KVM: x86: Report error when setting CPUID if Hyper-V allocation fails

Return -ENOMEM back to userspace if allocating the Hyper-V vCPU struct
fails when enabling Hyper-V in guest CPUID.  Silently ignoring failure
means that KVM will not have an up-to-date CPUID cache if allocating the
struct succeeds later on, e.g. when activating SynIC.

Rejecting the CPUID operation also guarantess that vcpu->arch.hyperv is
non-NULL if hyperv_enabled is true, which will allow for additional
cleanup, e.g. in the eVMCS code.

Note, the initialization needs to be done before CPUID is set, and more
subtly before kvm_check_cpuid(), which potentially enables dynamic
XFEATURES.  Sadly, there's no easy way to avoid exposing Hyper-V details
to CPUID or vice versa.  Expose kvm_hv_vcpu_init() and the Hyper-V CPUID
signature to CPUID instead of exposing cpuid_entry2_find() outside of
CPUID code.  It's hard to envision kvm_hv_vcpu_init() being misused,
whereas cpuid_entry2_find() absolutely shouldn't be used outside of core
CPUID code.

Fixes: 10d7bf1e46dc ("KVM: x86: hyper-v: Cache guest CPUID leaves determining features availability")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Link: https://lore.kernel.org/r/20220830133737.1539624-6-vkuznets@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 14 insertions, 16 deletions

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 8aadd31ed058..bf4729e8cc80 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -38,9 +38,6 @@
 #include "irq.h"
 #include "fpu.h"
 
-/* "Hv#1" signature */
-#define HYPERV_CPUID_SIGNATURE_EAX 0x31237648
-
 #define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_VCPUS, 64)
 
 static void stimer_mark_pending(struct kvm_vcpu_hv_stimer *stimer,
@@ -934,7 +931,7 @@ static void stimer_init(struct kvm_vcpu_hv_stimer *stimer, int timer_index)
 	stimer_prepare_msg(stimer);
 }
 
-static int kvm_hv_vcpu_init(struct kvm_vcpu *vcpu)
+int kvm_hv_vcpu_init(struct kvm_vcpu *vcpu)
 {
 	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
 	int i;
@@ -1984,26 +1981,27 @@ ret_success:
 	return HV_STATUS_SUCCESS;
 }
 
-void kvm_hv_set_cpuid(struct kvm_vcpu *vcpu)
+void kvm_hv_set_cpuid(struct kvm_vcpu *vcpu, bool hyperv_enabled)
 {
+	struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
 	struct kvm_cpuid_entry2 *entry;
-	struct kvm_vcpu_hv *hv_vcpu;
 
-	entry = kvm_find_cpuid_entry(vcpu, HYPERV_CPUID_INTERFACE);
-	if (entry && entry->eax == HYPERV_CPUID_SIGNATURE_EAX) {
-		vcpu->arch.hyperv_enabled = true;
-	} else {
-		vcpu->arch.hyperv_enabled = false;
-		return;
-	}
+	vcpu->arch.hyperv_enabled = hyperv_enabled;
 
-	if (kvm_hv_vcpu_init(vcpu))
+	if (!hv_vcpu) {
+		/*
+		 * KVM should have already allocated kvm_vcpu_hv if Hyper-V is
+		 * enabled in CPUID.
+		 */
+		WARN_ON_ONCE(vcpu->arch.hyperv_enabled);
 		return;
-
-	hv_vcpu = to_hv_vcpu(vcpu);
+	}
 
 	memset(&hv_vcpu->cpuid_cache, 0, sizeof(hv_vcpu->cpuid_cache));
 
+	if (!vcpu->arch.hyperv_enabled)
+		return;
+
 	entry = kvm_find_cpuid_entry(vcpu, HYPERV_CPUID_FEATURES);
 	if (entry) {
 		hv_vcpu->cpuid_cache.features_eax = entry->eax;