enable kubelet server to dynamically load tls certificate files

pkg/kubelet/certificate/kubelet.go

@@ -17,22 +17,26 @@ limitations under the License.
 package certificate
 
 import (
+	"context"
 	"crypto/tls"
 	"crypto/x509"
 	"crypto/x509/pkix"
 	"fmt"
 	"math"
 	"net"
 	"sort"
+	"sync/atomic"
 	"time"
 
 	certificates "k8s.io/api/certificates/v1"
 	v1 "k8s.io/api/core/v1"
 	"k8s.io/apimachinery/pkg/types"
+	"k8s.io/apiserver/pkg/server/dynamiccertificates"
 	clientset "k8s.io/client-go/kubernetes"
 	"k8s.io/client-go/util/certificate"
 	compbasemetrics "k8s.io/component-base/metrics"
 	"k8s.io/component-base/metrics/legacyregistry"
+	"k8s.io/klog/v2"
 	kubeletconfig "k8s.io/kubernetes/pkg/kubelet/apis/config"
 	"k8s.io/kubernetes/pkg/kubelet/metrics"
 	netutils "k8s.io/utils/net"
@@ -234,3 +238,66 @@ func NewKubeletClientCertificateManager(
 
 	return m, nil
 }
+
+// NewKubeletServerCertificateDynamicFileManager creates a certificate manager based on reading and watching certificate and key files.
+// The returned struct implements certificate.Manager interface, enabling using it like other CertificateManager in this package.
+// But the struct doesn't communicate with API server to perform certificate request at all.
+func NewKubeletServerCertificateDynamicFileManager(certFile, keyFile string) (certificate.Manager, error) {
+	c, err := dynamiccertificates.NewDynamicServingContentFromFiles("kubelet-server-cert-files", certFile, keyFile)
+	if err != nil {
+		return nil, fmt.Errorf("unable to set up dynamic certificate manager for kubelet server cert files: %w", err)
+	}
+	m := &kubeletServerCertificateDynamicFileManager{
+		dynamicCertificateContent: c,
+		certFile:                  certFile,
+		keyFile:                   keyFile,
+	}
+	m.Enqueue()
+	c.AddListener(m)
+	return m, nil
+}
+
+// kubeletServerCertificateDynamicFileManager uses a dynamic CertKeyContentProvider based on cert and key files.
+type kubeletServerCertificateDynamicFileManager struct {
+	cancelFn                  context.CancelFunc
+	certFile                  string
+	keyFile                   string
+	dynamicCertificateContent *dynamiccertificates.DynamicCertKeyPairContent
+	currentTLSCertificate     atomic.Pointer[tls.Certificate]
+}
+
+// Enqueue implements the functions to be notified when the serving cert content changes.
+func (m *kubeletServerCertificateDynamicFileManager) Enqueue() {
+	certContent, keyContent := m.dynamicCertificateContent.CurrentCertKeyContent()
+	cert, err := tls.X509KeyPair(certContent, keyContent)
+	if err != nil {
+		klog.ErrorS(err, "invalid certificate and key pair from file", "certFile", m.certFile, "keyFile", m.keyFile)
+		return
+	}
+	m.currentTLSCertificate.Store(&cert)
+	klog.V(4).InfoS("loaded certificate and key pair in kubelet server certificate manager", "certFile", m.certFile, "keyFile", m.keyFile)
+}
+
+// Current returns the last valid certificate key pair loaded from files.
+func (m *kubeletServerCertificateDynamicFileManager) Current() *tls.Certificate {
+	return m.currentTLSCertificate.Load()
+}
+
+// Start starts watching the certificate and key files
+func (m *kubeletServerCertificateDynamicFileManager) Start() {
+	var ctx context.Context
+	ctx, m.cancelFn = context.WithCancel(context.Background())
+	go m.dynamicCertificateContent.Run(ctx, 1)
+}
+
+// Stop stops watching the certificate and key files
+func (m *kubeletServerCertificateDynamicFileManager) Stop() {
+	if m.cancelFn != nil {
+		m.cancelFn()
+	}
+}
+
+// ServerHealthy always returns true since the file manager doesn't communicate with any server
+func (m *kubeletServerCertificateDynamicFileManager) ServerHealthy() bool {
+	return true
+}

pkg/kubelet/kubelet.go

@@ -774,17 +774,28 @@ func NewMainKubelet(kubeCfg *kubeletconfiginternal.KubeletConfiguration,
 	}
 	klet.imageManager = imageManager
 
-	if kubeCfg.ServerTLSBootstrap && kubeDeps.TLSOptions != nil && utilfeature.DefaultFeatureGate.Enabled(features.RotateKubeletServerCertificate) {
-		klet.serverCertificateManager, err = kubeletcertificate.NewKubeletServerCertificateManager(klet.kubeClient, kubeCfg, klet.nodeName, klet.getLastObservedNodeAddresses, certDirectory)
-		if err != nil {
-			return nil, fmt.Errorf("failed to initialize certificate manager: %v", err)
+	if kubeDeps.TLSOptions != nil {
+		if kubeCfg.ServerTLSBootstrap && utilfeature.DefaultFeatureGate.Enabled(features.RotateKubeletServerCertificate) {
+			klet.serverCertificateManager, err = kubeletcertificate.NewKubeletServerCertificateManager(klet.kubeClient, kubeCfg, klet.nodeName, klet.getLastObservedNodeAddresses, certDirectory)
+			if err != nil {
+				return nil, fmt.Errorf("failed to initialize certificate manager: %w", err)
+			}
+
+		} else if kubeDeps.TLSOptions.CertFile != "" && kubeDeps.TLSOptions.KeyFile != "" && utilfeature.DefaultFeatureGate.Enabled(features.ReloadKubeletServerCertificateFile) {
+			klet.serverCertificateManager, err = kubeletcertificate.NewKubeletServerCertificateDynamicFileManager(kubeDeps.TLSOptions.CertFile, kubeDeps.TLSOptions.KeyFile)
+			if err != nil {
+				return nil, fmt.Errorf("failed to initialize file based certificate manager: %w", err)
+			}
 		}
-		kubeDeps.TLSOptions.Config.GetCertificate = func(*tls.ClientHelloInfo) (*tls.Certificate, error) {
-			cert := klet.serverCertificateManager.Current()
-			if cert == nil {
-				return nil, fmt.Errorf("no serving certificate available for the kubelet")
+
+		if klet.serverCertificateManager != nil {
+			kubeDeps.TLSOptions.Config.GetCertificate = func(*tls.ClientHelloInfo) (*tls.Certificate, error) {
+				cert := klet.serverCertificateManager.Current()
+				if cert == nil {
+					return nil, fmt.Errorf("no serving certificate available for the kubelet")
+				}
+				return cert, nil
 			}
-			return cert, nil
 		}
 	}