Define ClusterTrustBundlePEM projected volume

This commit defines the ClusterTrustBundlePEM projected volume types.
These types have been renamed from the KEP (PEMTrustAnchors) in order to
leave open the possibility of a similar projection drawing from a
yet-to-exist namespaced-scoped TrustBundle object, which came up during
KEP discussion.

* Add the projection field to internal and v1 APIs.
* Add validation to ensure that usages of the project must specify a
  name and path.
* Add TODO covering admission control to forbid mirror pods from using
  the projection.

Part of KEP-3257.

pkg/api/pod/util.go

@@ -549,6 +549,7 @@ func dropDisabledFields(
 	dropDisabledMatchLabelKeysFieldInTopologySpread(podSpec, oldPodSpec)
 	dropDisabledMatchLabelKeysFieldInPodAffinity(podSpec, oldPodSpec)
 	dropDisabledDynamicResourceAllocationFields(podSpec, oldPodSpec)
+	dropDisabledClusterTrustBundleProjection(podSpec, oldPodSpec)
 
 	if !utilfeature.DefaultFeatureGate.Enabled(features.InPlacePodVerticalScaling) && !inPlacePodVerticalScalingInUse(oldPodSpec) {
 		// Drop ResizePolicy fields. Don't drop updates to Resources field as template.spec.resources
@@ -969,6 +970,53 @@ func restartableInitContainersInUse(podSpec *api.PodSpec) bool {
 	return inUse
 }
 
+func clusterTrustBundleProjectionInUse(podSpec *api.PodSpec) bool {
+	if podSpec == nil {
+		return false
+	}
+	for _, v := range podSpec.Volumes {
+		if v.Projected == nil {
+			continue
+		}
+
+		for _, s := range v.Projected.Sources {
+			if s.ClusterTrustBundle != nil {
+				return true
+			}
+		}
+	}
+
+	return false
+}
+
+func dropDisabledClusterTrustBundleProjection(podSpec, oldPodSpec *api.PodSpec) {
+	if utilfeature.DefaultFeatureGate.Enabled(features.ClusterTrustBundleProjection) {
+		return
+	}
+	if podSpec == nil {
+		return
+	}
+
+	// If the pod was already using it, it can keep using it.
+	if clusterTrustBundleProjectionInUse(oldPodSpec) {
+		return
+	}
+
+	for _, v := range podSpec.Volumes {
+		if v.Projected == nil {
+			continue
+		}
+
+		filteredSources := []api.VolumeProjection{}
+		for _, s := range v.Projected.Sources {
+			if s.ClusterTrustBundle == nil {
+				filteredSources = append(filteredSources, s)
+			}
+		}
+		v.Projected.Sources = filteredSources
+	}
+}
+
 func hasInvalidLabelValueInAffinitySelector(spec *api.PodSpec) bool {
 	if spec.Affinity != nil {
 		if spec.Affinity.PodAffinity != nil {

pkg/apis/certificates/validation/validation.go

@@ -21,14 +21,11 @@ import (
 	"crypto/x509"
 	"encoding/pem"
 	"fmt"
-	"strings"
 
 	"github.com/google/go-cmp/cmp"
 	v1 "k8s.io/api/core/v1"
 	apiequality "k8s.io/apimachinery/pkg/api/equality"
-	apimachineryvalidation "k8s.io/apimachinery/pkg/api/validation"
 	"k8s.io/apimachinery/pkg/util/sets"
-	utilvalidation "k8s.io/apimachinery/pkg/util/validation"
 	"k8s.io/apimachinery/pkg/util/validation/field"
 	utilcert "k8s.io/client-go/util/cert"
 	"k8s.io/kubernetes/pkg/apis/certificates"
@@ -198,7 +195,7 @@ func validateCertificateSigningRequest(csr *certificates.CertificateSigningReque
 	if !opts.allowLegacySignerName && csr.Spec.SignerName == certificates.LegacyUnknownSignerName {
 		allErrs = append(allErrs, field.Invalid(specPath.Child("signerName"), csr.Spec.SignerName, "the legacy signerName is not allowed via this API version"))
 	} else {
-		allErrs = append(allErrs, ValidateSignerName(specPath.Child("signerName"), csr.Spec.SignerName)...)
+		allErrs = append(allErrs, apivalidation.ValidateSignerName(specPath.Child("signerName"), csr.Spec.SignerName)...)
 	}
 	if csr.Spec.ExpirationSeconds != nil && *csr.Spec.ExpirationSeconds < 600 {
 		allErrs = append(allErrs, field.Invalid(specPath.Child("expirationSeconds"), *csr.Spec.ExpirationSeconds, "may not specify a duration less than 600 seconds (10 minutes)"))
@@ -266,82 +263,6 @@ func validateConditions(fldPath *field.Path, csr *certificates.CertificateSignin
 	return allErrs
 }
 
-// ensure signerName is of the form domain.com/something and up to 571 characters.
-// This length and format is specified to accommodate signerNames like:
-// <fqdn>/<resource-namespace>.<resource-name>.
-// The max length of a FQDN is 253 characters (DNS1123Subdomain max length)
-// The max length of a namespace name is 63 characters (DNS1123Label max length)
-// The max length of a resource name is 253 characters (DNS1123Subdomain max length)
-// We then add an additional 2 characters to account for the one '.' and one '/'.
-func ValidateSignerName(fldPath *field.Path, signerName string) field.ErrorList {
-	var el field.ErrorList
-	if len(signerName) == 0 {
-		el = append(el, field.Required(fldPath, ""))
-		return el
-	}
-
-	segments := strings.Split(signerName, "/")
-	// validate that there is one '/' in the signerName.
-	// we do this after validating the domain segment to provide more info to the user.
-	if len(segments) != 2 {
-		el = append(el, field.Invalid(fldPath, signerName, "must be a fully qualified domain and path of the form 'example.com/signer-name'"))
-		// return early here as we should not continue attempting to validate a missing or malformed path segment
-		// (i.e. one containing multiple or zero `/`)
-		return el
-	}
-
-	// validate that segments[0] is less than 253 characters altogether
-	maxDomainSegmentLength := utilvalidation.DNS1123SubdomainMaxLength
-	if len(segments[0]) > maxDomainSegmentLength {
-		el = append(el, field.TooLong(fldPath, segments[0], maxDomainSegmentLength))
-	}
-	// validate that segments[0] consists of valid DNS1123 labels separated by '.'
-	domainLabels := strings.Split(segments[0], ".")
-	for _, lbl := range domainLabels {
-		// use IsDNS1123Label as we want to ensure the max length of any single label in the domain
-		// is 63 characters
-		if errs := utilvalidation.IsDNS1123Label(lbl); len(errs) > 0 {
-			for _, err := range errs {
-				el = append(el, field.Invalid(fldPath, segments[0], fmt.Sprintf("validating label %q: %s", lbl, err)))
-			}
-			// if we encounter any errors whilst parsing the domain segment, break from
-			// validation as any further error messages will be duplicates, and non-distinguishable
-			// from each other, confusing users.
-			break
-		}
-	}
-
-	// validate that there is at least one '.' in segments[0]
-	if len(domainLabels) < 2 {
-		el = append(el, field.Invalid(fldPath, segments[0], "should be a domain with at least two segments separated by dots"))
-	}
-
-	// validate that segments[1] consists of valid DNS1123 subdomains separated by '.'.
-	pathLabels := strings.Split(segments[1], ".")
-	for _, lbl := range pathLabels {
-		// use IsDNS1123Subdomain because it enforces a length restriction of 253 characters
-		// which is required in order to fit a full resource name into a single 'label'
-		if errs := utilvalidation.IsDNS1123Subdomain(lbl); len(errs) > 0 {
-			for _, err := range errs {
-				el = append(el, field.Invalid(fldPath, segments[1], fmt.Sprintf("validating label %q: %s", lbl, err)))
-			}
-			// if we encounter any errors whilst parsing the path segment, break from
-			// validation as any further error messages will be duplicates, and non-distinguishable
-			// from each other, confusing users.
-			break
-		}
-	}
-
-	// ensure that segments[1] can accommodate a dns label + dns subdomain + '.'
-	maxPathSegmentLength := utilvalidation.DNS1123SubdomainMaxLength + utilvalidation.DNS1123LabelMaxLength + 1
-	maxSignerNameLength := maxDomainSegmentLength + maxPathSegmentLength + 1
-	if len(signerName) > maxSignerNameLength {
-		el = append(el, field.TooLong(fldPath, signerName, maxSignerNameLength))
-	}
-
-	return el
-}
-
 func ValidateCertificateSigningRequestUpdate(newCSR, oldCSR *certificates.CertificateSigningRequest) field.ErrorList {
 	opts := getValidationOptions(newCSR, oldCSR)
 	return validateCertificateSigningRequestUpdate(newCSR, oldCSR, opts)
@@ -539,24 +460,6 @@ func hasDuplicateUsage(usages []certificates.KeyUsage) bool {
 	return false
 }
 
-// We require your name to be prefixed by .spec.signerName
-func validateClusterTrustBundleName(signerName string) func(name string, prefix bool) []string {
-	return func(name string, isPrefix bool) []string {
-		if signerName == "" {
-			if strings.Contains(name, ":") {
-				return []string{"ClusterTrustBundle without signer name must not have \":\" in its name"}
-			}
-			return apimachineryvalidation.NameIsDNSSubdomain(name, isPrefix)
-		}
-
-		requiredPrefix := strings.ReplaceAll(signerName, "/", ":") + ":"
-		if !strings.HasPrefix(name, requiredPrefix) {
-			return []string{fmt.Sprintf("ClusterTrustBundle for signerName %s must be named with prefix %s", signerName, requiredPrefix)}
-		}
-		return apimachineryvalidation.NameIsDNSSubdomain(strings.TrimPrefix(name, requiredPrefix), isPrefix)
-	}
-}
-
 type ValidateClusterTrustBundleOptions struct {
 	SuppressBundleParsing bool
 }
@@ -565,11 +468,11 @@ type ValidateClusterTrustBundleOptions struct {
 func ValidateClusterTrustBundle(bundle *certificates.ClusterTrustBundle, opts ValidateClusterTrustBundleOptions) field.ErrorList {
 	var allErrors field.ErrorList
 
-	metaErrors := apivalidation.ValidateObjectMeta(&bundle.ObjectMeta, false, validateClusterTrustBundleName(bundle.Spec.SignerName), field.NewPath("metadata"))
+	metaErrors := apivalidation.ValidateObjectMeta(&bundle.ObjectMeta, false, apivalidation.ValidateClusterTrustBundleName(bundle.Spec.SignerName), field.NewPath("metadata"))
 	allErrors = append(allErrors, metaErrors...)
 
 	if bundle.Spec.SignerName != "" {
-		signerNameErrors := ValidateSignerName(field.NewPath("spec", "signerName"), bundle.Spec.SignerName)
+		signerNameErrors := apivalidation.ValidateSignerName(field.NewPath("spec", "signerName"), bundle.Spec.SignerName)
 		allErrors = append(allErrors, signerNameErrors...)
 	}
 

pkg/apis/core/types.go

@@ -1759,6 +1759,29 @@ type ServiceAccountTokenProjection struct {
 	Path string
 }
 
+// ClusterTrustBundleProjection allows a pod to access the
+// `.spec.trustBundle` field of a ClusterTrustBundle object in an auto-updating
+// file.
+type ClusterTrustBundleProjection struct {
+	// Select a single ClusterTrustBundle by object name.   Mutually-exclusive
+	// with SignerName and LabelSelector.
+	Name *string
+
+	// Select all ClusterTrustBundles for this signer that match LabelSelector.
+	// Mutually-exclusive with Name.
+	SignerName *string
+
+	// Select all ClusterTrustBundles that match this LabelSelecotr.
+	// Mutually-exclusive with Name.
+	LabelSelector *metav1.LabelSelector
+
+	// Block pod startup if the selected ClusterTrustBundle(s) aren't available?
+	Optional *bool
+
+	// Relative path from the volume root to write the bundle.
+	Path string
+}
+
 // ProjectedVolumeSource represents a projected volume source
 type ProjectedVolumeSource struct {
 	// list of volume projections
@@ -1784,6 +1807,8 @@ type VolumeProjection struct {
 	ConfigMap *ConfigMapProjection
 	// information about the serviceAccountToken data to project
 	ServiceAccountToken *ServiceAccountTokenProjection
+	// information about the ClusterTrustBundle data to project
+	ClusterTrustBundle *ClusterTrustBundleProjection
 }
 
 // KeyToPath maps a string key to a path within a volume.

pkg/apis/core/validation/names.go

@@ -0,0 +1,132 @@
+/*
+Copyright 2023 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 validation
+
+import (
+	"fmt"
+	"strings"
+
+	apimachineryvalidation "k8s.io/apimachinery/pkg/api/validation"
+	"k8s.io/apimachinery/pkg/util/validation"
+	"k8s.io/apimachinery/pkg/util/validation/field"
+)
+
+// ValidateSignerName checks that signerName is syntactically valid.
+//
+// ensure signerName is of the form domain.com/something and up to 571 characters.
+// This length and format is specified to accommodate signerNames like:
+// <fqdn>/<resource-namespace>.<resource-name>.
+// The max length of a FQDN is 253 characters (DNS1123Subdomain max length)
+// The max length of a namespace name is 63 characters (DNS1123Label max length)
+// The max length of a resource name is 253 characters (DNS1123Subdomain max length)
+// We then add an additional 2 characters to account for the one '.' and one '/'.
+func ValidateSignerName(fldPath *field.Path, signerName string) field.ErrorList {
+	var el field.ErrorList
+	if len(signerName) == 0 {
+		el = append(el, field.Required(fldPath, ""))
+		return el
+	}
+
+	segments := strings.Split(signerName, "/")
+	// validate that there is one '/' in the signerName.
+	// we do this after validating the domain segment to provide more info to the user.
+	if len(segments) != 2 {
+		el = append(el, field.Invalid(fldPath, signerName, "must be a fully qualified domain and path of the form 'example.com/signer-name'"))
+		// return early here as we should not continue attempting to validate a missing or malformed path segment
+		// (i.e. one containing multiple or zero `/`)
+		return el
+	}
+
+	// validate that segments[0] is less than 253 characters altogether
+	maxDomainSegmentLength := validation.DNS1123SubdomainMaxLength
+	if len(segments[0]) > maxDomainSegmentLength {
+		el = append(el, field.TooLong(fldPath, segments[0], maxDomainSegmentLength))
+	}
+	// validate that segments[0] consists of valid DNS1123 labels separated by '.'
+	domainLabels := strings.Split(segments[0], ".")
+	for _, lbl := range domainLabels {
+		// use IsDNS1123Label as we want to ensure the max length of any single label in the domain
+		// is 63 characters
+		if errs := validation.IsDNS1123Label(lbl); len(errs) > 0 {
+			for _, err := range errs {
+				el = append(el, field.Invalid(fldPath, segments[0], fmt.Sprintf("validating label %q: %s", lbl, err)))
+			}
+			// if we encounter any errors whilst parsing the domain segment, break from
+			// validation as any further error messages will be duplicates, and non-distinguishable
+			// from each other, confusing users.
+			break
+		}
+	}
+
+	// validate that there is at least one '.' in segments[0]
+	if len(domainLabels) < 2 {
+		el = append(el, field.Invalid(fldPath, segments[0], "should be a domain with at least two segments separated by dots"))
+	}
+
+	// validate that segments[1] consists of valid DNS1123 subdomains separated by '.'.
+	pathLabels := strings.Split(segments[1], ".")
+	for _, lbl := range pathLabels {
+		// use IsDNS1123Subdomain because it enforces a length restriction of 253 characters
+		// which is required in order to fit a full resource name into a single 'label'
+		if errs := validation.IsDNS1123Subdomain(lbl); len(errs) > 0 {
+			for _, err := range errs {
+				el = append(el, field.Invalid(fldPath, segments[1], fmt.Sprintf("validating label %q: %s", lbl, err)))
+			}
+			// if we encounter any errors whilst parsing the path segment, break from
+			// validation as any further error messages will be duplicates, and non-distinguishable
+			// from each other, confusing users.
+			break
+		}
+	}
+
+	// ensure that segments[1] can accommodate a dns label + dns subdomain + '.'
+	maxPathSegmentLength := validation.DNS1123SubdomainMaxLength + validation.DNS1123LabelMaxLength + 1
+	maxSignerNameLength := maxDomainSegmentLength + maxPathSegmentLength + 1
+	if len(signerName) > maxSignerNameLength {
+		el = append(el, field.TooLong(fldPath, signerName, maxSignerNameLength))
+	}
+
+	return el
+}
+
+// ValidateClusterTrustBundleName checks that a ClusterTrustBundle name conforms
+// to the rules documented on the type.
+func ValidateClusterTrustBundleName(signerName string) func(name string, prefix bool) []string {
+	return func(name string, isPrefix bool) []string {
+		if signerName == "" {
+			if strings.Contains(name, ":") {
+				return []string{"ClusterTrustBundle without signer name must not have \":\" in its name"}
+			}
+			return apimachineryvalidation.NameIsDNSSubdomain(name, isPrefix)
+		}
+
+		requiredPrefix := strings.ReplaceAll(signerName, "/", ":") + ":"
+		if !strings.HasPrefix(name, requiredPrefix) {
+			return []string{fmt.Sprintf("ClusterTrustBundle for signerName %s must be named with prefix %s", signerName, requiredPrefix)}
+		}
+		return apimachineryvalidation.NameIsDNSSubdomain(strings.TrimPrefix(name, requiredPrefix), isPrefix)
+	}
+}
+
+func extractSignerNameFromClusterTrustBundleName(name string) (string, bool) {
+	if splitPoint := strings.LastIndex(name, ":"); splitPoint != -1 {
+		// This looks like it refers to a signerName trustbundle.
+		return strings.ReplaceAll(name[:splitPoint], ":", "/"), true
+	} else {
+		return "", false
+	}
+}

pkg/apis/core/validation/validation.go

@@ -1155,6 +1155,69 @@ func validateProjectionSources(projection *core.ProjectedVolumeSource, projectio
 				allErrs = append(allErrs, field.Required(fldPath.Child("path"), ""))
 			}
 		}
+		if projPath := srcPath.Child("clusterTrustBundlePEM"); source.ClusterTrustBundle != nil {
+			numSources++
+
+			usingName := source.ClusterTrustBundle.Name != nil
+			usingSignerName := source.ClusterTrustBundle.SignerName != nil
+
+			switch {
+			case usingName && usingSignerName:
+				allErrs = append(allErrs, field.Invalid(projPath, source.ClusterTrustBundle, "only one of name and signerName may be used"))
+			case usingName:
+				if *source.ClusterTrustBundle.Name == "" {
+					allErrs = append(allErrs, field.Required(projPath.Child("name"), "must be a valid object name"))
+				}
+
+				name := *source.ClusterTrustBundle.Name
+				if signerName, ok := extractSignerNameFromClusterTrustBundleName(name); ok {
+					validationFunc := ValidateClusterTrustBundleName(signerName)
+					errMsgs := validationFunc(name, false)
+					for _, msg := range errMsgs {
+						allErrs = append(allErrs, field.Invalid(projPath.Child("name"), name, fmt.Sprintf("not a valid clustertrustbundlename: %v", msg)))
+					}
+				} else {
+					validationFunc := ValidateClusterTrustBundleName("")
+					errMsgs := validationFunc(name, false)
+					for _, msg := range errMsgs {
+						allErrs = append(allErrs, field.Invalid(projPath.Child("name"), name, fmt.Sprintf("not a valid clustertrustbundlename: %v", msg)))
+					}
+				}
+
+				if source.ClusterTrustBundle.LabelSelector != nil {
+					allErrs = append(allErrs, field.Invalid(projPath.Child("labelSelector"), source.ClusterTrustBundle.LabelSelector, "labelSelector must be unset if name is specified"))
+				}
+			case usingSignerName:
+				if *source.ClusterTrustBundle.SignerName == "" {
+					allErrs = append(allErrs, field.Required(projPath.Child("signerName"), "must be a valid signer name"))
+				}
+
+				allErrs = append(allErrs, ValidateSignerName(projPath.Child("signerName"), *source.ClusterTrustBundle.SignerName)...)
+
+				labelSelectorErrs := unversionedvalidation.ValidateLabelSelector(
+					source.ClusterTrustBundle.LabelSelector,
+					unversionedvalidation.LabelSelectorValidationOptions{AllowInvalidLabelValueInSelector: false},
+					projPath.Child("labelSelector"),
+				)
+				allErrs = append(allErrs, labelSelectorErrs...)
+
+			default:
+				allErrs = append(allErrs, field.Required(projPath, "either name or signerName must be specified"))
+			}
+
+			if source.ClusterTrustBundle.Path == "" {
+				allErrs = append(allErrs, field.Required(projPath.Child("path"), ""))
+			}
+
+			allErrs = append(allErrs, validateLocalNonReservedPath(source.ClusterTrustBundle.Path, projPath.Child("path"))...)
+
+			curPath := source.ClusterTrustBundle.Path
+			if !allPaths.Has(curPath) {
+				allPaths.Insert(curPath)
+			} else {
+				allErrs = append(allErrs, field.Invalid(fldPath, curPath, "conflicting duplicate paths"))
+			}
+		}
 		if numSources > 1 {
 			allErrs = append(allErrs, field.Forbidden(srcPath, "may not specify more than 1 volume type"))
 		}

pkg/apis/core/validation/validation_test.go

@@ -10414,6 +10414,63 @@ func TestValidatePod(t *testing.T) {
 				}},
 			},
 		},
+		"valid ClusterTrustBundlePEM projected volume referring to a CTB by name": {
+			ObjectMeta: metav1.ObjectMeta{Name: "valid-extended", Namespace: "ns"},
+			Spec: core.PodSpec{
+				ServiceAccountName: "some-service-account",
+				Containers:         []core.Container{{Name: "ctr", Image: "image", ImagePullPolicy: "IfNotPresent", TerminationMessagePolicy: "File"}},
+				RestartPolicy:      core.RestartPolicyAlways,
+				DNSPolicy:          core.DNSClusterFirst,
+				Volumes: []core.Volume{
+					{
+						Name: "projected-volume",
+						VolumeSource: core.VolumeSource{
+							Projected: &core.ProjectedVolumeSource{
+								Sources: []core.VolumeProjection{
+									{
+										ClusterTrustBundle: &core.ClusterTrustBundleProjection{
+											Path: "foo-path",
+											Name: utilpointer.String("foo"),
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+		},
+		"valid ClusterTrustBundlePEM projected volume referring to a CTB by signer name": {
+			ObjectMeta: metav1.ObjectMeta{Name: "valid-extended", Namespace: "ns"},
+			Spec: core.PodSpec{
+				ServiceAccountName: "some-service-account",
+				Containers:         []core.Container{{Name: "ctr", Image: "image", ImagePullPolicy: "IfNotPresent", TerminationMessagePolicy: "File"}},
+				RestartPolicy:      core.RestartPolicyAlways,
+				DNSPolicy:          core.DNSClusterFirst,
+				Volumes: []core.Volume{
+					{
+						Name: "projected-volume",
+						VolumeSource: core.VolumeSource{
+							Projected: &core.ProjectedVolumeSource{
+								Sources: []core.VolumeProjection{
+									{
+										ClusterTrustBundle: &core.ClusterTrustBundleProjection{
+											Path:       "foo-path",
+											SignerName: utilpointer.String("example.com/foo"),
+											LabelSelector: &metav1.LabelSelector{
+												MatchLabels: map[string]string{
+													"version": "live",
+												},
+											},
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+		},
 		"ephemeral volume + PVC, no conflict between them": {
 			ObjectMeta: metav1.ObjectMeta{Name: "123", Namespace: "ns"},
 			Spec: core.PodSpec{
@@ -12024,6 +12081,133 @@ func TestValidatePod(t *testing.T) {
 				},
 			},
 		},
+		"ClusterTrustBundlePEM projected volume using both byName and bySigner": {
+			expectedError: "only one of name and signerName may be used",
+			spec: core.Pod{
+				ObjectMeta: metav1.ObjectMeta{Name: "valid-extended", Namespace: "ns"},
+				Spec: core.PodSpec{
+					ServiceAccountName: "some-service-account",
+					Containers:         []core.Container{{Name: "ctr", Image: "image", ImagePullPolicy: "IfNotPresent", TerminationMessagePolicy: "File"}},
+					RestartPolicy:      core.RestartPolicyAlways,
+					DNSPolicy:          core.DNSClusterFirst,
+					Volumes: []core.Volume{
+						{
+							Name: "projected-volume",
+							VolumeSource: core.VolumeSource{
+								Projected: &core.ProjectedVolumeSource{
+									Sources: []core.VolumeProjection{
+										{
+											ClusterTrustBundle: &core.ClusterTrustBundleProjection{
+												Path:       "foo-path",
+												SignerName: utilpointer.String("example.com/foo"),
+												LabelSelector: &metav1.LabelSelector{
+													MatchLabels: map[string]string{
+														"version": "live",
+													},
+												},
+												Name: utilpointer.String("foo"),
+											},
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+		},
+		"ClusterTrustBundlePEM projected volume byName with no name": {
+			expectedError: "must be a valid object name",
+			spec: core.Pod{
+				ObjectMeta: metav1.ObjectMeta{Name: "valid-extended", Namespace: "ns"},
+				Spec: core.PodSpec{
+					ServiceAccountName: "some-service-account",
+					Containers:         []core.Container{{Name: "ctr", Image: "image", ImagePullPolicy: "IfNotPresent", TerminationMessagePolicy: "File"}},
+					RestartPolicy:      core.RestartPolicyAlways,
+					DNSPolicy:          core.DNSClusterFirst,
+					Volumes: []core.Volume{
+						{
+							Name: "projected-volume",
+							VolumeSource: core.VolumeSource{
+								Projected: &core.ProjectedVolumeSource{
+									Sources: []core.VolumeProjection{
+										{
+											ClusterTrustBundle: &core.ClusterTrustBundleProjection{
+												Path: "foo-path",
+												Name: utilpointer.String(""),
+											},
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+		},
+		"ClusterTrustBundlePEM projected volume bySigner with no signer name": {
+			expectedError: "must be a valid signer name",
+			spec: core.Pod{
+				ObjectMeta: metav1.ObjectMeta{Name: "valid-extended", Namespace: "ns"},
+				Spec: core.PodSpec{
+					ServiceAccountName: "some-service-account",
+					Containers:         []core.Container{{Name: "ctr", Image: "image", ImagePullPolicy: "IfNotPresent", TerminationMessagePolicy: "File"}},
+					RestartPolicy:      core.RestartPolicyAlways,
+					DNSPolicy:          core.DNSClusterFirst,
+					Volumes: []core.Volume{
+						{
+							Name: "projected-volume",
+							VolumeSource: core.VolumeSource{
+								Projected: &core.ProjectedVolumeSource{
+									Sources: []core.VolumeProjection{
+										{
+											ClusterTrustBundle: &core.ClusterTrustBundleProjection{
+												Path:       "foo-path",
+												SignerName: utilpointer.String(""),
+												LabelSelector: &metav1.LabelSelector{
+													MatchLabels: map[string]string{
+														"foo": "bar",
+													},
+												},
+											},
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+		},
+		"ClusterTrustBundlePEM projected volume bySigner with invalid signer name": {
+			expectedError: "must be a fully qualified domain and path of the form",
+			spec: core.Pod{
+				ObjectMeta: metav1.ObjectMeta{Name: "valid-extended", Namespace: "ns"},
+				Spec: core.PodSpec{
+					ServiceAccountName: "some-service-account",
+					Containers:         []core.Container{{Name: "ctr", Image: "image", ImagePullPolicy: "IfNotPresent", TerminationMessagePolicy: "File"}},
+					RestartPolicy:      core.RestartPolicyAlways,
+					DNSPolicy:          core.DNSClusterFirst,
+					Volumes: []core.Volume{
+						{
+							Name: "projected-volume",
+							VolumeSource: core.VolumeSource{
+								Projected: &core.ProjectedVolumeSource{
+									Sources: []core.VolumeProjection{
+										{
+											ClusterTrustBundle: &core.ClusterTrustBundleProjection{
+												Path:       "foo-path",
+												SignerName: utilpointer.String("example.com/foo/invalid"),
+											},
+										},
+									},
+								},
+							},
+						},
+					},
+				},
+			},
+		},
 		"final PVC name for ephemeral volume must be valid": {
 			expectedError: "spec.volumes[1].name: Invalid value: \"" + longVolName + "\": PVC name \"" + longPodName + "-" + longVolName + "\": must be no more than 253 characters",
 			spec: core.Pod{

plugin/pkg/admission/serviceaccount/admission.go

@@ -210,6 +210,9 @@ func (s *Plugin) Validate(ctx context.Context, a admission.Attributes, o admissi
 					if projSource.ServiceAccountToken != nil {
 						return admission.NewForbidden(a, fmt.Errorf("a mirror pod may not use ServiceAccountToken volume projections"))
 					}
+					if projSource.ClusterTrustBundle != nil {
+						return admission.NewForbidden(a, fmt.Errorf("a mirror pod may not use ClusterTrustBundle volume projections"))
+					}
 				}
 			}
 		}

staging/src/k8s.io/api/core/v1/types.go

@@ -1842,22 +1842,31 @@ type ServiceAccountTokenProjection struct {
 // filesystem.
 type ClusterTrustBundleProjection struct {
 	// Select a single ClusterTrustBundle by object name.  Mutually-exclusive
-	// with SignerName and LabelSelector.
+	// with signerName and labelSelector.
 	// +optional
 	Name *string `json:"name,omitempty" protobuf:"bytes,1,rep,name=name"`
 
 	// Select all ClusterTrustBundles that match this signer name.
-	// Mutually-exclusive with Name.
+	// Mutually-exclusive with name.  The contents of all selected
+	// ClusterTrustBundles will be unified and deduplicated.
 	// +optional
 	SignerName *string `json:"signerName,omitempty" protobuf:"bytes,2,rep,name=signerName"`
 
-	// Select all ClusterTrustBundles that match this label selector.  Must not
-	// be null or empty if SignerName is provided.  Mutually-exclusive with
-	// Name.
-	//
+	// Select all ClusterTrustBundles that match this label selector.  Only has
+	// effect if signerName is set.  Mutually-exclusive with name.  If unset,
+	// interpreted as "match nothing".  If set but empty, interpreted as "match
+	// everything".
 	// +optional
 	LabelSelector *metav1.LabelSelector `json:"labelSelector,omitempty" protobuf:"bytes,3,rep,name=labelSelector"`
 
+	// If true, don't block pod startup if the referenced ClusterTrustBundle(s)
+	// aren't available.  If using name, then the named ClusterTrustBundle is
+	// allowed not to exist.  If using signerName, then the combination of
+	// signerName and labelSelector is allowed to match zero
+	// ClusterTrustBundles.
+	// +optional
+	Optional *bool `json:"optional,omitempty"`
+
 	// Relative path from the volume root to write the bundle.
 	Path string `json:"path" protobuf:"bytes,4,rep,name=path"`
 }
@@ -1895,26 +1904,20 @@ type VolumeProjection struct {
 	ServiceAccountToken *ServiceAccountTokenProjection `json:"serviceAccountToken,omitempty" protobuf:"bytes,4,opt,name=serviceAccountToken"`
 
 	// ClusterTrustBundle allows a pod to access the `.spec.trustBundle` field
-	// of a ClusterTrustBundle object in an auto-updating file.
+	// of ClusterTrustBundle objects in an auto-updating file.
 	//
 	// Alpha, gated by the ClusterTrustBundleProjection feature gate.
 	//
 	// ClusterTrustBundle objects can either be selected by name, or by the
 	// combination of signer name and a label selector.
 	//
-	// When selecting by name, the referenced ClusterTrustBundle object must
-	// have an empty spec.signerName field.
-	//
-	// When selecting by signer name, the contents of all ClusterTrustBundle
-	// objects associated with the signer and matching the label will be unified
-	// and deduplicated.
-	//
 	// Kubelet performs aggressive normalization of the PEM contents written
 	// into the pod filesystem.  Esoteric PEM features such as inter-block
 	// comments and block headers are stripped.  Certificates are deduplicated.
 	// The ordering of certificates within the file is arbitrary, and Kubelet
 	// may change the order over time.
 	//
+	// +featureGate=ClusterTrustBundleProjection
 	// +optional
 	ClusterTrustBundle *ClusterTrustBundleProjection `json:"clusterTrustBundle,omitempty" protobuf:"bytes,5,opt,name=clusterTrustBundle"`
 }

staging/src/k8s.io/client-go/applyconfigurations/core/v1/clustertrustbundlepemprojection.go

@@ -0,0 +1,48 @@
+/*
+Copyright 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.
+*/
+
+// Code generated by applyconfiguration-gen. DO NOT EDIT.
+
+package v1
+
+// ClusterTrustBundlePEMProjectionApplyConfiguration represents an declarative configuration of the ClusterTrustBundlePEMProjection type for use
+// with apply.
+type ClusterTrustBundlePEMProjectionApplyConfiguration struct {
+	Name *string `json:"name,omitempty"`
+	Path *string `json:"path,omitempty"`
+}
+
+// ClusterTrustBundlePEMProjectionApplyConfiguration constructs an declarative configuration of the ClusterTrustBundlePEMProjection type for use with
+// apply.
+func ClusterTrustBundlePEMProjection() *ClusterTrustBundlePEMProjectionApplyConfiguration {
+	return &ClusterTrustBundlePEMProjectionApplyConfiguration{}
+}
+
+// WithName sets the Name field in the declarative configuration to the given value
+// and returns the receiver, so that objects can be built by chaining "With" function invocations.
+// If called multiple times, the Name field is set to the value of the last call.
+func (b *ClusterTrustBundlePEMProjectionApplyConfiguration) WithName(value string) *ClusterTrustBundlePEMProjectionApplyConfiguration {
+	b.Name = &value
+	return b
+}
+
+// WithPath sets the Path field in the declarative configuration to the given value
+// and returns the receiver, so that objects can be built by chaining "With" function invocations.
+// If called multiple times, the Path field is set to the value of the last call.
+func (b *ClusterTrustBundlePEMProjectionApplyConfiguration) WithPath(value string) *ClusterTrustBundlePEMProjectionApplyConfiguration {
+	b.Path = &value
+	return b
+}