Add digest resolution to cosigned.

This change introduces a mutating webhook to complement our validating webhook.

The validating webhook in #799 began rejecting tag reference because tags are mutable and can drift between validation and resolution by the kubelet.  This change introduces a mutating webhook that resolves tags to digests as resources are created, so that users aren't necessarily forced to provide digests, but we get the benefits of them nonetheless.

Fixes: #784
Signed-off-by: Matt Moore <mattomata@gmail.com>

.github/workflows/kind-e2e-cosigned.yaml

@@ -64,6 +64,10 @@ jobs:
         sudo mv ko /usr/local/bin
         echo '::endgroup::'
 
+        echo '::group:: install yq'
+        go get github.com/mikefarah/yq/v4
+        echo '::endgroup::'
+
     - name: Check out code onto GOPATH
       uses: actions/checkout@v2
       with:

cmd/cosign/webhook/main.go

@@ -31,6 +31,7 @@ import (
 	"knative.dev/pkg/webhook"
 	"knative.dev/pkg/webhook/certificates"
 	"knative.dev/pkg/webhook/resourcesemantics"
+	"knative.dev/pkg/webhook/resourcesemantics/defaulting"
 	"knative.dev/pkg/webhook/resourcesemantics/validation"
 
 	cwebhook "github.com/sigstore/cosign/pkg/cosign/kubernetes/webhook"
@@ -58,9 +59,20 @@ func main() {
 	sharedmain.MainWithContext(ctx, "cosigned",
 		certificates.NewController,
 		NewValidatingAdmissionController,
+		NewMutatingAdmissionController,
 	)
 }
 
+var types = map[schema.GroupVersionKind]resourcesemantics.GenericCRD{
+	corev1.SchemeGroupVersion.WithKind("Pod"): &duckv1.Pod{},
+
+	appsv1.SchemeGroupVersion.WithKind("ReplicaSet"):  &duckv1.WithPod{},
+	appsv1.SchemeGroupVersion.WithKind("Deployment"):  &duckv1.WithPod{},
+	appsv1.SchemeGroupVersion.WithKind("StatefulSet"): &duckv1.WithPod{},
+	appsv1.SchemeGroupVersion.WithKind("DaemonSet"):   &duckv1.WithPod{},
+	batchv1.SchemeGroupVersion.WithKind("Job"):        &duckv1.WithPod{},
+}
+
 func NewValidatingAdmissionController(ctx context.Context, cmw configmap.Watcher) *controller.Impl {
 	validator := cwebhook.NewValidator(ctx, *secretName)
 
@@ -72,15 +84,7 @@ func NewValidatingAdmissionController(ctx context.Context, cmw configmap.Watcher
 		"/validations",
 
 		// The resources to validate.
-		map[schema.GroupVersionKind]resourcesemantics.GenericCRD{
-			corev1.SchemeGroupVersion.WithKind("Pod"): &duckv1.Pod{},
-
-			appsv1.SchemeGroupVersion.WithKind("ReplicaSet"):  &duckv1.WithPod{},
-			appsv1.SchemeGroupVersion.WithKind("Deployment"):  &duckv1.WithPod{},
-			appsv1.SchemeGroupVersion.WithKind("StatefulSet"): &duckv1.WithPod{},
-			appsv1.SchemeGroupVersion.WithKind("DaemonSet"):   &duckv1.WithPod{},
-			batchv1.SchemeGroupVersion.WithKind("Job"):        &duckv1.WithPod{},
-		},
+		types,
 
 		// A function that infuses the context passed to Validate/SetDefaults with custom metadata.
 		func(ctx context.Context) context.Context {
@@ -97,3 +101,29 @@ func NewValidatingAdmissionController(ctx context.Context, cmw configmap.Watcher
 		nil,
 	)
 }
+
+func NewMutatingAdmissionController(ctx context.Context, cmw configmap.Watcher) *controller.Impl {
+	validator := cwebhook.NewValidator(ctx, *secretName)
+
+	return defaulting.NewAdmissionController(ctx,
+		// Name of the resource webhook.
+		webhookName,
+
+		// The path on which to serve the webhook.
+		"/mutations",
+
+		// The resources to validate.
+		types,
+
+		// A function that infuses the context passed to Validate/SetDefaults with custom metadata.
+		func(ctx context.Context) context.Context {
+			ctx = duckv1.WithPodDefaulter(ctx, validator.ResolvePod)
+			ctx = duckv1.WithPodSpecDefaulter(ctx, validator.ResolvePodSpecable)
+			return ctx
+		},
+
+		// Whether to disallow unknown fields.
+		// We pass false because we're using partial schemas.
+		false,
+	)
+}

config/200-clusterrole.yaml

@@ -21,12 +21,12 @@ rules:
     resources: ["events"]
     verbs: ["create"]
 
-  # Allow the reconciliation of exactly our validating webhook.
+  # Allow the reconciliation of exactly our validating and mutating webhooks.
   - apiGroups: ["admissionregistration.k8s.io"]
-    resources: ["validatingwebhookconfigurations"]
+    resources: ["validatingwebhookconfigurations", "mutatingwebhookconfigurations"]
     verbs: ["list", "watch"]
   - apiGroups: ["admissionregistration.k8s.io"]
-    resources: ["validatingwebhookconfigurations"]
+    resources: ["validatingwebhookconfigurations", "mutatingwebhookconfigurations"]
     verbs: ["get", "update"]
     resourceNames: ["cosigned.sigstore.dev"]
 

config/500-webhook-configuration.yaml

@@ -32,6 +32,27 @@ webhooks:
   failurePolicy: Fail
   sideEffects: None
 
+---
+apiVersion: admissionregistration.k8s.io/v1
+kind: MutatingWebhookConfiguration
+metadata:
+  name: cosigned.sigstore.dev
+webhooks:
+- name: cosigned.sigstore.dev
+  namespaceSelector:
+    # The webhook should only apply to things that opt-in
+    matchExpressions:
+    - key: cosigned.sigstore.dev/include
+      operator: In
+      values: ["true"]
+  admissionReviewVersions: [v1]
+  clientConfig:
+    service:
+      name: webhook
+      namespace: cosign-system
+  failurePolicy: Fail
+  sideEffects: None
+
 ---
 apiVersion: v1
 kind: Secret

pkg/cosign/kubernetes/webhook/validator.go

@@ -20,11 +20,13 @@ import (
 	"fmt"
 
 	"github.com/google/go-containerregistry/pkg/name"
+	"github.com/sigstore/cosign/pkg/oci/remote"
 	corev1 "k8s.io/api/core/v1"
 	listersv1 "k8s.io/client-go/listers/core/v1"
 	"knative.dev/pkg/apis"
 	duckv1 "knative.dev/pkg/apis/duck/v1"
 	secretinformer "knative.dev/pkg/injection/clients/namespacedkube/informers/core/v1/secret"
+	"knative.dev/pkg/logging"
 	"knative.dev/pkg/system"
 )
 
@@ -91,3 +93,42 @@ func (v *Validator) validatePodSpec(ctx context.Context, ps *corev1.PodSpec) (er
 
 	return errs
 }
+
+// ResolvePodSpecable implements duckv1.PodSpecValidator
+func (v *Validator) ResolvePodSpecable(ctx context.Context, wp *duckv1.WithPod) {
+	v.resolvePodSpec(ctx, &wp.Spec.Template.Spec)
+}
+
+// ResolvePod implements duckv1.PodValidator
+func (v *Validator) ResolvePod(ctx context.Context, p *duckv1.Pod) {
+	v.resolvePodSpec(ctx, &p.Spec)
+}
+
+// For testing
+var remoteResolveDigest = remote.ResolveDigest
+
+func (v *Validator) resolvePodSpec(ctx context.Context, ps *corev1.PodSpec) {
+	resolveContainers := func(cs []corev1.Container) {
+		for i, c := range cs {
+			ref, err := name.ParseReference(c.Image)
+			if err != nil {
+				logging.FromContext(ctx).Debugf("Unable to parse reference: %v", err)
+				continue
+			}
+
+			// If we are in the context of a mutating webhook, then resolve the tag to a digest.
+			switch {
+			case apis.IsInCreate(ctx), apis.IsInUpdate(ctx):
+				digest, err := remoteResolveDigest(ref)
+				if err != nil {
+					logging.FromContext(ctx).Debugf("Unable to resolve digest %q: %v", ref.String(), err)
+					continue
+				}
+				cs[i].Image = digest.String()
+			}
+		}
+	}
+
+	resolveContainers(ps.InitContainers)
+	resolveContainers(ps.Containers)
+}

pkg/cosign/kubernetes/webhook/validator_test.go

@@ -20,9 +20,11 @@ import (
 	"errors"
 	"testing"
 
+	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-containerregistry/pkg/name"
 	"github.com/sigstore/cosign/pkg/cosign"
 	"github.com/sigstore/cosign/pkg/oci"
+	"github.com/sigstore/cosign/pkg/oci/remote"
 	"github.com/sigstore/cosign/pkg/oci/static"
 	corev1 "k8s.io/api/core/v1"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
@@ -147,3 +149,143 @@ UoJou2P8sbDxpLiE/v3yLw1/jyOrCPWYHWFXnyyeGlkgSVefG54tNoK7Uw==
 		})
 	}
 }
+
+func TestResolvePodSpec(t *testing.T) {
+	tag := name.MustParseReference("gcr.io/distroless/static:nonroot")
+	// Resolved via crane digest on 2021/09/25
+	digest := name.MustParseReference("gcr.io/distroless/static:nonroot@sha256:be5d77c62dbe7fedfb0a4e5ec2f91078080800ab1f18358e5f31fcc8faa023c4")
+
+	ctx, _ := rtesting.SetupFakeContext(t)
+	si := fakesecret.Get(ctx)
+
+	secretName := "blah"
+
+	si.Informer().GetIndexer().Add(&corev1.Secret{
+		ObjectMeta: metav1.ObjectMeta{
+			Namespace: system.Namespace(),
+			Name:      secretName,
+		},
+		Data: map[string][]byte{
+			// Random public key (cosign generate-key-pair) 2021-09-25
+			"cosign.pub": []byte(`-----BEGIN PUBLIC KEY-----
+MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEapTW568kniCbL0OXBFIhuhOboeox
+UoJou2P8sbDxpLiE/v3yLw1/jyOrCPWYHWFXnyyeGlkgSVefG54tNoK7Uw==
+-----END PUBLIC KEY-----
+`),
+		},
+	})
+
+	v := NewValidator(ctx, secretName)
+
+	rrd := remoteResolveDigest
+	defer func() {
+		remoteResolveDigest = rrd
+	}()
+	resolve := func(ref name.Reference, opts ...remote.Option) (name.Digest, error) {
+		return digest.(name.Digest), nil
+	}
+
+	tests := []struct {
+		name string
+		ps   *corev1.PodSpec
+		want *corev1.PodSpec
+		wc   func(context.Context) context.Context
+		rrd  func(name.Reference, ...remote.Option) (name.Digest, error)
+	}{{
+		name: "nothing changed (not the right update)",
+		ps: &corev1.PodSpec{
+			InitContainers: []corev1.Container{{
+				Name:  "setup-stuff",
+				Image: tag.String(),
+			}},
+			Containers: []corev1.Container{{
+				Name:  "user-container",
+				Image: tag.String(),
+			}},
+		},
+		want: &corev1.PodSpec{
+			InitContainers: []corev1.Container{{
+				Name:  "setup-stuff",
+				Image: tag.String(),
+			}},
+			Containers: []corev1.Container{{
+				Name:  "user-container",
+				Image: tag.String(),
+			}},
+		},
+		rrd: resolve,
+	}, {
+		name: "nothing changed (bad reference)",
+		ps: &corev1.PodSpec{
+			Containers: []corev1.Container{{
+				Name:  "user-container",
+				Image: "in@valid",
+			}},
+		},
+		want: &corev1.PodSpec{
+			Containers: []corev1.Container{{
+				Name:  "user-container",
+				Image: "in@valid",
+			}},
+		},
+		wc:  apis.WithinCreate,
+		rrd: resolve,
+	}, {
+		name: "nothing changed (unable to resolve)",
+		ps: &corev1.PodSpec{
+			Containers: []corev1.Container{{
+				Name:  "user-container",
+				Image: tag.String(),
+			}},
+		},
+		want: &corev1.PodSpec{
+			Containers: []corev1.Container{{
+				Name:  "user-container",
+				Image: tag.String(),
+			}},
+		},
+		wc: apis.WithinCreate,
+		rrd: func(r name.Reference, o ...remote.Option) (name.Digest, error) {
+			return name.Digest{}, errors.New("boom")
+		},
+	}, {
+		name: "digests resolve (in create)",
+		ps: &corev1.PodSpec{
+			InitContainers: []corev1.Container{{
+				Name:  "setup-stuff",
+				Image: tag.String(),
+			}},
+			Containers: []corev1.Container{{
+				Name:  "user-container",
+				Image: tag.String(),
+			}},
+		},
+		want: &corev1.PodSpec{
+			InitContainers: []corev1.Container{{
+				Name:  "setup-stuff",
+				Image: digest.String(),
+			}},
+			Containers: []corev1.Container{{
+				Name:  "user-container",
+				Image: digest.String(),
+			}},
+		},
+		wc:  apis.WithinCreate,
+		rrd: resolve,
+	}}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			remoteResolveDigest = test.rrd
+			got := test.ps.DeepCopy()
+			ctx := context.Background()
+			if test.wc != nil {
+				ctx = test.wc(context.Background())
+			}
+			v.resolvePodSpec(ctx, got)
+			if !cmp.Equal(got, test.want) {
+				t.Errorf("resolvePodSpec = %s", cmp.Diff(got, test.want))
+			}
+		})
+	}
+}