k8s.io/kubernetes@v1.31.0-alpha.0.0.20240520171757-56147500dadc/cmd/kubeadm/app/phases/kubeconfig/kubeconfig.go

/*
Copyright 2018 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package kubeconfig

import (
	"bytes"
	"context"
	"crypto"
	"crypto/x509"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"time"

	"github.com/pkg/errors"

	rbac "k8s.io/api/rbac/v1"
	apierrors "k8s.io/apimachinery/pkg/api/errors"
	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
	"k8s.io/apimachinery/pkg/util/wait"
	clientset "k8s.io/client-go/kubernetes"
	"k8s.io/client-go/tools/clientcmd"
	clientcmdapi "k8s.io/client-go/tools/clientcmd/api"
	certutil "k8s.io/client-go/util/cert"
	"k8s.io/client-go/util/keyutil"
	"k8s.io/klog/v2"

	kubeadmapi "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm"
	kubeadmconstants "k8s.io/kubernetes/cmd/kubeadm/app/constants"
	certsphase "k8s.io/kubernetes/cmd/kubeadm/app/phases/certs"
	kubeadmutil "k8s.io/kubernetes/cmd/kubeadm/app/util"
	kubeconfigutil "k8s.io/kubernetes/cmd/kubeadm/app/util/kubeconfig"
	"k8s.io/kubernetes/cmd/kubeadm/app/util/pkiutil"
)

const (
	errInvalid = "invalid argument"
	errExist   = "file already exists"
)

// clientCertAuth struct holds info required to build a client certificate to provide authentication info in a kubeconfig object
type clientCertAuth struct {
	CAKey         crypto.Signer
	Organizations []string
}

// tokenAuth struct holds info required to use a token to provide authentication info in a kubeconfig object
type tokenAuth struct {
	Token string `datapolicy:"token"`
}

// kubeConfigSpec struct holds info required to build a KubeConfig object
type kubeConfigSpec struct {
	CACert             *x509.Certificate
	APIServer          string
	ClientName         string
	ClientCertNotAfter time.Time
	TokenAuth          *tokenAuth      `datapolicy:"token"`
	ClientCertAuth     *clientCertAuth `datapolicy:"security-key"`
}

// CreateJoinControlPlaneKubeConfigFiles will create and write to disk the kubeconfig files required by kubeadm
// join --control-plane workflow, plus the admin kubeconfig file used by the administrator and kubeadm itself; the
// kubelet.conf file must not be created because it will be created and signed by the kubelet TLS bootstrap process.
// When not using external CA mode, if a kubeconfig file already exists it is used only if evaluated equal,
// otherwise an error is returned. For external CA mode, the creation of kubeconfig files is skipped.
func CreateJoinControlPlaneKubeConfigFiles(outDir string, cfg *kubeadmapi.InitConfiguration) error {
	var externalCA bool
	caKeyPath := filepath.Join(cfg.CertificatesDir, kubeadmconstants.CAKeyName)
	if _, err := os.Stat(caKeyPath); os.IsNotExist(err) {
		externalCA = true
	}

	files := []string{
		kubeadmconstants.AdminKubeConfigFileName,
		kubeadmconstants.ControllerManagerKubeConfigFileName,
		kubeadmconstants.SchedulerKubeConfigFileName,
	}

	for _, file := range files {
		if externalCA {
			fmt.Printf("[kubeconfig] External CA mode: Using user provided %s\n", file)
			continue
		}
		if err := createKubeConfigFiles(outDir, cfg, file); err != nil {
			return err
		}
	}
	return nil
}

// CreateKubeConfigFile creates a kubeconfig file.
// If the kubeconfig file already exists, it is used only if evaluated equal; otherwise an error is returned.
func CreateKubeConfigFile(kubeConfigFileName string, outDir string, cfg *kubeadmapi.InitConfiguration) error {
	klog.V(1).Infof("creating kubeconfig file for %s", kubeConfigFileName)
	return createKubeConfigFiles(outDir, cfg, kubeConfigFileName)
}

// createKubeConfigFiles creates all the requested kubeconfig files.
// If kubeconfig files already exists, they are used only if evaluated equal; otherwise an error is returned.
func createKubeConfigFiles(outDir string, cfg *kubeadmapi.InitConfiguration, kubeConfigFileNames ...string) error {

	// gets the KubeConfigSpecs, actualized for the current InitConfiguration
	specs, err := getKubeConfigSpecs(cfg)
	if err != nil {
		return err
	}

	for _, kubeConfigFileName := range kubeConfigFileNames {
		// retrieves the KubeConfigSpec for given kubeConfigFileName
		spec, exists := specs[kubeConfigFileName]
		if !exists {
			return errors.Errorf("couldn't retrieve KubeConfigSpec for %s", kubeConfigFileName)
		}

		// builds the KubeConfig object
		config, err := buildKubeConfigFromSpec(spec, cfg.ClusterName)
		if err != nil {
			return err
		}

		// writes the kubeconfig to disk if it does not exist
		if err = createKubeConfigFileIfNotExists(outDir, kubeConfigFileName, config); err != nil {
			return err
		}
	}

	return nil
}

// getKubeConfigSpecs returns all KubeConfigSpecs actualized to the context of the current InitConfiguration
// NB. this method holds the information about how kubeadm creates kubeconfig files.
func getKubeConfigSpecs(cfg *kubeadmapi.InitConfiguration) (map[string]*kubeConfigSpec, error) {
	caCert, caKey, err := pkiutil.TryLoadCertAndKeyFromDisk(cfg.CertificatesDir, kubeadmconstants.CACertAndKeyBaseName)
	if os.IsNotExist(errors.Cause(err)) {
		return nil, errors.Wrap(err, "the CA files do not exist, please run `kubeadm init phase certs ca` to generate it")
	}
	if err != nil {
		return nil, errors.Wrap(err, "couldn't create a kubeconfig; the CA files couldn't be loaded")
	}
	// Validate period
	certsphase.CheckCertificatePeriodValidity(kubeadmconstants.CACertAndKeyBaseName, caCert)

	configs, err := getKubeConfigSpecsBase(cfg)
	if err != nil {
		return nil, err
	}
	for _, spec := range configs {
		spec.CACert = caCert
		spec.ClientCertAuth.CAKey = caKey
	}
	return configs, nil
}

// buildKubeConfigFromSpec creates a kubeconfig object for the given kubeConfigSpec
func buildKubeConfigFromSpec(spec *kubeConfigSpec, clustername string) (*clientcmdapi.Config, error) {

	// If this kubeconfig should use token
	if spec.TokenAuth != nil {
		// create a kubeconfig with a token
		return kubeconfigutil.CreateWithToken(
			spec.APIServer,
			clustername,
			spec.ClientName,
			pkiutil.EncodeCertPEM(spec.CACert),
			spec.TokenAuth.Token,
		), nil
	}

	// otherwise, create a client cert
	clientCertConfig := newClientCertConfigFromKubeConfigSpec(spec)

	clientCert, clientKey, err := pkiutil.NewCertAndKey(spec.CACert, spec.ClientCertAuth.CAKey, &clientCertConfig)
	if err != nil {
		return nil, errors.Wrapf(err, "failure while creating %s client certificate", spec.ClientName)
	}

	encodedClientKey, err := keyutil.MarshalPrivateKeyToPEM(clientKey)
	if err != nil {
		return nil, errors.Wrapf(err, "failed to marshal private key to PEM")
	}
	// create a kubeconfig with the client certs
	return kubeconfigutil.CreateWithCerts(
		spec.APIServer,
		clustername,
		spec.ClientName,
		pkiutil.EncodeCertPEM(spec.CACert),
		encodedClientKey,
		pkiutil.EncodeCertPEM(clientCert),
	), nil
}

func newClientCertConfigFromKubeConfigSpec(spec *kubeConfigSpec) pkiutil.CertConfig {
	return pkiutil.CertConfig{
		Config: certutil.Config{
			CommonName:   spec.ClientName,
			Organization: spec.ClientCertAuth.Organizations,
			Usages:       []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},
		},
		NotAfter: spec.ClientCertNotAfter,
	}
}

// validateKubeConfig check if the kubeconfig file exist and has the expected CA and server URL
func validateKubeConfig(outDir, filename string, config *clientcmdapi.Config) error {
	kubeConfigFilePath := filepath.Join(outDir, filename)

	if _, err := os.Stat(kubeConfigFilePath); err != nil {
		return err
	}

	// The kubeconfig already exists, let's check if it has got the same CA and server URL
	currentConfig, err := clientcmd.LoadFromFile(kubeConfigFilePath)
	if err != nil {
		return errors.Wrapf(err, "failed to load kubeconfig file %s that already exists on disk", kubeConfigFilePath)
	}

	expectedCtx, exists := config.Contexts[config.CurrentContext]
	if !exists {
		return errors.Errorf("failed to find expected context %s", config.CurrentContext)
	}
	expectedCluster := expectedCtx.Cluster
	currentCtx, exists := currentConfig.Contexts[currentConfig.CurrentContext]
	if !exists {
		return errors.Errorf("failed to find CurrentContext in Contexts of the kubeconfig file %s", kubeConfigFilePath)
	}
	currentCluster := currentCtx.Cluster
	if currentConfig.Clusters[currentCluster] == nil {
		return errors.Errorf("failed to find the given CurrentContext Cluster in Clusters of the kubeconfig file %s", kubeConfigFilePath)
	}

	// Make sure the compared CAs are whitespace-trimmed. The function clientcmd.LoadFromFile() just decodes
	// the base64 CA and places it raw in the v1.Config object. In case the user has extra whitespace
	// in the CA they used to create a kubeconfig this comparison to a generated v1.Config will otherwise fail.
	caCurrent := bytes.TrimSpace(currentConfig.Clusters[currentCluster].CertificateAuthorityData)
	if len(caCurrent) == 0 {
		// fallback to load CA cert data from external CA file
		clusterCAFilePath := currentConfig.Clusters[currentCluster].CertificateAuthority
		if len(clusterCAFilePath) > 0 {
			clusterCABytes, err := os.ReadFile(clusterCAFilePath)
			if err != nil {
				klog.Warningf("failed to load CA cert from %q for kubeconfig %q, %v", clusterCAFilePath, kubeConfigFilePath, err)
			} else {
				caCurrent = bytes.TrimSpace(clusterCABytes)
			}
		}
	}
	caExpected := bytes.TrimSpace(config.Clusters[expectedCluster].CertificateAuthorityData)

	// If the current CA cert on disk doesn't match the expected CA cert, error out because we have a file, but it's stale
	if !bytes.Equal(caCurrent, caExpected) {
		return errors.Errorf("a kubeconfig file %q exists already but has got the wrong CA cert", kubeConfigFilePath)
	}
	// If the current API Server location on disk doesn't match the expected API server, show a warning
	if currentConfig.Clusters[currentCluster].Server != config.Clusters[expectedCluster].Server {
		klog.Warningf("a kubeconfig file %q exists already but has an unexpected API Server URL: expected: %s, got: %s",
			kubeConfigFilePath, config.Clusters[expectedCluster].Server, currentConfig.Clusters[currentCluster].Server)
	}

	return nil
}

// createKubeConfigFileIfNotExists saves the KubeConfig object into a file if there isn't any file at the given path.
// If there already is a kubeconfig file at the given path; kubeadm tries to load it and check if the values in the
// existing and the expected config equals. If they do; kubeadm will just skip writing the file as it's up-to-date,
// but if a file exists but has old content or isn't a kubeconfig file, this function returns an error.
func createKubeConfigFileIfNotExists(outDir, filename string, config *clientcmdapi.Config) error {
	kubeConfigFilePath := filepath.Join(outDir, filename)

	err := validateKubeConfig(outDir, filename, config)
	if err != nil {
		// Check if the file exist, and if it doesn't, just write it to disk
		if !os.IsNotExist(err) {
			return err
		}
		fmt.Printf("[kubeconfig] Writing %q kubeconfig file\n", filename)
		err = kubeconfigutil.WriteToDisk(kubeConfigFilePath, config)
		return errors.Wrapf(err, "failed to save kubeconfig file %q on disk", kubeConfigFilePath)
	}
	// kubeadm doesn't validate the existing kubeconfig file more than this (kubeadm trusts the client certs to be valid)
	// Basically, if we find a kubeconfig file with the same path; the same CA cert and the same server URL;
	// kubeadm thinks those files are equal and doesn't bother writing a new file
	fmt.Printf("[kubeconfig] Using existing kubeconfig file: %q\n", kubeConfigFilePath)

	return nil
}

// WriteKubeConfigWithClientCert writes a kubeconfig file - with a client certificate as authentication info  - to the given writer.
func WriteKubeConfigWithClientCert(out io.Writer, cfg *kubeadmapi.InitConfiguration, clientName string, organizations []string, notAfter time.Time) error {

	// creates the KubeConfigSpecs, actualized for the current InitConfiguration
	caCert, caKey, err := pkiutil.TryLoadCertAndKeyFromDisk(cfg.CertificatesDir, kubeadmconstants.CACertAndKeyBaseName)
	if err != nil {
		return errors.Wrap(err, "couldn't create a kubeconfig; the CA files couldn't be loaded")
	}
	// Validate period
	certsphase.CheckCertificatePeriodValidity(kubeadmconstants.CACertAndKeyBaseName, caCert)

	controlPlaneEndpoint, err := kubeadmutil.GetControlPlaneEndpoint(cfg.ControlPlaneEndpoint, &cfg.LocalAPIEndpoint)
	if err != nil {
		return err
	}

	spec := &kubeConfigSpec{
		ClientName: clientName,
		APIServer:  controlPlaneEndpoint,
		CACert:     caCert,
		ClientCertAuth: &clientCertAuth{
			CAKey:         caKey,
			Organizations: organizations,
		},
		ClientCertNotAfter: notAfter,
	}

	return writeKubeConfigFromSpec(out, spec, cfg.ClusterName)
}

// WriteKubeConfigWithToken writes a kubeconfig file - with a token as client authentication info - to the given writer.
func WriteKubeConfigWithToken(out io.Writer, cfg *kubeadmapi.InitConfiguration, clientName, token string, notAfter time.Time) error {

	// creates the KubeConfigSpecs, actualized for the current InitConfiguration
	caCert, _, err := pkiutil.TryLoadCertAndKeyFromDisk(cfg.CertificatesDir, kubeadmconstants.CACertAndKeyBaseName)
	if err != nil {
		return errors.Wrap(err, "couldn't create a kubeconfig; the CA files couldn't be loaded")
	}
	// Validate period
	certsphase.CheckCertificatePeriodValidity(kubeadmconstants.CACertAndKeyBaseName, caCert)

	controlPlaneEndpoint, err := kubeadmutil.GetControlPlaneEndpoint(cfg.ControlPlaneEndpoint, &cfg.LocalAPIEndpoint)
	if err != nil {
		return err
	}

	spec := &kubeConfigSpec{
		ClientName: clientName,
		APIServer:  controlPlaneEndpoint,
		CACert:     caCert,
		TokenAuth: &tokenAuth{
			Token: token,
		},
		ClientCertNotAfter: notAfter,
	}

	return writeKubeConfigFromSpec(out, spec, cfg.ClusterName)
}

// writeKubeConfigFromSpec creates a kubeconfig object from a kubeConfigSpec and writes it to the given writer.
func writeKubeConfigFromSpec(out io.Writer, spec *kubeConfigSpec, clustername string) error {

	// builds the KubeConfig object
	config, err := buildKubeConfigFromSpec(spec, clustername)
	if err != nil {
		return err
	}

	// writes the kubeconfig to disk if it not exists
	configBytes, err := clientcmd.Write(*config)
	if err != nil {
		return errors.Wrap(err, "failure while serializing admin kubeconfig")
	}

	fmt.Fprintln(out, string(configBytes))
	return nil
}

// ValidateKubeconfigsForExternalCA check if the kubeconfig file exist and has the expected CA and server URL using kubeadmapi.InitConfiguration.
func ValidateKubeconfigsForExternalCA(outDir string, cfg *kubeadmapi.InitConfiguration) error {
	// Creates a kubeconfig file with the target CA and server URL
	// to be used as a input for validating user provided kubeconfig files
	caCert, err := pkiutil.TryLoadCertFromDisk(cfg.CertificatesDir, kubeadmconstants.CACertAndKeyBaseName)
	if err != nil {
		return errors.Wrapf(err, "the CA file couldn't be loaded")
	}
	// Validate period
	certsphase.CheckCertificatePeriodValidity(kubeadmconstants.CACertAndKeyBaseName, caCert)

	// validate user provided kubeconfig files for the scheduler and controller-manager
	localAPIEndpoint, err := kubeadmutil.GetLocalAPIEndpoint(&cfg.LocalAPIEndpoint)
	if err != nil {
		return err
	}

	validationConfigLocal := kubeconfigutil.CreateBasic(localAPIEndpoint, "dummy", "dummy", pkiutil.EncodeCertPEM(caCert))
	kubeConfigFileNamesLocal := []string{
		kubeadmconstants.ControllerManagerKubeConfigFileName,
		kubeadmconstants.SchedulerKubeConfigFileName,
	}

	for _, kubeConfigFileName := range kubeConfigFileNamesLocal {
		if err = validateKubeConfig(outDir, kubeConfigFileName, validationConfigLocal); err != nil {
			return errors.Wrapf(err, "the %s file does not exists or it is not valid", kubeConfigFileName)
		}
	}

	// validate user provided kubeconfig files for the kubelet and admin
	controlPlaneEndpoint, err := kubeadmutil.GetControlPlaneEndpoint(cfg.ControlPlaneEndpoint, &cfg.LocalAPIEndpoint)
	if err != nil {
		return err
	}

	validationConfigCPE := kubeconfigutil.CreateBasic(controlPlaneEndpoint, "dummy", "dummy", pkiutil.EncodeCertPEM(caCert))
	kubeConfigFileNamesCPE := []string{
		kubeadmconstants.AdminKubeConfigFileName,
		kubeadmconstants.SuperAdminKubeConfigFileName,
		kubeadmconstants.KubeletKubeConfigFileName,
	}

	for _, kubeConfigFileName := range kubeConfigFileNamesCPE {
		if err = validateKubeConfig(outDir, kubeConfigFileName, validationConfigCPE); err != nil {
			return errors.Wrapf(err, "the %s file does not exists or it is not valid", kubeConfigFileName)
		}
	}

	return nil
}

func getKubeConfigSpecsBase(cfg *kubeadmapi.InitConfiguration) (map[string]*kubeConfigSpec, error) {
	controlPlaneEndpoint, err := kubeadmutil.GetControlPlaneEndpoint(cfg.ControlPlaneEndpoint, &cfg.LocalAPIEndpoint)
	if err != nil {
		return nil, err
	}
	localAPIEndpoint, err := kubeadmutil.GetLocalAPIEndpoint(&cfg.LocalAPIEndpoint)
	if err != nil {
		return nil, err
	}

	startTime := kubeadmutil.StartTimeUTC()
	notAfter := startTime.Add(kubeadmconstants.CertificateValidityPeriod)
	if cfg.ClusterConfiguration.CertificateValidityPeriod != nil {
		notAfter = startTime.Add(cfg.ClusterConfiguration.CertificateValidityPeriod.Duration)
	}

	return map[string]*kubeConfigSpec{
		kubeadmconstants.AdminKubeConfigFileName: {
			APIServer:  controlPlaneEndpoint,
			ClientName: "kubernetes-admin",
			ClientCertAuth: &clientCertAuth{
				Organizations: []string{kubeadmconstants.ClusterAdminsGroupAndClusterRoleBinding},
			},
			ClientCertNotAfter: notAfter,
		},
		kubeadmconstants.SuperAdminKubeConfigFileName: {
			APIServer:  controlPlaneEndpoint,
			ClientName: "kubernetes-super-admin",
			ClientCertAuth: &clientCertAuth{
				Organizations: []string{kubeadmconstants.SystemPrivilegedGroup},
			},
			ClientCertNotAfter: notAfter,
		},
		kubeadmconstants.KubeletKubeConfigFileName: {
			APIServer:  controlPlaneEndpoint,
			ClientName: fmt.Sprintf("%s%s", kubeadmconstants.NodesUserPrefix, cfg.NodeRegistration.Name),
			ClientCertAuth: &clientCertAuth{
				Organizations: []string{kubeadmconstants.NodesGroup},
			},
			ClientCertNotAfter: notAfter,
		},
		kubeadmconstants.ControllerManagerKubeConfigFileName: {
			APIServer:          localAPIEndpoint,
			ClientName:         kubeadmconstants.ControllerManagerUser,
			ClientCertAuth:     &clientCertAuth{},
			ClientCertNotAfter: notAfter,
		},
		kubeadmconstants.SchedulerKubeConfigFileName: {
			APIServer:          localAPIEndpoint,
			ClientName:         kubeadmconstants.SchedulerUser,
			ClientCertAuth:     &clientCertAuth{},
			ClientCertNotAfter: notAfter,
		},
	}, nil
}

func createKubeConfigAndCSR(kubeConfigDir string, kubeadmConfig *kubeadmapi.InitConfiguration, name string, spec *kubeConfigSpec) error {
	if kubeConfigDir == "" {
		return errors.Errorf("%s: kubeConfigDir was empty", errInvalid)
	}
	if kubeadmConfig == nil {
		return errors.Errorf("%s: kubeadmConfig was nil", errInvalid)
	}
	if name == "" {
		return errors.Errorf("%s: name was empty", errInvalid)
	}
	if spec == nil {
		return errors.Errorf("%s: spec was nil", errInvalid)
	}
	kubeConfigPath := filepath.Join(kubeConfigDir, name)
	if _, err := os.Stat(kubeConfigPath); err == nil {
		return errors.Errorf("%s: kube config: %s", errExist, kubeConfigPath)
	} else if !os.IsNotExist(err) {
		return errors.Wrapf(err, "unexpected error while checking if file exists: %s", kubeConfigPath)
	}
	if pkiutil.CSROrKeyExist(kubeConfigDir, name) {
		return errors.Errorf("%s: csr: %s", errExist, kubeConfigPath)
	}

	clientCertConfig := newClientCertConfigFromKubeConfigSpec(spec)

	clientKey, err := pkiutil.NewPrivateKey(clientCertConfig.EncryptionAlgorithm)
	if err != nil {
		return err
	}
	clientCSR, err := pkiutil.NewCSR(clientCertConfig, clientKey)
	if err != nil {
		return err
	}

	encodedClientKey, err := keyutil.MarshalPrivateKeyToPEM(clientKey)
	if err != nil {
		return err
	}

	var (
		emptyCACert     []byte
		emptyClientCert []byte
	)

	// create a kubeconfig with the client certs
	config := kubeconfigutil.CreateWithCerts(
		spec.APIServer,
		kubeadmConfig.ClusterName,
		spec.ClientName,
		emptyCACert,
		encodedClientKey,
		emptyClientCert,
	)

	if err := kubeconfigutil.WriteToDisk(kubeConfigPath, config); err != nil {
		return err
	}
	// Write CSR to disk
	if err := pkiutil.WriteCSR(kubeConfigDir, name, clientCSR); err != nil {
		return err
	}
	return nil
}

// CreateDefaultKubeConfigsAndCSRFiles is used in ExternalCA mode to create
// kubeconfig files and adjacent CSR files.
func CreateDefaultKubeConfigsAndCSRFiles(out io.Writer, kubeConfigDir string, kubeadmConfig *kubeadmapi.InitConfiguration) error {
	kubeConfigs, err := getKubeConfigSpecsBase(kubeadmConfig)
	if err != nil {
		return err
	}
	if out != nil {
		fmt.Fprintf(out, "generating keys and CSRs in %s\n", kubeConfigDir)
	}
	for name, spec := range kubeConfigs {
		if err := createKubeConfigAndCSR(kubeConfigDir, kubeadmConfig, name, spec); err != nil {
			return err
		}
		if out != nil {
			fmt.Fprintf(out, "  %s\n", name)
		}
	}
	return nil
}

// EnsureRBACFunc defines a function type that can be passed to EnsureAdminClusterRoleBinding().
type EnsureRBACFunc func(context.Context, clientset.Interface, clientset.Interface, time.Duration, time.Duration) (clientset.Interface, error)

// EnsureAdminClusterRoleBinding constructs a client from admin.conf and optionally
// constructs a client from super-admin.conf if the file exists. It then proceeds
// to pass the clients to EnsureAdminClusterRoleBindingImpl. The function returns a
// usable client from admin.conf with RBAC properly constructed or an error.
func EnsureAdminClusterRoleBinding(outDir string, ensureRBACFunc EnsureRBACFunc) (clientset.Interface, error) {
	var (
		err                           error
		adminClient, superAdminClient clientset.Interface
	)

	// Create a client from admin.conf.
	adminClient, err = kubeconfigutil.ClientSetFromFile(filepath.Join(outDir, kubeadmconstants.AdminKubeConfigFileName))
	if err != nil {
		return nil, err
	}

	// Create a client from super-admin.conf.
	superAdminPath := filepath.Join(outDir, kubeadmconstants.SuperAdminKubeConfigFileName)
	if _, err := os.Stat(superAdminPath); err == nil {
		superAdminClient, err = kubeconfigutil.ClientSetFromFile(superAdminPath)
		if err != nil {
			return nil, err
		}
	}

	if ensureRBACFunc == nil {
		ensureRBACFunc = EnsureAdminClusterRoleBindingImpl
	}

	ctx := context.Background()
	return ensureRBACFunc(
		ctx, adminClient, superAdminClient,
		kubeadmconstants.KubernetesAPICallRetryInterval, kubeadmapi.GetActiveTimeouts().KubernetesAPICall.Duration,
	)
}

// EnsureAdminClusterRoleBindingImpl first attempts to see if the ClusterRoleBinding
// kubeadm:cluster-admins exists by using adminClient. If it already exists,
// it would mean the adminClient is usable. If it does not, attempt to create
// the ClusterRoleBinding by using superAdminClient.
func EnsureAdminClusterRoleBindingImpl(ctx context.Context, adminClient, superAdminClient clientset.Interface,
	retryInterval, retryTimeout time.Duration) (clientset.Interface, error) {

	klog.V(1).Infof("ensuring that the ClusterRoleBinding for the %s Group exists",
		kubeadmconstants.ClusterAdminsGroupAndClusterRoleBinding)

	var (
		err, lastError     error
		crbExists          bool
		clusterRoleBinding = &rbac.ClusterRoleBinding{
			ObjectMeta: metav1.ObjectMeta{
				Name: kubeadmconstants.ClusterAdminsGroupAndClusterRoleBinding,
			},
			RoleRef: rbac.RoleRef{
				APIGroup: rbac.GroupName,
				Kind:     "ClusterRole",
				Name:     "cluster-admin",
			},
			Subjects: []rbac.Subject{
				{
					Kind: rbac.GroupKind,
					Name: kubeadmconstants.ClusterAdminsGroupAndClusterRoleBinding,
				},
			},
		}
	)

	// First try to create the CRB with the admin.conf client. If the admin.conf contains a User bound
	// to the built-in super-user group, this will pass. In all other cases an error will be returned.
	// The poll here is required to ensure the API server is reachable during "kubeadm init" workflows.
	err = wait.PollUntilContextTimeout(
		ctx,
		retryInterval,
		retryTimeout,
		true, func(ctx context.Context) (bool, error) {
			if _, err := adminClient.RbacV1().ClusterRoleBindings().Create(
				ctx,
				clusterRoleBinding,
				metav1.CreateOptions{},
			); err != nil {
				if apierrors.IsForbidden(err) {
					// If it encounters a forbidden error this means that the API server was reached
					// but the CRB is missing - i.e. the admin.conf user does not have permissions
					// to create its own permission RBAC yet.
					return true, nil
				} else if apierrors.IsAlreadyExists(err) {
					// If the CRB exists it means the admin.conf already has the right
					// permissions; return.
					crbExists = true
					return true, nil
				} else {
					// Retry on any other error type.
					lastError = errors.Wrap(err, "unable to create ClusterRoleBinding")
					return false, nil
				}
			}
			crbExists = true
			return true, nil
		})
	if err != nil {
		return nil, lastError
	}

	// The CRB was created or already existed; return the admin.conf client.
	if crbExists {
		return adminClient, nil
	}

	// If the superAdminClient is nil at this point we cannot proceed creating the CRB; return an error.
	if superAdminClient == nil {
		return nil, errors.Errorf("the ClusterRoleBinding for the %s Group is missing but there is no %s to create it",
			kubeadmconstants.ClusterAdminsGroupAndClusterRoleBinding,
			kubeadmconstants.SuperAdminKubeConfigFileName)
	}

	// Create the ClusterRoleBinding with the super-admin.conf client.
	klog.V(1).Infof("creating the ClusterRoleBinding for the %s Group by using %s",
		kubeadmconstants.ClusterAdminsGroupAndClusterRoleBinding,
		kubeadmconstants.SuperAdminKubeConfigFileName)

	err = wait.PollUntilContextTimeout(
		ctx,
		retryInterval,
		retryTimeout,
		true, func(ctx context.Context) (bool, error) {
			if _, err := superAdminClient.RbacV1().ClusterRoleBindings().Create(
				ctx,
				clusterRoleBinding,
				metav1.CreateOptions{},
			); err != nil {
				lastError = err
				if apierrors.IsAlreadyExists(err) {
					klog.V(5).Infof("ClusterRoleBinding %s already exists.", kubeadmconstants.ClusterAdminsGroupAndClusterRoleBinding)
					return true, nil
				}
				// Retry on any other type of error.
				return false, nil
			}
			return true, nil
		})
	if err != nil {
		return nil, errors.Wrapf(lastError, "unable to create the %s ClusterRoleBinding by using %s",
			kubeadmconstants.ClusterAdminsGroupAndClusterRoleBinding,
			kubeadmconstants.SuperAdminKubeConfigFileName)
	}

	// Once the CRB is in place, start using the admin.conf client.
	return adminClient, nil
}