[PATCH v11 35/35] KVM: SEV: Provide support for SNP_EXTENDED_GUEST_REQUEST NAE event

Michael Roth
2023-12-30T17:34:29Z

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 that firmware will use when signing attestation reports,
make use of the transaction/synchronization mechanisms added by the
SNP_SET_CONFIG_{START,END} SEV device ioctls such that the guest will be
told to retry the request when an update to reported TCB or
userspace-supplied certificates may have occurred or is in progress
while an extended guest request is being processed.

Signed-off-by: Michael Roth <michael.roth@amd.com>
---
Documentation/virt/kvm/api.rst | 26 ++++++++++++
arch/x86/include/asm/sev.h | 4 ++
arch/x86/kvm/svm/sev.c | 75 ++++++++++++++++++++++++++++++++++
arch/x86/kvm/svm/svm.h | 3 ++
arch/x86/virt/svm/sev.c | 20 +++++++++
include/uapi/linux/kvm.h | 6 +++
6 files changed, 134 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 2a526b4f8e06..960e2153d468 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7037,6 +7037,7 @@ 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 {
@@ -7050,6 +7051,11 @@ 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;
};
};

@@ -7079,6 +7085,26 @@ 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 925578ad34e6..25f533827d62 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -270,6 +270,8 @@ int rmp_make_shared(u64 pfn, enum pg_level level);
void snp_leak_pages(u64 pfn, unsigned int npages);
u64 snp_config_transaction_start(void);
u64 snp_config_transaction_end(void);
+u64 snp_config_transaction_get_id(void);
+bool snp_config_transaction_is_stale(u64 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; }
@@ -284,6 +286,8 @@ static inline int rmp_make_shared(u64 pfn, enum pg_level level) { return -ENODEV
static inline void snp_leak_pages(u64 pfn, unsigned int npages) {}
static inline u64 snp_config_transaction_start(void) { return 0; }
static inline u64 snp_config_transaction_end(void) { return 0; }
+static inline u64 snp_config_transaction_get_id(void) { return 0; }
+static inline bool snp_config_transaction_is_stale(u64 id) { return false; }
#endif

#endif
diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 05051e36926d..dda195999c42 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3111,6 +3111,7 @@ 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;
@@ -3618,6 +3619,77 @@ 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;
+
+ if (!__snp_handle_guest_req(kvm, control->exit_info_1, control->exit_info_2,
+ &fw_err))
+ vmm_ret = SNP_GUEST_VMM_ERR_GENERIC;
+
+ /*
+ * Give errors related to stale transactions precedence to provide more
+ * potential options for servicing firmware while guests are running.
+ */
+ if (snp_config_transaction_is_stale(svm->snp_transaction_id))
+ vmm_ret = SNP_GUEST_VMM_ERR_BUSY;
+
+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_config_transaction_get_id();
+ if (snp_config_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;
@@ -3882,6 +3954,9 @@ 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 a56109e100ac..a2ac6dc3a79a 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -307,6 +307,9 @@ 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 fc9e1b7fc187..fee273a84030 100644
--- a/arch/x86/virt/svm/sev.c
+++ b/arch/x86/virt/svm/sev.c
@@ -542,3 +542,23 @@ u64 snp_config_transaction_end(void)
return id;
}
EXPORT_SYMBOL_GPL(snp_config_transaction_end);
+
+u64 snp_config_transaction_get_id(void)
+{
+ return snp_transaction_id;
+}
+EXPORT_SYMBOL_GPL(snp_config_transaction_get_id);
+
+bool snp_config_transaction_is_stale(u64 id)
+{
+ bool stale = false;
+
+ mutex_lock(&snp_transaction_lock);
+ if (snp_transaction_pending ||
+ id != snp_transaction_id)
+ stale = true;
+ mutex_unlock(&snp_transaction_lock);
+
+ return stale;
+}
+EXPORT_SYMBOL_GPL(snp_config_transaction_is_stale);
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index e0599144387b..fe8994b95de9 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -170,6 +170,7 @@ 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 {
@@ -183,6 +184,11 @@ 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