admission.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 imagepolicy contains an admission controller that configures a webhook to which policy
// decisions are delegated.
package imagepolicy

import (
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"strings"
	"time"

	"github.com/golang/glog"

	"k8s.io/api/imagepolicy/v1alpha1"
	apierrors "k8s.io/apimachinery/pkg/api/errors"
	"k8s.io/apimachinery/pkg/runtime/schema"
	"k8s.io/apimachinery/pkg/util/cache"
	"k8s.io/apimachinery/pkg/util/yaml"
	"k8s.io/apiserver/pkg/admission"
	"k8s.io/apiserver/pkg/util/webhook"
	"k8s.io/client-go/rest"
	"k8s.io/kubernetes/pkg/api/legacyscheme"
	api "k8s.io/kubernetes/pkg/apis/core"

	// install the clientgo image policy API for use with api registry
	_ "k8s.io/kubernetes/pkg/apis/imagepolicy/install"
)

// PluginName indicates name of admission plugin.
const PluginName = "ImagePolicyWebhook"

var (
	groupVersions = []schema.GroupVersion{v1alpha1.SchemeGroupVersion}
)

// Register registers a plugin
func Register(plugins *admission.Plugins) {
	plugins.Register(PluginName, func(config io.Reader) (admission.Interface, error) {
		newImagePolicyWebhook, err := NewImagePolicyWebhook(config)
		if err != nil {
			return nil, err
		}
		return newImagePolicyWebhook, nil
	})
}

// Plugin is an implementation of admission.Interface.
type Plugin struct {
	*admission.Handler
	webhook       *webhook.GenericWebhook
	responseCache *cache.LRUExpireCache
	allowTTL      time.Duration
	denyTTL       time.Duration
	retryBackoff  time.Duration
	defaultAllow  bool
}

var _ admission.ValidationInterface = &Plugin{}

func (a *Plugin) statusTTL(status v1alpha1.ImageReviewStatus) time.Duration {
	if status.Allowed {
		return a.allowTTL
	}
	return a.denyTTL
}

// Filter out annotations that don't match *.image-policy.k8s.io/*
func (a *Plugin) filterAnnotations(allAnnotations map[string]string) map[string]string {
	annotations := make(map[string]string)
	for k, v := range allAnnotations {
		if strings.Contains(k, ".image-policy.k8s.io/") {
			annotations[k] = v
		}
	}
	return annotations
}

// Function to call on webhook failure; behavior determined by defaultAllow flag
func (a *Plugin) webhookError(pod *api.Pod, attributes admission.Attributes, err error) error {
	if err != nil {
		glog.V(2).Infof("error contacting webhook backend: %s", err)
		if a.defaultAllow {
			annotations := pod.GetAnnotations()
			if annotations == nil {
				annotations = make(map[string]string)
			}
			annotations[api.ImagePolicyFailedOpenKey] = "true"
			pod.ObjectMeta.SetAnnotations(annotations)
			glog.V(2).Infof("resource allowed in spite of webhook backend failure")
			return nil
		}
		glog.V(2).Infof("resource not allowed due to webhook backend failure ")
		return admission.NewForbidden(attributes, err)
	}
	return nil
}

// Validate makes an admission decision based on the request attributes
func (a *Plugin) Validate(attributes admission.Attributes) (err error) {
	// Ignore all calls to subresources or resources other than pods.
	if attributes.GetSubresource() != "" || attributes.GetResource().GroupResource() != api.Resource("pods") {
		return nil
	}

	pod, ok := attributes.GetObject().(*api.Pod)
	if !ok {
		return apierrors.NewBadRequest("Resource was marked with kind Pod but was unable to be converted")
	}

	// Build list of ImageReviewContainerSpec
	var imageReviewContainerSpecs []v1alpha1.ImageReviewContainerSpec
	containers := make([]api.Container, 0, len(pod.Spec.Containers)+len(pod.Spec.InitContainers))
	containers = append(containers, pod.Spec.Containers...)
	containers = append(containers, pod.Spec.InitContainers...)
	for _, c := range containers {
		imageReviewContainerSpecs = append(imageReviewContainerSpecs, v1alpha1.ImageReviewContainerSpec{
			Image: c.Image,
		})
	}
	imageReview := v1alpha1.ImageReview{
		Spec: v1alpha1.ImageReviewSpec{
			Containers:  imageReviewContainerSpecs,
			Annotations: a.filterAnnotations(pod.Annotations),
			Namespace:   attributes.GetNamespace(),
		},
	}
	if err := a.admitPod(pod, attributes, &imageReview); err != nil {
		return admission.NewForbidden(attributes, err)
	}
	return nil
}

func (a *Plugin) admitPod(pod *api.Pod, attributes admission.Attributes, review *v1alpha1.ImageReview) error {
	cacheKey, err := json.Marshal(review.Spec)
	if err != nil {
		return err
	}
	if entry, ok := a.responseCache.Get(string(cacheKey)); ok {
		review.Status = entry.(v1alpha1.ImageReviewStatus)
	} else {
		result := a.webhook.WithExponentialBackoff(func() rest.Result {
			return a.webhook.RestClient.Post().Body(review).Do()
		})

		if err := result.Error(); err != nil {
			return a.webhookError(pod, attributes, err)
		}
		var statusCode int
		if result.StatusCode(&statusCode); statusCode < 200 || statusCode >= 300 {
			return a.webhookError(pod, attributes, fmt.Errorf("Error contacting webhook: %d", statusCode))
		}

		if err := result.Into(review); err != nil {
			return a.webhookError(pod, attributes, err)
		}

		a.responseCache.Add(string(cacheKey), review.Status, a.statusTTL(review.Status))
	}

	if !review.Status.Allowed {
		if len(review.Status.Reason) > 0 {
			return fmt.Errorf("image policy webhook backend denied one or more images: %s", review.Status.Reason)
		}
		return errors.New("one or more images rejected by webhook backend")
	}

	return nil
}

// NewImagePolicyWebhook a new ImagePolicyWebhook plugin from the provided config file.
// The config file is specified by --admission-control-config-file and has the
// following format for a webhook:
//
//   {
//     "imagePolicy": {
//        "kubeConfigFile": "path/to/kubeconfig/for/backend",
//        "allowTTL": 30,           # time in s to cache approval
//        "denyTTL": 30,            # time in s to cache denial
//        "retryBackoff": 500,      # time in ms to wait between retries
//        "defaultAllow": true      # determines behavior if the webhook backend fails
//     }
//   }
//
// The config file may be json or yaml.
//
// The kubeconfig property refers to another file in the kubeconfig format which
// specifies how to connect to the webhook backend.
//
// The kubeconfig's cluster field is used to refer to the remote service, user refers to the returned authorizer.
//
//     # clusters refers to the remote service.
//     clusters:
//     - name: name-of-remote-imagepolicy-service
//       cluster:
//         certificate-authority: /path/to/ca.pem      # CA for verifying the remote service.
//         server: https://images.example.com/policy # URL of remote service to query. Must use 'https'.
//
//     # users refers to the API server's webhook configuration.
//     users:
//     - name: name-of-api-server
//       user:
//         client-certificate: /path/to/cert.pem # cert for the webhook plugin to use
//         client-key: /path/to/key.pem          # key matching the cert
//
// For additional HTTP configuration, refer to the kubeconfig documentation
// http://kubernetes.io/v1.1/docs/user-guide/kubeconfig-file.html.
func NewImagePolicyWebhook(configFile io.Reader) (*Plugin, error) {
	if configFile == nil {
		return nil, fmt.Errorf("no config specified")
	}

	// TODO: move this to a versioned configuration file format
	var config AdmissionConfig
	d := yaml.NewYAMLOrJSONDecoder(configFile, 4096)
	err := d.Decode(&config)
	if err != nil {
		return nil, err
	}

	whConfig := config.ImagePolicyWebhook
	if err := normalizeWebhookConfig(&whConfig); err != nil {
		return nil, err
	}

	gw, err := webhook.NewGenericWebhook(legacyscheme.Scheme, legacyscheme.Codecs, whConfig.KubeConfigFile, groupVersions, whConfig.RetryBackoff)
	if err != nil {
		return nil, err
	}
	return &Plugin{
		Handler:       admission.NewHandler(admission.Create, admission.Update),
		webhook:       gw,
		responseCache: cache.NewLRUExpireCache(1024),
		allowTTL:      whConfig.AllowTTL,
		denyTTL:       whConfig.DenyTTL,
		defaultAllow:  whConfig.DefaultAllow,
	}, nil
}