KVM: x86: hyper-v: Honor HV_DEBUGGING privilege bit

Hyper-V partition must possess 'HV_DEBUGGING' privilege to issue
HVCALL_POST_DEBUG_DATA/HVCALL_RETRIEVE_DEBUG_DATA/
HVCALL_RESET_DEBUG_SESSION hypercalls.

Note, when SynDBG is disabled hv_check_hypercall_access() returns
'true' (like for any other unknown hypercall) so the result will
be HV_STATUS_INVALID_HYPERCALL_CODE and not HV_STATUS_ACCESS_DENIED.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210521095204.2161214-25-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -2103,6 +2103,15 @@ static bool hv_check_hypercall_access(struct kvm_vcpu_hv *hv_vcpu, u16 code)
 		return hv_vcpu->cpuid_cache.features_ebx & HV_POST_MESSAGES;
 	case HVCALL_SIGNAL_EVENT:
 		return hv_vcpu->cpuid_cache.features_ebx & HV_SIGNAL_EVENTS;
+	case HVCALL_POST_DEBUG_DATA:
+	case HVCALL_RETRIEVE_DEBUG_DATA:
+	case HVCALL_RESET_DEBUG_SESSION:
+		/*
+		 * Return 'true' when SynDBG is disabled so the resulting code
+		 * will be HV_STATUS_INVALID_HYPERCALL_CODE.
+		 */
+		return !kvm_hv_is_syndbg_enabled(hv_vcpu->vcpu) ||
+			hv_vcpu->cpuid_cache.features_ebx & HV_DEBUGGING;
 	default:
 		break;
 	}