kvm_dirty_regs is not set.
 		struct kvm_user_vmgexit {
 		#define KVM_USER_VMGEXIT_PSC_MSR	1
 		#define KVM_USER_VMGEXIT_PSC		2
+		#define KVM_USER_VMGEXIT_EXT_GUEST_REQ	3
 			__u32 type; /* KVM_USER_VMGEXIT_* type */
 			union {
 				struct {
@@ -7079,6 +7080,11 @@ values in kvm_run even if the corresponding bit in kvm_dirty_regs is not set.
 					__u64 shared_gpa;
 					__u64 ret;
 				} psc;
+				struct {
+					__u64 data_gpa;
+					__u64 data_npages;
+					__u32 ret;
+				} ext_guest_req;
 			};
 		};
 
@@ -7108,6 +7114,26 @@ private/shared state. Userspace will return a value in 'ret' that is in
 agreement with the GHCB-defined return values that the guest will expect
 in the SW_EXITINFO2 field of the GHCB in response to these requests.
 
+For the KVM_USER_VMGEXIT_EXT_GUEST_REQ type, the ext_guest_req union type
+is used. The kernel will supply in 'data_gpa' the value the guest supplies
+via the RAX field of the GHCB when issued extended guest requests.
+'data_npages' will similarly contain the value the guest supplies in RBX
+denoting the number of shared pages available to write the certificate
+data into.
+
+  - If the supplied number of pages is sufficient, userspace should write
+    the certificate data blob (in the format defined by the GHCB spec) in
+    the address indicated by 'data_gpa' and set 'ret' to 0.
+
+  - If the number of pages supplied is not sufficient, userspace must write
+    the required number of pages in 'data_npages' and then set 'ret' to 1.
+
+  - If userspace is temporarily unable to handle the request, 'ret' should
+    be set to 2 to inform the guest to retry later.
+
+  - If some other error occurred, userspace should set 'ret' to a non-zero
+    value that is distinct from the specific return values mentioned above.
+
 6. Capabilities that can be enabled on vCPUs
 ============================================
 
diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index ee24ef815e35..dfc28ac4dd0e 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -276,6 +276,9 @@ void snp_leak_pages(u64 pfn, unsigned int npages);
 void kdump_sev_callback(void);
 int snp_pause_attestation(u64 *transaction_id);
 void snp_resume_attestation(u64 *transaction_id);
+u64 snp_transaction_get_id(void);
+bool __snp_transaction_is_stale(u64 transaction_id);
+bool snp_transaction_is_stale(u64 transaction_id);
 #else
 static inline bool snp_probe_rmptable_info(void) { return false; }
 static inline int snp_lookup_rmpentry(u64 pfn, bool *assigned, int *level) { return -ENODEV; }
@@ -291,6 +294,9 @@ static inline void snp_leak_pages(u64 pfn, unsigned int npages) {}
 static inline void kdump_sev_callback(void) { }
 static inline int snp_pause_attestation(u64 *transaction_id) { return 0; }
 static inline void snp_resume_attestation(u64 *transaction_id) {}
+static inline u64 snp_transaction_get_id(void) { return 0; }
+static inline bool __snp_transaction_is_stale(u64 transaction_id) { return false; }
+static inline bool snp_transaction_is_stale(u64 transaction_id) { return false; }
 #endif
 
 #endif
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 68db390b19d0..1cec466e593b 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3292,6 +3292,7 @@ static int sev_es_validate_vmgexit(struct vcpu_svm *svm)
 	case SVM_VMGEXIT_PSC:
 	case SVM_VMGEXIT_TERM_REQUEST:
 	case SVM_VMGEXIT_GUEST_REQUEST:
+	case SVM_VMGEXIT_EXT_GUEST_REQUEST:
 		break;
 	default:
 		reason = GHCB_ERR_INVALID_EVENT;
@@ -3812,6 +3813,84 @@ static void snp_handle_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t resp
 	ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, SNP_GUEST_ERR(vmm_ret, fw_err));
 }
 
+static int snp_complete_ext_guest_req(struct kvm_vcpu *vcpu)
+{
+	struct vcpu_svm *svm = to_svm(vcpu);
+	struct vmcb_control_area *control;
+	struct kvm *kvm = vcpu->kvm;
+	sev_ret_code fw_err = 0;
+	int vmm_ret;
+
+	vmm_ret = vcpu->run->vmgexit.ext_guest_req.ret;
+	if (vmm_ret) {
+		if (vmm_ret == SNP_GUEST_VMM_ERR_INVALID_LEN)
+			vcpu->arch.regs[VCPU_REGS_RBX] =
+				vcpu->run->vmgexit.ext_guest_req.data_npages;
+		goto abort_request;
+	}
+
+	control = &svm->vmcb->control;
+
+	/*
+	 * To avoid the message sequence number getting out of sync between the
+	 * actual value seen by firmware verses the value expected by the guest,
+	 * make sure attestations can't get paused on the write-side at this
+	 * point by holding the lock for the entire duration of the firmware
+	 * request so that there is no situation where SNP_GUEST_VMM_ERR_BUSY
+	 * would need to be returned after firmware sees the request.
+	 */
+	mutex_lock(&snp_pause_attestation_lock);
+
+	if (__snp_transaction_is_stale(svm->snp_transaction_id))
+		vmm_ret = SNP_GUEST_VMM_ERR_BUSY;
+	else if (!__snp_handle_guest_req(kvm, control->exit_info_1,
+					 control->exit_info_2, &fw_err))
+		vmm_ret = SNP_GUEST_VMM_ERR_GENERIC;
+
+	mutex_unlock(&snp_pause_attestation_lock);
+
+abort_request:
+	ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, SNP_GUEST_ERR(vmm_ret, fw_err));
+
+	return 1; /* resume guest */
+}
+
+static int snp_begin_ext_guest_req(struct kvm_vcpu *vcpu)
+{
+	int vmm_ret = SNP_GUEST_VMM_ERR_GENERIC;
+	struct vcpu_svm *svm = to_svm(vcpu);
+	unsigned long data_npages;
+	sev_ret_code fw_err;
+	gpa_t data_gpa;
+
+	if (!sev_snp_guest(vcpu->kvm))
+		goto abort_request;
+
+	data_gpa = vcpu->arch.regs[VCPU_REGS_RAX];
+	data_npages = vcpu->arch.regs[VCPU_REGS_RBX];
+
+	if (!IS_ALIGNED(data_gpa, PAGE_SIZE))
+		goto abort_request;
+
+	svm->snp_transaction_id = snp_transaction_get_id();
+	if (snp_transaction_is_stale(svm->snp_transaction_id)) {
+		vmm_ret = SNP_GUEST_VMM_ERR_BUSY;
+		goto abort_request;
+	}
+
+	vcpu->run->exit_reason = KVM_EXIT_VMGEXIT;
+	vcpu->run->vmgexit.type = KVM_USER_VMGEXIT_EXT_GUEST_REQ;
+	vcpu->run->vmgexit.ext_guest_req.data_gpa = data_gpa;
+	vcpu->run->vmgexit.ext_guest_req.data_npages = data_npages;
+	vcpu->arch.complete_userspace_io = snp_complete_ext_guest_req;
+
+	return 0; /* forward request to userspace */
+
+abort_request:
+	ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, SNP_GUEST_ERR(vmm_ret, fw_err));
+	return 1; /* resume guest */
+}
+
 static int sev_handle_vmgexit_msr_protocol(struct vcpu_svm *svm)
 {
 	struct vmcb_control_area *control = &svm->vmcb->control;
@@ -4076,6 +4155,9 @@ int sev_handle_vmgexit(struct kvm_vcpu *vcpu)
 		snp_handle_guest_req(svm, control->exit_info_1, control->exit_info_2);
 		ret = 1;
 		break;
+	case SVM_VMGEXIT_EXT_GUEST_REQUEST:
+		ret = snp_begin_ext_guest_req(vcpu);
+		break;
 	case SVM_VMGEXIT_UNSUPPORTED_EVENT:
 		vcpu_unimpl(vcpu,
 			    "vmgexit: unsupported event - exit_info_1=%#llx, exit_info_2=%#llx\n",
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 8a8ee475ad86..28140bc8af27 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -303,6 +303,9 @@ struct vcpu_svm {
 
 	/* Guest GIF value, used when vGIF is not enabled */
 	bool guest_gif;
+
+	/* Transaction ID associated with SNP config updates */
+	u64 snp_transaction_id;
 };
 
 struct svm_cpu_data {
diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
index b75f2e7d4012..f1f7486a3dcf 100644
--- a/arch/x86/virt/svm/sev.c
+++ b/arch/x86/virt/svm/sev.c
@@ -72,6 +72,7 @@ static unsigned long snp_nr_leaked_pages;
 
 /* For synchronizing TCB/certificate updates with extended guest requests */
 DEFINE_MUTEX(snp_pause_attestation_lock);
+EXPORT_SYMBOL_GPL(snp_pause_attestation_lock);
 static u64 snp_transaction_id;
 static bool snp_attestation_paused;
 
@@ -611,3 +612,39 @@ void snp_resume_attestation(u64 *transaction_id)
 	mutex_unlock(&snp_pause_attestation_lock);
 }
 EXPORT_SYMBOL_GPL(snp_resume_attestation);
+
+u64 snp_transaction_get_id(void)
+{
+	u64 id;
+
+	mutex_lock(&snp_pause_attestation_lock);
+	id = snp_transaction_id;
+	mutex_unlock(&snp_pause_attestation_lock);
+
+	return id;
+}
+EXPORT_SYMBOL_GPL(snp_transaction_get_id);
+
+/* Must be called with snp_pause_attestion_lock held */
+bool __snp_transaction_is_stale(u64 transaction_id)
+{
+	lockdep_assert_held(&snp_pause_attestation_lock);
+
+	return (snp_attestation_paused ||
+		transaction_id != snp_transaction_id);
+}
+EXPORT_SYMBOL_GPL(__snp_transaction_is_stale);
+
+bool snp_transaction_is_stale(u64 transaction_id)
+{
+	bool stale;
+
+	mutex_lock(&snp_pause_attestation_lock);
+
+	stale = __snp_transaction_is_stale(transaction_id);
+
+	mutex_unlock(&snp_pause_attestation_lock);
+
+	return stale;
+}
+EXPORT_SYMBOL_GPL(snp_transaction_is_stale);
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index e33c48bfbd67..585de3a2591e 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -138,6 +138,7 @@ struct kvm_xen_exit {
 struct kvm_user_vmgexit {
 #define KVM_USER_VMGEXIT_PSC_MSR	1
 #define KVM_USER_VMGEXIT_PSC		2
+#define KVM_USER_VMGEXIT_EXT_GUEST_REQ	3
 	__u32 type; /* KVM_USER_VMGEXIT_* type */
 	union {
 		struct {
@@ -151,6 +152,11 @@ struct kvm_user_vmgexit {
 			__u64 shared_gpa;
 			__u64 ret;
 		} psc;
+		struct {
+			__u64 data_gpa;
+			__u64 data_npages;
+			__u32 ret;
+		} ext_guest_req;
 	};
 };
 
-- 
2.25.1