Update the DOKS admission controller webhook check

DOKS has improved handling of webhooks such that the only webhooks that
cause problems are those that:

* Have failurePolicy set to Fail,
* Target a service other than the Kubernetes apiserver, and
* Apply kube-system, and
* Applies to the namespace of the targeted service or are in a
  single-node cluster.

Update the webhook check to reflect this improvement.

checks.md

@@ -209,7 +209,10 @@ spec:
 - Name: `admission-controller-webhook`
 - Groups: `doks`
 
-When you configure admission controllers with webhooks that have a `failurePolicy` set to `Fail`, it prevents managed components like Cilium, kube-proxy, and CoreDNS from starting on a new node during an upgrade. This will result in the cluster upgrade failing.
+Admission control webhooks can disrupt upgrade and node replacement operations by preventing system components from starting. Specifically, this happens when an admission control webhook:
+* has failurePolicy set to Fail,
+* targets a service other than the Kubernetes apiserver, and
+* applies to both kube-system and the namespace of the targeted service.
 
 ### Example
 
@@ -233,7 +236,7 @@ webhooks:
     scope: "Namespaced"
   clientConfig:
     service:
-      namespace: kube-system
+      namespace: webhook
       name: webhook-server
       path: /pods
   admissionReviewVersions:
@@ -244,8 +247,23 @@ webhooks:
 
 ### How to Fix
 
+There are a few options:
+1. Use the `Ignore` `failurePolicy`.
+2. Use an apiserver extension as your webhook service.
+3. Explicitly exclude the kube-system namespace.
+4. Explicitly exclude the webhook service's namespace.
+
 ```yaml
-# Recommended: Exclude objects in the `kube-system` namespace by explicitly specifying a namespaceSelector or objectSelector
+# Recommended: Exclude objects in the `webhook` namespace by explicitly specifying a namespaceSelector.
+
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: webhook
+  labels:
+    skip-webhooks: "yes"
+
+---
 apiVersion: admissionregistration.k8s.io/v1beta1
 kind: ValidatingWebhookConfiguration
 metadata:
@@ -264,7 +282,7 @@ webhooks:
     scope: "Namespaced"
   clientConfig:
     service:
-      namespace: kube-system
+      namespace: webhook
       name: webhook-server
       path: /pods
   admissionReviewVersions:
@@ -273,8 +291,8 @@ webhooks:
   failurePolicy: Fail
   namespaceSelector:
     matchExpressions:
-      - key: "doks.digitalocean.com/webhook"
+      - key: "skip-webhooks"
-        operator: "Exists"
+        operator: "DoesNotExist"
 ```
 
 ## Pod State

checks/doks/admission_controller_webhook.go

@@ -17,10 +17,13 @@ limitations under the License.
 package doks
 
 import (
+	"errors"
+
 	"github.com/digitalocean/clusterlint/checks"
 	"github.com/digitalocean/clusterlint/kube"
 	ar "k8s.io/api/admissionregistration/v1beta1"
 	corev1 "k8s.io/api/core/v1"
+	v1 "k8s.io/api/core/v1"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 )
 
@@ -43,46 +46,112 @@ func (w *webhookCheck) Groups() []string {
 // Description returns a detailed human-readable description of what this check
 // does.
 func (w *webhookCheck) Description() string {
-	return "Check for admission controllers that could prevent managed components from starting"
+	return "Check for admission control webhooks that could cause problems during upgrades"
 }
 
-// Run runs this check on a set of Kubernetes objects. It can return warnings
+// Run runs this check on a set of Kubernetes objects.
-// (low-priority problems) and errors (high-priority problems) as well as an
-// error value indicating that the check failed to run.
 func (w *webhookCheck) Run(objects *kube.Objects) ([]checks.Diagnostic, error) {
+	const apiserverServiceName = "kubernetes"
+
 	var diagnostics []checks.Diagnostic
 
 	for _, config := range objects.ValidatingWebhookConfigurations.Items {
-		for _, validatingWebhook := range config.Webhooks {
+		for _, wh := range config.Webhooks {
-			if *validatingWebhook.FailurePolicy == ar.Fail &&
+			if *wh.FailurePolicy == ar.Ignore {
-				validatingWebhook.ClientConfig.Service != nil &&
+				// Webhooks with failurePolicy: Ignore are fine.
-				selectorMatchesNamespace(validatingWebhook.NamespaceSelector, objects.SystemNamespace) {
+				continue
+			}
+			if wh.ClientConfig.Service == nil {
+				// Webhooks whose targets are external to the cluster are fine.
+				continue
+			}
+			if wh.ClientConfig.Service.Namespace == metav1.NamespaceDefault &&
+				wh.ClientConfig.Service.Name == apiserverServiceName {
+				// Webhooks that target the kube-apiserver are fine.
+				continue
+			}
+			if !selectorMatchesNamespace(wh.NamespaceSelector, objects.SystemNamespace) {
+				// Webhooks that don't apply to kube-system are fine.
+				continue
+			}
+			var svcNamespace *v1.Namespace
+			for _, ns := range objects.Namespaces.Items {
+				if ns.Name == wh.ClientConfig.Service.Namespace {
+					svcNamespace = &ns
+				}
+			}
+			if svcNamespace == nil {
+				return nil, errors.New("webhook refers to service in non-existent namespace")
+			}
+			if !selectorMatchesNamespace(wh.NamespaceSelector, svcNamespace) && len(objects.Nodes.Items) > 1 {
+				// Webhooks that don't apply to their own namespace are fine, as
+				// long as there's more than one node in the cluster.
+				continue
+			}
+
 			d := checks.Diagnostic{
 				Severity: checks.Error,
-					Message:  "Webhook matches objects in the kube-system namespace. This can cause problems when upgrading the cluster.",
+				Message:  "Validating webhook is configured in such a way that it may be problematic during upgrades.",
 				Kind:     checks.ValidatingWebhookConfiguration,
 				Object:   &config.ObjectMeta,
 				Owners:   config.ObjectMeta.GetOwnerReferences(),
 			}
 			diagnostics = append(diagnostics, d)
-			}
+
+			// We don't want to produce diagnostics for multiple webhooks in the
+			// same webhook configuration, so break out of the inner loop if we
+			// get here.
+			break
 		}
 	}
 
 	for _, config := range objects.MutatingWebhookConfigurations.Items {
-		for _, mutatingWebhook := range config.Webhooks {
+		for _, wh := range config.Webhooks {
-			if *mutatingWebhook.FailurePolicy == ar.Fail &&
+			if *wh.FailurePolicy == ar.Ignore {
-				mutatingWebhook.ClientConfig.Service != nil &&
+				// Webhooks with failurePolicy: Ignore are fine.
-				selectorMatchesNamespace(mutatingWebhook.NamespaceSelector, objects.SystemNamespace) {
+				continue
+			}
+			if wh.ClientConfig.Service == nil {
+				// Webhooks whose targets are external to the cluster are fine.
+				continue
+			}
+			if wh.ClientConfig.Service.Namespace == metav1.NamespaceDefault &&
+				wh.ClientConfig.Service.Name == apiserverServiceName {
+				// Webhooks that target the kube-apiserver are fine.
+				continue
+			}
+			if !selectorMatchesNamespace(wh.NamespaceSelector, objects.SystemNamespace) {
+				// Webhooks that don't apply to kube-system are fine.
+				continue
+			}
+			var svcNamespace *v1.Namespace
+			for _, ns := range objects.Namespaces.Items {
+				if ns.Name == wh.ClientConfig.Service.Namespace {
+					svcNamespace = &ns
+				}
+			}
+			if svcNamespace == nil {
+				return nil, errors.New("webhook refers to service in non-existent namespace")
+			}
+			if !selectorMatchesNamespace(wh.NamespaceSelector, svcNamespace) && len(objects.Nodes.Items) > 1 {
+				// Webhooks that don't apply to their own namespace are fine, as
+				// long as there's more than one node in the cluster.
+				continue
+			}
+
 			d := checks.Diagnostic{
 				Severity: checks.Error,
-					Message:  "Webhook matches objects in the kube-system namespace. This can cause problems when upgrading the cluster.",
+				Message:  "Mutating webhook is configured in such a way that it may be problematic during upgrades.",
 				Kind:     checks.MutatingWebhookConfiguration,
 				Object:   &config.ObjectMeta,
 				Owners:   config.ObjectMeta.GetOwnerReferences(),
 			}
 			diagnostics = append(diagnostics, d)
-			}
+
+			// We don't want to produce diagnostics for multiple webhooks in the
+			// same webhook configuration, so break out of the inner loop if we
+			// get here.
+			break
 		}
 	}
 	return diagnostics, nil

checks/doks/admission_controller_webhook_test.go

@@ -27,6 +27,8 @@ import (
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 )
 
+var webhookURL = "https://example.com/webhook"
+
 func TestWebhookCheckMeta(t *testing.T) {
 	webhookCheck := webhookCheck{}
 	assert.Equal(t, "admission-controller-webhook", webhookCheck.Name())
@@ -58,69 +60,180 @@ func TestWebhookError(t *testing.T) {
 		},
 		{
 			name: "failure policy is ignore",
-			objs:     initObjects(ar.Ignore),
+			objs: webhookTestObjects(
+				ar.Ignore,
+				&metav1.LabelSelector{},
+				ar.WebhookClientConfig{
+					Service: &ar.ServiceReference{
+						Namespace: "webhook",
+						Name:      "webhook-service",
+					},
+				},
+				2,
+			),
 			expected: nil,
 		},
 		{
 			name: "webook does not use service",
-			objs:     webhookURL(),
+			objs: webhookTestObjects(
+				ar.Fail,
+				&metav1.LabelSelector{},
+				ar.WebhookClientConfig{
+					URL: &webhookURL,
+				},
+				2,
+			),
 			expected: nil,
 		},
 		{
-			name:     "namespace selector is empty",
+			name: "webook service is apiserver",
-			objs:     initObjects(ar.Fail),
+			objs: webhookTestObjects(
-			expected: webhookErrors(),
+				ar.Fail,
+				&metav1.LabelSelector{},
+				ar.WebhookClientConfig{
+					Service: &ar.ServiceReference{
+						Namespace: "default",
+						Name:      "kubernetes",
 					},
-		{
-			name:     "namespace selector matches label",
-			objs:     label(map[string]string{"doks_key": "bar"}),
-			expected: webhookErrors(),
 				},
-		{
+				2,
-			name:     "namespace selector does not match label",
+			),
-			objs:     label(map[string]string{"non-existent-label-on-namespace": "bar"}),
 			expected: nil,
 		},
 		{
-			name:     "namespace selector matches OpExists expression",
+			name: "namespace label selector does not match kube-system",
-			objs:     expr("doks_key", []string{}, metav1.LabelSelectorOpExists),
+			objs: webhookTestObjects(
-			expected: webhookErrors(),
+				ar.Fail,
+				&metav1.LabelSelector{
+					MatchLabels: map[string]string{"non-existent-label-on-namespace": "bar"},
 				},
-		{
+				ar.WebhookClientConfig{
-			name:     "namespace selector matches OpDoesNotExist expression",
+					Service: &ar.ServiceReference{
-			objs:     expr("random", []string{}, metav1.LabelSelectorOpDoesNotExist),
+						Namespace: "webhook",
-			expected: webhookErrors(),
+						Name:      "webhook-service",
 					},
-		{
-			name:     "namespace selector matches OpIn expression",
-			objs:     expr("doks_key", []string{"bar"}, metav1.LabelSelectorOpIn),
-			expected: webhookErrors(),
 				},
-		{
+				2,
-			name:     "namespace selector matches OpNotIn expression",
+			),
-			objs:     expr("doks_key", []string{"non-existent"}, metav1.LabelSelectorOpNotIn),
-			expected: webhookErrors(),
-		},
-		{
-			name:     "namespace selector does not match OpExists expression",
-			objs:     expr("non-existent", []string{}, metav1.LabelSelectorOpExists),
 			expected: nil,
 		},
 		{
-			name:     "namespace selector does not match OpDoesNotExist expression",
+			name: "namespace OpExists expression selector does not match kube-system",
-			objs:     expr("doks_key", []string{}, metav1.LabelSelectorOpDoesNotExist),
+			objs: webhookTestObjects(
+				ar.Fail,
+				&metav1.LabelSelector{
+					MatchExpressions: expr("non-existent", []string{}, metav1.LabelSelectorOpExists),
+				},
+				ar.WebhookClientConfig{
+					Service: &ar.ServiceReference{
+						Namespace: "webhook",
+						Name:      "webhook-service",
+					},
+				},
+				2,
+			),
 			expected: nil,
 		},
 		{
-			name:     "namespace selector does not match OpIn expression",
+			name: "namespace OpDoesNotExist expression selector does not match kube-system",
-			objs:     expr("doks_key", []string{"non-existent"}, metav1.LabelSelectorOpIn),
+			objs: webhookTestObjects(
+				ar.Fail,
+				&metav1.LabelSelector{
+					MatchExpressions: expr("doks_key", []string{}, metav1.LabelSelectorOpDoesNotExist),
+				},
+				ar.WebhookClientConfig{
+					Service: &ar.ServiceReference{
+						Namespace: "webhook",
+						Name:      "webhook-service",
+					},
+				},
+				2,
+			),
 			expected: nil,
 		},
 		{
-			name:     "namespace selector does not match OpNotIn expression",
+			name: "namespace OpIn expression selector does not match kube-system",
-			objs:     expr("doks_key", []string{"bar"}, metav1.LabelSelectorOpNotIn),
+			objs: webhookTestObjects(
+				ar.Fail,
+				&metav1.LabelSelector{
+					MatchExpressions: expr("doks_key", []string{"non-existent"}, metav1.LabelSelectorOpIn),
+				},
+				ar.WebhookClientConfig{
+					Service: &ar.ServiceReference{
+						Namespace: "webhook",
+						Name:      "webhook-service",
+					},
+				},
+				2,
+			),
 			expected: nil,
 		},
+		{
+			name: "namespace OpNotIn expression selector does not match kube-system",
+			objs: webhookTestObjects(
+				ar.Fail,
+				&metav1.LabelSelector{
+					MatchExpressions: expr("doks_key", []string{"bar"}, metav1.LabelSelectorOpNotIn),
+				},
+				ar.WebhookClientConfig{
+					Service: &ar.ServiceReference{
+						Namespace: "webhook",
+						Name:      "webhook-service",
+					},
+				},
+				2,
+			),
+			expected: nil,
+		},
+		{
+			name: "namespace label selector does not match own namespace",
+			objs: webhookTestObjects(
+				ar.Fail,
+				&metav1.LabelSelector{
+					MatchLabels: map[string]string{"doks_key": "bar"},
+				},
+				ar.WebhookClientConfig{
+					Service: &ar.ServiceReference{
+						Namespace: "webhook",
+						Name:      "webhook-service",
+					},
+				},
+				2,
+			),
+			expected: nil,
+		},
+		{
+			name: "single-node cluster",
+			objs: webhookTestObjects(
+				ar.Fail,
+				&metav1.LabelSelector{
+					MatchLabels: map[string]string{"doks_key": "bar"},
+				},
+				ar.WebhookClientConfig{
+					Service: &ar.ServiceReference{
+						Namespace: "webhook",
+						Name:      "webhook-service",
+					},
+				},
+				1,
+			),
+			expected: webhookErrors(),
+		},
+		{
+			name: "webhook applies to its own namespace and kube-system",
+			objs: webhookTestObjects(
+				ar.Fail,
+				&metav1.LabelSelector{},
+				ar.WebhookClientConfig{
+					Service: &ar.ServiceReference{
+						Namespace: "webhook",
+						Name:      "webhook-service",
+					},
+				},
+				2,
+			),
+			expected: webhookErrors(),
+		},
 	}
 
 	webhookCheck := webhookCheck{}
@@ -134,13 +247,45 @@ func TestWebhookError(t *testing.T) {
 	}
 }
 
-func initObjects(failurePolicyType ar.FailurePolicyType) *kube.Objects {
+func expr(key string, values []string, op metav1.LabelSelectorOperator) []metav1.LabelSelectorRequirement {
+	return []metav1.LabelSelectorRequirement{{
+		Key:      key,
+		Operator: op,
+		Values:   values,
+	}}
+}
+
+func webhookTestObjects(
+	failurePolicyType ar.FailurePolicyType,
+	nsSelector *metav1.LabelSelector,
+	clientConfig ar.WebhookClientConfig,
+	numNodes int,
+) *kube.Objects {
 	objs := &kube.Objects{
 		SystemNamespace: &corev1.Namespace{
 			TypeMeta: metav1.TypeMeta{Kind: "Namespace", APIVersion: "v1"},
 			ObjectMeta: metav1.ObjectMeta{
 				Name:   "kube-system",
-				Labels: map[string]string{"doks_key": "bar"}},
+				Labels: map[string]string{"doks_key": "bar"},
+			},
+		},
+		Namespaces: &corev1.NamespaceList{
+			Items: []corev1.Namespace{
+				{
+					TypeMeta: metav1.TypeMeta{Kind: "Namespace", APIVersion: "v1"},
+					ObjectMeta: metav1.ObjectMeta{
+						Name:   "kube-system",
+						Labels: map[string]string{"doks_key": "bar"},
+					},
+				},
+				{
+					TypeMeta: metav1.TypeMeta{Kind: "Namespace", APIVersion: "v1"},
+					ObjectMeta: metav1.ObjectMeta{
+						Name:   "webhook",
+						Labels: map[string]string{"doks_key": "xyzzy"},
+					},
+				},
+			},
 		},
 		MutatingWebhookConfigurations: &ar.MutatingWebhookConfigurationList{
 			Items: []ar.MutatingWebhookConfiguration{
@@ -153,13 +298,8 @@ func initObjects(failurePolicyType ar.FailurePolicyType) *kube.Objects {
 						{
 							Name:              "mw_foo",
 							FailurePolicy:     &failurePolicyType,
-							NamespaceSelector: &metav1.LabelSelector{},
+							NamespaceSelector: nsSelector,
-							ClientConfig: ar.WebhookClientConfig{
+							ClientConfig:      clientConfig,
-								Service: &ar.ServiceReference{
-									Name:      "some-svc",
-									Namespace: "k8s",
-								},
-							},
 						},
 					},
 				},
@@ -176,83 +316,38 @@ func initObjects(failurePolicyType ar.FailurePolicyType) *kube.Objects {
 						{
 							Name:              "vw_foo",
 							FailurePolicy:     &failurePolicyType,
-							NamespaceSelector: &metav1.LabelSelector{},
+							NamespaceSelector: nsSelector,
-							ClientConfig: ar.WebhookClientConfig{
+							ClientConfig:      clientConfig,
-								Service: &ar.ServiceReference{
-									Name:      "some-svc",
-									Namespace: "k8s",
-								},
-							},
 						},
 					},
 				},
 			},
 		},
 	}
-	return objs
-}
 
-func webhookURL() *kube.Objects {
+	objs.Nodes = &corev1.NodeList{}
-	var url = "https://example.com/webhook/action"
+	for i := 0; i < numNodes; i++ {
-	objs := initObjects(ar.Fail)
+		objs.Nodes.Items = append(objs.Nodes.Items, corev1.Node{})
-	objs.ValidatingWebhookConfigurations.Items[0].Webhooks[0].ClientConfig = ar.WebhookClientConfig{
-		URL: &url,
-	}
-	objs.MutatingWebhookConfigurations.Items[0].Webhooks[0].ClientConfig = ar.WebhookClientConfig{
-		URL: &url,
-	}
-	return objs
-}
-
-func label(label map[string]string) *kube.Objects {
-	objs := initObjects(ar.Fail)
-	objs.ValidatingWebhookConfigurations.Items[0].Webhooks[0].NamespaceSelector = &metav1.LabelSelector{
-		MatchLabels: label,
-	}
-	objs.MutatingWebhookConfigurations.Items[0].Webhooks[0].NamespaceSelector = &metav1.LabelSelector{
-		MatchLabels: label,
-	}
-	return objs
-}
-
-func expr(key string, values []string, labelOperator metav1.LabelSelectorOperator) *kube.Objects {
-	objs := initObjects(ar.Fail)
-	objs.ValidatingWebhookConfigurations.Items[0].Webhooks[0].NamespaceSelector = &metav1.LabelSelector{
-		MatchExpressions: []metav1.LabelSelectorRequirement{
-			{
-				Key:      key,
-				Operator: labelOperator,
-				Values:   values,
-			},
-		},
-	}
-	objs.MutatingWebhookConfigurations.Items[0].Webhooks[0].NamespaceSelector = &metav1.LabelSelector{
-		MatchExpressions: []metav1.LabelSelectorRequirement{
-			{
-				Key:      key,
-				Operator: labelOperator,
-				Values:   values,
-			},
-		},
 	}
 	return objs
 }
 
 func webhookErrors() []checks.Diagnostic {
-	objs := initObjects(ar.Fail)
+	objs := webhookTestObjects(ar.Fail, nil, ar.WebhookClientConfig{}, 0)
 	validatingConfig := objs.ValidatingWebhookConfigurations.Items[0]
 	mutatingConfig := objs.MutatingWebhookConfigurations.Items[0]
+
 	diagnostics := []checks.Diagnostic{
 		{
 			Severity: checks.Error,
-			Message:  "Webhook matches objects in the kube-system namespace. This can cause problems when upgrading the cluster.",
+			Message:  "Validating webhook is configured in such a way that it may be problematic during upgrades.",
 			Kind:     checks.ValidatingWebhookConfiguration,
 			Object:   &validatingConfig.ObjectMeta,
 			Owners:   validatingConfig.ObjectMeta.GetOwnerReferences(),
 		},
 		{
 			Severity: checks.Error,
-			Message:  "Webhook matches objects in the kube-system namespace. This can cause problems when upgrading the cluster.",
+			Message:  "Mutating webhook is configured in such a way that it may be problematic during upgrades.",
 			Kind:     checks.MutatingWebhookConfiguration,
 			Object:   &mutatingConfig.ObjectMeta,
 			Owners:   mutatingConfig.ObjectMeta.GetOwnerReferences(),

checks/doks/node_name_pod_selector_test.go

@@ -56,7 +56,7 @@ func TestNodeNameError(t *testing.T) {
 		{
 			name:     "node name used in node selector",
 			objs:     invalidPod(),
-			expected: warnings(invalidPod(), podSelectorCheck.Name()),
+			expected: expectedWarnings(invalidPod(), podSelectorCheck.Name()),
 		},
 	}
 
@@ -89,7 +89,7 @@ func invalidPod() *kube.Objects {
 	return objs
 }
 
-func warnings(objs *kube.Objects, name string) []checks.Diagnostic {
+func expectedWarnings(objs *kube.Objects, name string) []checks.Diagnostic {
 	pod := objs.Pods.Items[0]
 	diagnostics := []checks.Diagnostic{
 		{

kube/objects.go

@@ -54,6 +54,7 @@ type Objects struct {
 	LimitRanges                     *corev1.LimitRangeList
 	MutatingWebhookConfigurations   *ar.MutatingWebhookConfigurationList
 	ValidatingWebhookConfigurations *ar.ValidatingWebhookConfigurationList
+	Namespaces                      *corev1.NamespaceList
 }
 
 // Client encapsulates a client for a Kubernetes cluster.
@@ -131,6 +132,10 @@ func (c *Client) FetchObjects(ctx context.Context) (*Objects, error) {
 		objects.ValidatingWebhookConfigurations, err = admissionControllerClient.ValidatingWebhookConfigurations().List(opts)
 		return
 	})
+	g.Go(func() (err error) {
+		objects.Namespaces, err = client.Namespaces().List(opts)
+		return
+	})
 
 	err := g.Wait()
 	if err != nil {