Lock subPath volumes

Users must not be allowed to step outside the volume with subPath.
Therefore the final subPath directory must be "locked" somehow
and checked if it's inside volume.

On Windows, we lock the directories. On Linux, we bind-mount the final
subPath into /var/lib/kubelet/pods/<uid>/volume-subpaths/<container name>/<subPathName>,
it can't be changed to symlink user once it's bind-mounted.

pkg/kubelet/util/util.go

@@ -19,6 +19,8 @@ package util
 import (
 	"fmt"
 	"net/url"
+	"path/filepath"
+	"strings"
 
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 )
@@ -45,3 +47,14 @@ func parseEndpoint(endpoint string) (string, string, error) {
 		return u.Scheme, "", fmt.Errorf("protocol %q not supported", u.Scheme)
 	}
 }
+
+func pathWithinBase(fullPath, basePath string) bool {
+	rel, err := filepath.Rel(basePath, fullPath)
+	if err != nil {
+		return false
+	}
+	if strings.HasPrefix(rel, "..") {
+		return false
+	}
+	return true
+}