Add DenyPSALabel admission plugin

pkg/admission/denypsalabel/admission.go

@@ -0,0 +1,92 @@
+/*
+Copyright 2020 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 denypsalabel
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"strings"
+
+	"k8s.io/apimachinery/pkg/api/errors"
+	"k8s.io/apiserver/pkg/admission"
+	"k8s.io/klog/v2"
+	"k8s.io/kubernetes/pkg/apis/core"
+)
+
+const (
+	// PluginName is the name of this admission controller plugin
+	PluginName  = "DenyPSALabel"
+	labelPrefix = "pod-security.kubernetes.io/"
+)
+
+// Register registers a plugin
+func Register(plugins *admission.Plugins) {
+	plugins.Register(PluginName, func(config io.Reader) (admission.Interface, error) {
+		plugin := newPlugin()
+		return plugin, nil
+	})
+}
+
+// psaLabelDenialPlugin holds state for and implements the admission plugin.
+type psaLabelDenialPlugin struct {
+	*admission.Handler
+}
+
+var _ admission.Interface = &psaLabelDenialPlugin{}
+var _ admission.ValidationInterface = &psaLabelDenialPlugin{}
+
+// newPlugin creates a new admission plugin.
+func newPlugin() *psaLabelDenialPlugin {
+	return &psaLabelDenialPlugin{
+		Handler: admission.NewHandler(admission.Create, admission.Update),
+	}
+}
+
+// Validate ensures that applying PSA label to namespaces is denied
+func (plug *psaLabelDenialPlugin) Validate(ctx context.Context, attr admission.Attributes, o admission.ObjectInterfaces) error {
+	if attr.GetResource().GroupResource() != core.Resource("namespaces") {
+		return nil
+	}
+
+	if len(attr.GetSubresource()) != 0 {
+		return nil
+	}
+
+	// if we can't convert then we don't handle this object so just return
+	newNS, ok := attr.GetObject().(*core.Namespace)
+	if !ok {
+		klog.V(3).Infof("Expected namespace resource, got: %v", attr.GetKind())
+		return errors.NewInternalError(fmt.Errorf("Expected Namespace resource, got: %v", attr.GetKind()))
+	}
+
+	if !isPSALabel(newNS) {
+		return nil
+	}
+
+	klog.V(4).Infof("Denying use of label with %s prefix on Namespace %s", labelPrefix, newNS.Name)
+	return admission.NewForbidden(attr, fmt.Errorf("Use of label with %s prefix on Namespace is denied by admission control", labelPrefix))
+}
+
+func isPSALabel(newNS *core.Namespace) bool {
+	for labelName := range newNS.Labels {
+		if strings.HasPrefix(labelName, labelPrefix) {
+			return true
+		}
+	}
+	return false
+}

pkg/cli/cmds/server.go

@@ -109,6 +109,7 @@ type Server struct {
 	EtcdS3Timeout            time.Duration
 	EtcdS3Insecure           bool
 	ServiceLBNamespace       string
+	DenyPSALabel             bool
 }
 
 var (
@@ -586,6 +587,12 @@ var ServerFlags = []cli.Flag{
 		Usage:  "(flags) Customized flag for kube-cloud-controller-manager process",
 		Value:  &ServerConfig.ExtraCloudControllerArgs,
 	},
+	&cli.BoolFlag{
+		Name:        "deny-psa-label",
+		Usage:       "(experimental) Deny use of pod-security.kubernetes.io labels on Namespaces",
+		Hidden:      true,
+		Destination: &ServerConfig.DenyPSALabel,
+	},
 }
 
 func NewServerCommand(action func(*cli.Context) error) cli.Command {

pkg/cli/server/server.go

@@ -180,6 +180,7 @@ func run(app *cli.Context, cfg *cmds.Server, leaderControllers server.CustomCont
 	serverConfig.ControlConfig.SupervisorMetrics = cfg.SupervisorMetrics
 	serverConfig.ControlConfig.VLevel = cmds.LogConfig.VLevel
 	serverConfig.ControlConfig.VModule = cmds.LogConfig.VModule
+	serverConfig.ControlConfig.DenyPSALabel = cfg.DenyPSALabel
 
 	if !cfg.EtcdDisableSnapshots || cfg.ClusterReset {
 		serverConfig.ControlConfig.EtcdSnapshotCompress = cfg.EtcdSnapshotCompress

pkg/daemons/config/types.go

@@ -245,6 +245,7 @@ type Control struct {
 	ServerNodeName           string
 	VLevel                   int
 	VModule                  string
+	DenyPSALabel             bool
 
 	BindAddress string
 	SANs        []string

pkg/daemons/control/server.go

@@ -9,6 +9,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/k3s-io/k3s/pkg/admission/denypsalabel"
 	"github.com/k3s-io/k3s/pkg/authenticator"
 	"github.com/k3s-io/k3s/pkg/cluster"
 	"github.com/k3s-io/k3s/pkg/daemons/config"
@@ -20,8 +21,10 @@ import (
 	"github.com/sirupsen/logrus"
 	authorizationv1 "k8s.io/api/authorization/v1"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apiserver/pkg/admission"
 	logsapi "k8s.io/component-base/logs/api/v1"
 	"k8s.io/kubernetes/pkg/kubeapiserver/authorizer/modes"
+	"k8s.io/kubernetes/pkg/kubeapiserver/options"
 	"k8s.io/kubernetes/pkg/registry/core/node"
 
 	// for client metric registration
@@ -228,6 +231,13 @@ func apiServer(ctx context.Context, cfg *config.Control) error {
 
 	args := config.GetArgs(argsMap, cfg.ExtraAPIArgs)
 
+	// Add extra admission plugins
+	if cfg.DenyPSALabel {
+		extraPlugins := make(map[string]func(*admission.Plugins))
+		extraPlugins[denypsalabel.PluginName] = denypsalabel.Register
+		options.AdmissionPlugins = extraPlugins
+	}
+
 	logrus.Infof("Running kube-apiserver %s", config.ArgString(args))
 
 	return executor.APIServer(ctx, runtime.ETCDReady, args)