PSA: allow procMount type Unmasked in baseline

a masked proc mount has traditionally been used to prevent untrusted containers from accessing leaky kernel APIs.
However, within a user namespace, typical ID checks protect better than masked proc. Further, allowing unmasked proc
with a user namespace gives access to a container mounting sub procs, which opens avenues for container-in-container use cases.

Update PSS for baseline to allow a container to access an unmasked /proc, if it's in a user namespace and if the UserNamespacesPodSecurityStandards feature is enabled.

Signed-off-by: Peter Hunt <pehunt@redhat.com>

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

@@ -35,6 +35,9 @@ spec.initContainers[*].securityContext.procMount
 
 **Allowed Values:** undefined/null, "Default"
 
+However, if the pod is in a user namespace (`hostUsers: false`), and the
+UserNamespacesPodSecurityStandards feature is enabled, all values are allowed.
+
 */
 
 func init() {
@@ -58,6 +61,14 @@ func CheckProcMount() Check {
 }
 
 func procMount_1_0(podMetadata *metav1.ObjectMeta, podSpec *corev1.PodSpec) CheckResult {
+	// TODO: When we remove the UserNamespacesPodSecurityStandards feature gate (and GA this relaxation),
+	// create a new policy version.
+	// Note: pod validation will check for well formed procMount type, so avoid double validation and allow everything
+	// here.
+	if relaxPolicyForUserNamespacePod(podSpec) {
+		return CheckResult{Allowed: true}
+	}
+
 	var badContainers []string
 	forbiddenProcMountTypes := sets.NewString()
 	visitContainers(podSpec, func(container *corev1.Container) {

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

@@ -33,6 +33,8 @@ func TestProcMount(t *testing.T) {
 		pod            *corev1.Pod
 		expectReason   string
 		expectDetail   string
+		expectAllowed  bool
+		relaxForUserNS bool
 	}{
 		{
 			name: "procMount",
@@ -47,15 +49,39 @@ func TestProcMount(t *testing.T) {
 				HostUsers: &hostUsers,
 			}},
 			expectReason:  `procMount`,
+			expectAllowed: false,
 			expectDetail:  `containers "d", "e" must not set securityContext.procMount to "Unmasked", "other"`,
 		},
+		{
+			name: "procMount",
+			pod: &corev1.Pod{Spec: corev1.PodSpec{
+				Containers: []corev1.Container{
+					{Name: "a", SecurityContext: nil},
+					{Name: "b", SecurityContext: &corev1.SecurityContext{}},
+					{Name: "c", SecurityContext: &corev1.SecurityContext{ProcMount: &defaultValue}},
+					{Name: "d", SecurityContext: &corev1.SecurityContext{ProcMount: &unmaskedValue}},
+					{Name: "e", SecurityContext: &corev1.SecurityContext{ProcMount: &otherValue}},
+				},
+				HostUsers: &hostUsers,
+			}},
+			expectReason:   "",
+			expectDetail:   "",
+			expectAllowed:  true,
+			relaxForUserNS: true,
+		},
 	}
 
 	for _, tc := range tests {
 		t.Run(tc.name, func(t *testing.T) {
+			if tc.relaxForUserNS {
+				RelaxPolicyForUserNamespacePods(true)
+				t.Cleanup(func() {
+					RelaxPolicyForUserNamespacePods(false)
+				})
+			}
 			result := procMount_1_0(&tc.pod.ObjectMeta, &tc.pod.Spec)
-			if result.Allowed {
+			if result.Allowed != tc.expectAllowed {
-				t.Fatal("expected disallowed")
+				t.Fatalf("expected Allowed to be %v was %v", tc.expectAllowed, result.Allowed)
 			}
 			if e, a := tc.expectReason, result.ForbiddenReason; e != a {
 				t.Errorf("expected\n%s\ngot\n%s", e, a)