KEP-127: Update PSS based on feature gate

pkg/features/kube_features.go

@@ -884,6 +884,18 @@ const (
 	// ImageMaximumGCAge enables the Kubelet configuration field of the same name, allowing an admin
 	// to specify the age after which an image will be garbage collected.
 	ImageMaximumGCAge featuregate.Feature = "ImageMaximumGCAge"
+
+	// owner: @saschagrunert
+	// alpha: v1.28
+	//
+	// Enables user namespace support for Pod Security Standards. Enabling this
+	// feature will modify all Pod Security Standard rules to allow setting:
+	// spec[.*].securityContext.[runAsNonRoot,runAsUser]
+	// This feature gate should only be enabled if all nodes in the cluster
+	// support the user namespace feature and have it enabled. The feature gate
+	// will not graduate or be enabled by default in future Kubernetes
+	// releases.
+	UserNamespacesPodSecurityStandards featuregate.Feature = "UserNamespacesPodSecurityStandards"
 )
 
 func init() {
@@ -1123,6 +1135,8 @@ var defaultKubernetesFeatureGates = map[featuregate.Feature]featuregate.FeatureS
 
 	ImageMaximumGCAge: {Default: false, PreRelease: featuregate.Alpha},
 
+	UserNamespacesPodSecurityStandards: {Default: false, PreRelease: featuregate.Alpha},
+
 	// inherited features from generic apiserver, relisted here to get a conflict if it is changed
 	// unintentionally on either side:
 

plugin/pkg/admission/security/podsecurity/admission.go

@@ -27,6 +27,7 @@ import (
 	_ "k8s.io/kubernetes/pkg/apis/apps/install"
 	_ "k8s.io/kubernetes/pkg/apis/batch/install"
 	_ "k8s.io/kubernetes/pkg/apis/core/install"
+	"k8s.io/kubernetes/pkg/features"
 
 	admissionv1 "k8s.io/api/admission/v1"
 	appsv1 "k8s.io/api/apps/v1"
@@ -151,6 +152,7 @@ func (p *Plugin) updateDelegate() {
 
 func (c *Plugin) InspectFeatureGates(featureGates featuregate.FeatureGate) {
 	c.inspectedFeatureGates = true
+	policy.RelaxPolicyForUserNamespacePods(featureGates.Enabled(features.UserNamespacesPodSecurityStandards))
 }
 
 // ValidateInitialization ensures all required options are set

staging/src/k8s.io/pod-security-admission/policy/check_runAsNonRoot.go

@@ -59,6 +59,11 @@ func CheckRunAsNonRoot() Check {
 }
 
 func runAsNonRoot_1_0(podMetadata *metav1.ObjectMeta, podSpec *corev1.PodSpec) CheckResult {
+	// See KEP-127: https://github.com/kubernetes/enhancements/blob/308ba8d/keps/sig-node/127-user-namespaces/README.md?plain=1#L411-L447
+	if relaxPolicyForUserNamespacePod(podSpec) {
+		return CheckResult{Allowed: true}
+	}
+
 	// things that explicitly set runAsNonRoot=false
 	var badSetters []string
 

staging/src/k8s.io/pod-security-admission/policy/check_runAsNonRoot_test.go

@@ -25,10 +25,12 @@ import (
 
 func TestRunAsNonRoot(t *testing.T) {
 	tests := []struct {
-		name         string
-		pod          *corev1.Pod
-		expectReason string
-		expectDetail string
+		name                                     string
+		pod                                      *corev1.Pod
+		expectReason                             string
+		expectDetail                             string
+		allowed                                  bool
+		enableUserNamespacesPodSecurityStandards bool
 	}{
 		{
 			name: "no explicit runAsNonRoot",
@@ -80,12 +82,36 @@ func TestRunAsNonRoot(t *testing.T) {
 			expectReason: `runAsNonRoot != true`,
 			expectDetail: `pod or containers "a", "b" must set securityContext.runAsNonRoot=true`,
 		},
+		{
+			name: "UserNamespacesPodSecurityStandards enabled without HostUsers",
+			pod: &corev1.Pod{Spec: corev1.PodSpec{
+				HostUsers: utilpointer.Bool(false),
+			}},
+			allowed:                                  true,
+			enableUserNamespacesPodSecurityStandards: true,
+		},
+		{
+			name: "UserNamespacesPodSecurityStandards enabled with HostUsers",
+			pod: &corev1.Pod{Spec: corev1.PodSpec{
+				Containers: []corev1.Container{
+					{Name: "a"},
+				},
+				HostUsers: utilpointer.Bool(true),
+			}},
+			expectReason:                             `runAsNonRoot != true`,
+			expectDetail:                             `pod or container "a" must set securityContext.runAsNonRoot=true`,
+			allowed:                                  false,
+			enableUserNamespacesPodSecurityStandards: true,
+		},
 	}
 
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
+			if tc.enableUserNamespacesPodSecurityStandards {
+				RelaxPolicyForUserNamespacePods(true)
+			}
 			result := runAsNonRoot_1_0(&tc.pod.ObjectMeta, &tc.pod.Spec)
-			if result.Allowed {
+			if result.Allowed && !tc.allowed {
 				t.Fatal("expected disallowed")
 			}
 			if e, a := tc.expectReason, result.ForbiddenReason; e != a {

staging/src/k8s.io/pod-security-admission/policy/check_runAsUser.go

@@ -60,6 +60,11 @@ func CheckRunAsUser() Check {
 }
 
 func runAsUser_1_23(podMetadata *metav1.ObjectMeta, podSpec *corev1.PodSpec) CheckResult {
+	// See KEP-127: https://github.com/kubernetes/enhancements/blob/308ba8d/keps/sig-node/127-user-namespaces/README.md?plain=1#L411-L447
+	if relaxPolicyForUserNamespacePod(podSpec) {
+		return CheckResult{Allowed: true}
+	}
+
 	// things that explicitly set runAsUser=0
 	var badSetters []string
 

staging/src/k8s.io/pod-security-admission/policy/check_runAsUser_test.go

@@ -25,11 +25,12 @@ import (
 
 func TestRunAsUser(t *testing.T) {
 	tests := []struct {
-		name         string
-		pod          *corev1.Pod
-		expectAllow  bool
-		expectReason string
-		expectDetail string
+		name                                     string
+		pod                                      *corev1.Pod
+		expectAllow                              bool
+		expectReason                             string
+		expectDetail                             string
+		enableUserNamespacesPodSecurityStandards bool
 	}{
 		{
 			name: "pod runAsUser=0",
@@ -90,10 +91,35 @@ func TestRunAsUser(t *testing.T) {
 			}},
 			expectAllow: true,
 		},
+		{
+			name: "UserNamespacesPodSecurityStandards enabled without HostUsers",
+			pod: &corev1.Pod{Spec: corev1.PodSpec{
+				HostUsers: utilpointer.Bool(false),
+			}},
+			expectAllow:                              true,
+			enableUserNamespacesPodSecurityStandards: true,
+		},
+		{
+			name: "UserNamespacesPodSecurityStandards enabled with HostUsers",
+			pod: &corev1.Pod{Spec: corev1.PodSpec{
+				SecurityContext: &corev1.PodSecurityContext{RunAsUser: utilpointer.Int64(0)},
+				Containers: []corev1.Container{
+					{Name: "a", SecurityContext: nil},
+				},
+				HostUsers: utilpointer.Bool(true),
+			}},
+			expectAllow:                              false,
+			expectReason:                             `runAsUser=0`,
+			expectDetail:                             `pod must not set runAsUser=0`,
+			enableUserNamespacesPodSecurityStandards: true,
+		},
 	}
 
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
+			if tc.enableUserNamespacesPodSecurityStandards {
+				RelaxPolicyForUserNamespacePods(true)
+			}
 			result := runAsUser_1_23(&tc.pod.ObjectMeta, &tc.pod.Spec)
 			if tc.expectAllow {
 				if !result.Allowed {

staging/src/k8s.io/pod-security-admission/policy/helpers.go

@@ -16,7 +16,12 @@ limitations under the License.
 
 package policy
 
-import "strings"
+import (
+	"strings"
+	"sync/atomic"
+
+	corev1 "k8s.io/api/core/v1"
+)
 
 func joinQuote(items []string) string {
 	if len(items) == 0 {
@@ -31,3 +36,21 @@ func pluralize(singular, plural string, count int) string {
 	}
 	return plural
 }
+
+var relaxPolicyForUserNamespacePods = &atomic.Bool{}
+
+// RelaxPolicyForUserNamespacePods allows opting into relaxing runAsUser /
+// runAsNonRoot restricted policies for user namespace pods, before the
+// usernamespace feature has reached GA and propagated to the oldest supported
+// nodes.
+// This should only be opted into in clusters where the administrator ensures
+// all nodes in the cluster enable the user namespace feature.
+func RelaxPolicyForUserNamespacePods(relax bool) {
+	relaxPolicyForUserNamespacePods.Store(relax)
+}
+
+// relaxPolicyForUserNamespacePod returns true if a policy should be relaxed
+// because of enabled user namespaces in the provided pod spec.
+func relaxPolicyForUserNamespacePod(podSpec *corev1.PodSpec) bool {
+	return relaxPolicyForUserNamespacePods.Load() && podSpec != nil && podSpec.HostUsers != nil && !*podSpec.HostUsers
+}