Add admission controller to force image pulls

Add an admission controller that forces every container's image pull policy to
Always when a pod is created.

cmd/kube-apiserver/app/plugins.go

@@ -25,6 +25,7 @@ import (
 
 	// Admission policies
 	_ "k8s.io/kubernetes/plugin/pkg/admission/admit"
+	_ "k8s.io/kubernetes/plugin/pkg/admission/alwayspullimages"
 	_ "k8s.io/kubernetes/plugin/pkg/admission/deny"
 	_ "k8s.io/kubernetes/plugin/pkg/admission/exec"
 	_ "k8s.io/kubernetes/plugin/pkg/admission/initialresources"

docs/admin/admission-controllers.md

@@ -42,6 +42,7 @@ Documentation for other releases can be found at
   - [How do I turn on an admission control plug-in?](#how-do-i-turn-on-an-admission-control-plug-in)
   - [What does each plug-in do?](#what-does-each-plug-in-do)
     - [AlwaysAdmit](#alwaysadmit)
+    - [AlwaysPullImages](#alwayspullimages)
     - [AlwaysDeny](#alwaysdeny)
     - [DenyExecOnPrivileged (deprecated)](#denyexeconprivileged-deprecated)
     - [DenyEscalatingExec](#denyescalatingexec)
@@ -90,6 +91,16 @@ ordered list of admission control choices to invoke prior to modifying objects i
 
 Use this plugin by itself to pass-through all requests.
 
+### AlwaysPullImages
+
+This plug-in modifies every new Pod to force the image pull policy to Always. This is useful in a
+multitenant cluster so that users can be assured that their private images can only be used by those
+who have the credentials to pull them. Without this plug-in, once an image has been pulled to a
+node, any pod from any user can use it simply by knowing the image's name (assuming the Pod is
+scheduled onto the right node), without any authorization check against the image. When this plug-in
+is enabled, images are always pulled prior to starting containers, which means valid credentials are
+required.
+
 ### AlwaysDeny
 
 Rejects all requests.  Used for testing.

docs/admin/kube-apiserver.md

@@ -50,7 +50,7 @@ kube-apiserver
 ### Options
 
 ```
-      --admission-control="AlwaysAdmit": Ordered list of plug-ins to do admission control of resources into cluster. Comma-delimited list of: AlwaysAdmit, AlwaysDeny, DenyEscalatingExec, DenyExecOnPrivileged, InitialResources, LimitRanger, NamespaceAutoProvision, NamespaceExists, NamespaceLifecycle, ResourceQuota, SecurityContextDeny, ServiceAccount
+      --admission-control="AlwaysAdmit": Ordered list of plug-ins to do admission control of resources into cluster. Comma-delimited list of: AlwaysAdmit, AlwaysDeny, AlwaysPullImages, DenyEscalatingExec, DenyExecOnPrivileged, InitialResources, LimitRanger, NamespaceAutoProvision, NamespaceExists, NamespaceLifecycle, ResourceQuota, SecurityContextDeny, ServiceAccount
       --admission-control-config-file="": File with admission control configuration.
       --advertise-address=<nil>: The IP address on which to advertise the apiserver to members of the cluster. This address must be reachable by the rest of the cluster. If blank, the --bind-address will be used. If --bind-address is unspecified, the host's default interface will be used.
       --allow-privileged[=false]: If true, allow privileged containers.
@@ -106,7 +106,7 @@ kube-apiserver
       --watch-cache[=true]: Enable watch caching in the apiserver
 ```
 
-###### Auto generated by spf13/cobra on 18-Dec-2015
+###### Auto generated by spf13/cobra on 22-Dec-2015
 
 
 <!-- BEGIN MUNGE: GENERATED_ANALYTICS -->

plugin/pkg/admission/alwayspullimages/admission.go

@@ -0,0 +1,70 @@
+/*
+Copyright 2015 The Kubernetes Authors All rights reserved.
+
+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 alwayspullimages contains an admission controller that modifies every new Pod to force
+// the image pull policy to Always. This is useful in a multitenant cluster so that users can be
+// assured that their private images can only be used by those who have the credentials to pull
+// them. Without this admission controller, once an image has been pulled to a node, any pod from
+// any user can use it simply by knowing the image's name (assuming the Pod is scheduled onto the
+// right node), without any authorization check against the image. With this admission controller
+// enabled, images are always pulled prior to starting containers, which means valid credentials are
+// required.
+package alwayspullimages
+
+import (
+	"io"
+
+	"k8s.io/kubernetes/pkg/admission"
+	"k8s.io/kubernetes/pkg/api"
+	apierrors "k8s.io/kubernetes/pkg/api/errors"
+	client "k8s.io/kubernetes/pkg/client/unversioned"
+)
+
+func init() {
+	admission.RegisterPlugin("AlwaysPullImages", func(client client.Interface, config io.Reader) (admission.Interface, error) {
+		return NewAlwaysPullImages(), nil
+	})
+}
+
+// alwaysPullImages is an implementation of admission.Interface.
+// It looks at all new pods and overrides each container's image pull policy to Always.
+type alwaysPullImages struct {
+	*admission.Handler
+}
+
+func (a *alwaysPullImages) Admit(attributes admission.Attributes) (err error) {
+	// Ignore all calls to subresources or resources other than pods.
+	if len(attributes.GetSubresource()) != 0 || attributes.GetResource() != 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")
+	}
+
+	for i := range pod.Spec.Containers {
+		pod.Spec.Containers[i].ImagePullPolicy = api.PullAlways
+	}
+
+	return nil
+}
+
+// NewAlwaysPullImages creates a new always pull images admission control handler
+func NewAlwaysPullImages() admission.Interface {
+	return &alwaysPullImages{
+		Handler: admission.NewHandler(admission.Create, admission.Update),
+	}
+}

plugin/pkg/admission/alwayspullimages/admission_test.go

@@ -0,0 +1,118 @@
+/*
+Copyright 2015 The Kubernetes Authors All rights reserved.
+
+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 alwayspullimages
+
+import (
+	"testing"
+
+	"k8s.io/kubernetes/pkg/admission"
+	"k8s.io/kubernetes/pkg/api"
+	"k8s.io/kubernetes/pkg/runtime"
+)
+
+// TestAdmission verifies all create requests for pods result in every container's image pull policy
+// set to Always
+func TestAdmission(t *testing.T) {
+	namespace := "test"
+	handler := &alwaysPullImages{}
+	pod := api.Pod{
+		ObjectMeta: api.ObjectMeta{Name: "123", Namespace: namespace},
+		Spec: api.PodSpec{
+			Containers: []api.Container{
+				{Name: "ctr1", Image: "image"},
+				{Name: "ctr2", Image: "image", ImagePullPolicy: api.PullNever},
+				{Name: "ctr3", Image: "image", ImagePullPolicy: api.PullIfNotPresent},
+				{Name: "ctr4", Image: "image", ImagePullPolicy: api.PullAlways},
+			},
+		},
+	}
+	err := handler.Admit(admission.NewAttributesRecord(&pod, api.Kind("Pod"), pod.Namespace, pod.Name, api.Resource("pods"), "", admission.Create, nil))
+	if err != nil {
+		t.Errorf("Unexpected error returned from admission handler")
+	}
+	for _, c := range pod.Spec.Containers {
+		if c.ImagePullPolicy != api.PullAlways {
+			t.Errorf("Container %s: expected pull always, got %v", c.ImagePullPolicy)
+		}
+	}
+}
+
+// TestOtherResources ensures that this admission controller is a no-op for other resources,
+// subresources, and non-pods.
+func TestOtherResources(t *testing.T) {
+	namespace := "testnamespace"
+	name := "testname"
+	pod := &api.Pod{
+		ObjectMeta: api.ObjectMeta{Name: name, Namespace: namespace},
+		Spec: api.PodSpec{
+			Containers: []api.Container{
+				{Name: "ctr2", Image: "image", ImagePullPolicy: api.PullNever},
+			},
+		},
+	}
+	tests := []struct {
+		name        string
+		kind        string
+		resource    string
+		subresource string
+		object      runtime.Object
+		expectError bool
+	}{
+		{
+			name:     "non-pod resource",
+			kind:     "Foo",
+			resource: "foos",
+			object:   pod,
+		},
+		{
+			name:        "pod subresource",
+			kind:        "Pod",
+			resource:    "pods",
+			subresource: "exec",
+			object:      pod,
+		},
+		{
+			name:        "non-pod object",
+			kind:        "Pod",
+			resource:    "pods",
+			object:      &api.Service{},
+			expectError: true,
+		},
+	}
+
+	for _, tc := range tests {
+		handler := &alwaysPullImages{}
+
+		err := handler.Admit(admission.NewAttributesRecord(tc.object, api.Kind(tc.kind), namespace, name, api.Resource(tc.resource), tc.subresource, admission.Create, nil))
+
+		if tc.expectError {
+			if err == nil {
+				t.Errorf("%s: unexpected nil error", tc.name)
+			}
+			continue
+		}
+
+		if err != nil {
+			t.Errorf("%s: unexpected error: %v", tc.name, err)
+			continue
+		}
+
+		if e, a := api.PullNever, pod.Spec.Containers[0].ImagePullPolicy; e != a {
+			t.Errorf("%s: image pull policy was changed to %s", tc.name, a)
+		}
+	}
+}