certificate/bootstrap/bootstrap.go

/*
Copyright 2016 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 bootstrap

import (
	"context"
	"crypto"
	"crypto/sha512"
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/base64"
	"errors"
	"fmt"
	"os"
	"path/filepath"
	"time"

	"k8s.io/klog/v2"

	certificatesv1 "k8s.io/api/certificates/v1"
	"k8s.io/apimachinery/pkg/types"
	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
	"k8s.io/apimachinery/pkg/util/wait"
	clientset "k8s.io/client-go/kubernetes"
	"k8s.io/client-go/kubernetes/scheme"
	restclient "k8s.io/client-go/rest"
	"k8s.io/client-go/tools/clientcmd"
	clientcmdapi "k8s.io/client-go/tools/clientcmd/api"
	"k8s.io/client-go/transport"
	certutil "k8s.io/client-go/util/cert"
	"k8s.io/client-go/util/certificate"
	"k8s.io/client-go/util/certificate/csr"
	"k8s.io/client-go/util/keyutil"
)

const tmpPrivateKeyFile = "kubelet-client.key.tmp"

// LoadClientConfig tries to load the appropriate client config for retrieving certs and for use by users.
// If bootstrapPath is empty, only kubeconfigPath is checked. If bootstrap path is set and the contents
// of kubeconfigPath are valid, both certConfig and userConfig will point to that file. Otherwise the
// kubeconfigPath on disk is populated based on bootstrapPath but pointing to the location of the client cert
// in certDir. This preserves the historical behavior of bootstrapping where on subsequent restarts the
// most recent client cert is used to request new client certs instead of the initial token.
func LoadClientConfig(kubeconfigPath, bootstrapPath, certDir string) (certConfig, userConfig *restclient.Config, err error) {
	if len(bootstrapPath) == 0 {
		clientConfig, err := loadRESTClientConfig(kubeconfigPath)
		if err != nil {
			return nil, nil, fmt.Errorf("unable to load kubeconfig: %v", err)
		}
		klog.V(2).InfoS("No bootstrapping requested, will use kubeconfig")
		return clientConfig, restclient.CopyConfig(clientConfig), nil
	}

	store, err := certificate.NewFileStore("kubelet-client", certDir, certDir, "", "")
	if err != nil {
		return nil, nil, fmt.Errorf("unable to build bootstrap cert store")
	}

	ok, err := isClientConfigStillValid(kubeconfigPath)
	if err != nil {
		return nil, nil, err
	}

	// use the current client config
	if ok {
		clientConfig, err := loadRESTClientConfig(kubeconfigPath)
		if err != nil {
			return nil, nil, fmt.Errorf("unable to load kubeconfig: %v", err)
		}
		klog.V(2).InfoS("Current kubeconfig file contents are still valid, no bootstrap necessary")
		return clientConfig, restclient.CopyConfig(clientConfig), nil
	}

	bootstrapClientConfig, err := loadRESTClientConfig(bootstrapPath)
	if err != nil {
		return nil, nil, fmt.Errorf("unable to load bootstrap kubeconfig: %v", err)
	}

	clientConfig := restclient.AnonymousClientConfig(bootstrapClientConfig)
	pemPath := store.CurrentPath()
	clientConfig.KeyFile = pemPath
	clientConfig.CertFile = pemPath
	if err := writeKubeconfigFromBootstrapping(clientConfig, kubeconfigPath, pemPath); err != nil {
		return nil, nil, err
	}
	klog.V(2).InfoS("Use the bootstrap credentials to request a cert, and set kubeconfig to point to the certificate dir")
	return bootstrapClientConfig, clientConfig, nil
}

// LoadClientCert requests a client cert for kubelet if the kubeconfigPath file does not exist.
// The kubeconfig at bootstrapPath is used to request a client certificate from the API server.
// On success, a kubeconfig file referencing the generated key and obtained certificate is written to kubeconfigPath.
// The certificate and key file are stored in certDir.
func LoadClientCert(ctx context.Context, kubeconfigPath, bootstrapPath, certDir string, nodeName types.NodeName) error {
	// Short-circuit if the kubeconfig file exists and is valid.
	ok, err := isClientConfigStillValid(kubeconfigPath)
	if err != nil {
		return err
	}
	if ok {
		klog.V(2).InfoS("Kubeconfig exists and is valid, skipping bootstrap", "path", kubeconfigPath)
		return nil
	}

	klog.V(2).InfoS("Using bootstrap kubeconfig to generate TLS client cert, key and kubeconfig file")

	bootstrapClientConfig, err := loadRESTClientConfig(bootstrapPath)
	if err != nil {
		return fmt.Errorf("unable to load bootstrap kubeconfig: %v", err)
	}

	bootstrapClient, err := clientset.NewForConfig(bootstrapClientConfig)
	if err != nil {
		return fmt.Errorf("unable to create certificates signing request client: %v", err)
	}

	store, err := certificate.NewFileStore("kubelet-client", certDir, certDir, "", "")
	if err != nil {
		return fmt.Errorf("unable to build bootstrap cert store")
	}

	var keyData []byte
	if cert, err := store.Current(); err == nil {
		if cert.PrivateKey != nil {
			keyData, err = keyutil.MarshalPrivateKeyToPEM(cert.PrivateKey)
			if err != nil {
				keyData = nil
			}
		}
	}
	// Cache the private key in a separate file until CSR succeeds. This has to
	// be a separate file because store.CurrentPath() points to a symlink
	// managed by the store.
	privKeyPath := filepath.Join(certDir, tmpPrivateKeyFile)
	if !verifyKeyData(keyData) {
		klog.V(2).InfoS("No valid private key and/or certificate found, reusing existing private key or creating a new one")
		// Note: always call LoadOrGenerateKeyFile so that private key is
		// reused on next startup if CSR request fails.
		keyData, _, err = keyutil.LoadOrGenerateKeyFile(privKeyPath)
		if err != nil {
			return err
		}
	}

	if err := waitForServer(ctx, *bootstrapClientConfig, 1*time.Minute); err != nil {
		klog.InfoS("Error waiting for apiserver to come up", "err", err)
	}

	certData, err := requestNodeCertificate(ctx, bootstrapClient, keyData, nodeName)
	if err != nil {
		return err
	}
	if _, err := store.Update(certData, keyData); err != nil {
		return err
	}
	if err := os.Remove(privKeyPath); err != nil && !os.IsNotExist(err) {
		klog.V(2).InfoS("Failed cleaning up private key file", "path", privKeyPath, "err", err)
	}

	return writeKubeconfigFromBootstrapping(bootstrapClientConfig, kubeconfigPath, store.CurrentPath())
}

func writeKubeconfigFromBootstrapping(bootstrapClientConfig *restclient.Config, kubeconfigPath, pemPath string) error {
	// Get the CA data from the bootstrap client config.
	caFile, caData := bootstrapClientConfig.CAFile, []byte{}
	if len(caFile) == 0 {
		caData = bootstrapClientConfig.CAData
	}

	// Build resulting kubeconfig.
	kubeconfigData := clientcmdapi.Config{
		// Define a cluster stanza based on the bootstrap kubeconfig.
		Clusters: map[string]*clientcmdapi.Cluster{"default-cluster": {
			Server:                   bootstrapClientConfig.Host,
			InsecureSkipTLSVerify:    bootstrapClientConfig.Insecure,
			CertificateAuthority:     caFile,
			CertificateAuthorityData: caData,
		}},
		// Define auth based on the obtained client cert.
		AuthInfos: map[string]*clientcmdapi.AuthInfo{"default-auth": {
			ClientCertificate: pemPath,
			ClientKey:         pemPath,
		}},
		// Define a context that connects the auth info and cluster, and set it as the default
		Contexts: map[string]*clientcmdapi.Context{"default-context": {
			Cluster:   "default-cluster",
			AuthInfo:  "default-auth",
			Namespace: "default",
		}},
		CurrentContext: "default-context",
	}

	// Marshal to disk
	return clientcmd.WriteToFile(kubeconfigData, kubeconfigPath)
}

func loadRESTClientConfig(kubeconfig string) (*restclient.Config, error) {
	// Load structured kubeconfig data from the given path.
	loader := &clientcmd.ClientConfigLoadingRules{ExplicitPath: kubeconfig}
	loadedConfig, err := loader.Load()
	if err != nil {
		return nil, err
	}
	// Flatten the loaded data to a particular restclient.Config based on the current context.
	return clientcmd.NewNonInteractiveClientConfig(
		*loadedConfig,
		loadedConfig.CurrentContext,
		&clientcmd.ConfigOverrides{},
		loader,
	).ClientConfig()
}

// isClientConfigStillValid checks the provided kubeconfig to see if it has a valid
// client certificate. It returns true if the kubeconfig is valid, or an error if bootstrapping
// should stop immediately.
func isClientConfigStillValid(kubeconfigPath string) (bool, error) {
	_, err := os.Stat(kubeconfigPath)
	if os.IsNotExist(err) {
		return false, nil
	}
	if err != nil {
		return false, fmt.Errorf("error reading existing bootstrap kubeconfig %s: %v", kubeconfigPath, err)
	}
	bootstrapClientConfig, err := loadRESTClientConfig(kubeconfigPath)
	if err != nil {
		utilruntime.HandleError(fmt.Errorf("unable to read existing bootstrap client config from %s: %v", kubeconfigPath, err))
		return false, nil
	}
	transportConfig, err := bootstrapClientConfig.TransportConfig()
	if err != nil {
		utilruntime.HandleError(fmt.Errorf("unable to load transport configuration from existing bootstrap client config read from %s: %v", kubeconfigPath, err))
		return false, nil
	}
	// has side effect of populating transport config data fields
	if _, err := transport.TLSConfigFor(transportConfig); err != nil {
		utilruntime.HandleError(fmt.Errorf("unable to load TLS configuration from existing bootstrap client config read from %s: %v", kubeconfigPath, err))
		return false, nil
	}
	certs, err := certutil.ParseCertsPEM(transportConfig.TLS.CertData)
	if err != nil {
		utilruntime.HandleError(fmt.Errorf("unable to load TLS certificates from existing bootstrap client config read from %s: %v", kubeconfigPath, err))
		return false, nil
	}
	if len(certs) == 0 {
		utilruntime.HandleError(fmt.Errorf("unable to read TLS certificates from existing bootstrap client config read from %s: %v", kubeconfigPath, err))
		return false, nil
	}
	now := time.Now()
	for _, cert := range certs {
		if now.After(cert.NotAfter) {
			utilruntime.HandleError(fmt.Errorf("part of the existing bootstrap client certificate in %s is expired: %v", kubeconfigPath, cert.NotAfter))
			return false, nil
		}
	}
	return true, nil
}

// verifyKeyData returns true if the provided data appears to be a valid private key.
func verifyKeyData(data []byte) bool {
	if len(data) == 0 {
		return false
	}
	_, err := keyutil.ParsePrivateKeyPEM(data)
	return err == nil
}

func waitForServer(ctx context.Context, cfg restclient.Config, deadline time.Duration) error {
	cfg.NegotiatedSerializer = scheme.Codecs.WithoutConversion()
	cfg.Timeout = 1 * time.Second
	cli, err := restclient.UnversionedRESTClientFor(&cfg)
	if err != nil {
		return fmt.Errorf("couldn't create client: %v", err)
	}

	ctx, cancel := context.WithTimeout(ctx, deadline)
	defer cancel()

	var connected bool
	wait.JitterUntil(func() {
		if _, err := cli.Get().AbsPath("/healthz").Do(ctx).Raw(); err != nil {
			klog.InfoS("Failed to connect to apiserver", "err", err)
			return
		}
		cancel()
		connected = true
	}, 2*time.Second, 0.2, true, ctx.Done())

	if !connected {
		return errors.New("timed out waiting to connect to apiserver")
	}
	return nil
}

// requestNodeCertificate will create a certificate signing request for a node
// (Organization and CommonName for the CSR will be set as expected for node
// certificates) and send it to API server, then it will watch the object's
// status, once approved by API server, it will return the API server's issued
// certificate (pem-encoded). If there is any errors, or the watch timeouts, it
// will return an error. This is intended for use on nodes (kubelet and
// kubeadm).
func requestNodeCertificate(ctx context.Context, client clientset.Interface, privateKeyData []byte, nodeName types.NodeName) (certData []byte, err error) {
	subject := &pkix.Name{
		Organization: []string{"system:nodes"},
		CommonName:   "system:node:" + string(nodeName),
	}

	privateKey, err := keyutil.ParsePrivateKeyPEM(privateKeyData)
	if err != nil {
		return nil, fmt.Errorf("invalid private key for certificate request: %v", err)
	}
	csrData, err := certutil.MakeCSR(privateKey, subject, nil, nil)
	if err != nil {
		return nil, fmt.Errorf("unable to generate certificate request: %v", err)
	}

	usages := []certificatesv1.KeyUsage{
		certificatesv1.UsageDigitalSignature,
		certificatesv1.UsageKeyEncipherment,
		certificatesv1.UsageClientAuth,
	}

	// The Signer interface contains the Public() method to get the public key.
	signer, ok := privateKey.(crypto.Signer)
	if !ok {
		return nil, fmt.Errorf("private key does not implement crypto.Signer")
	}

	name, err := digestedName(signer.Public(), subject, usages)
	if err != nil {
		return nil, err
	}

	reqName, reqUID, err := csr.RequestCertificate(client, csrData, name, certificatesv1.KubeAPIServerClientKubeletSignerName, nil, usages, privateKey)
	if err != nil {
		return nil, err
	}

	ctx, cancel := context.WithTimeout(ctx, 3600*time.Second)
	defer cancel()

	klog.V(2).InfoS("Waiting for client certificate to be issued")
	return csr.WaitForCertificate(ctx, client, reqName, reqUID)
}

// This digest should include all the relevant pieces of the CSR we care about.
// We can't directly hash the serialized CSR because of random padding that we
// regenerate every loop and we include usages which are not contained in the
// CSR. This needs to be kept up to date as we add new fields to the node
// certificates and with ensureCompatible.
func digestedName(publicKey interface{}, subject *pkix.Name, usages []certificatesv1.KeyUsage) (string, error) {
	hash := sha512.New512_256()

	// Here we make sure two different inputs can't write the same stream
	// to the hash. This delimiter is not in the base64.URLEncoding
	// alphabet so there is no way to have spill over collisions. Without
	// it 'CN:foo,ORG:bar' hashes to the same value as 'CN:foob,ORG:ar'
	const delimiter = '|'
	encode := base64.RawURLEncoding.EncodeToString

	write := func(data []byte) {
		hash.Write([]byte(encode(data)))
		hash.Write([]byte{delimiter})
	}

	publicKeyData, err := x509.MarshalPKIXPublicKey(publicKey)
	if err != nil {
		return "", err
	}
	write(publicKeyData)

	write([]byte(subject.CommonName))
	for _, v := range subject.Organization {
		write([]byte(v))
	}
	for _, v := range usages {
		write([]byte(v))
	}

	return fmt.Sprintf("node-csr-%s", encode(hash.Sum(nil))), nil
}