storage/kubernetes: guess namespace from the service account token

The in cluster kubernetes client currently requires using the
downward API to determine its namespace. However this value can be
determine by inspecting the service account token mounted into the
pod. As a fallback, use this to guess the current namespace.

storage/kubernetes/client.go

@@ -322,6 +322,32 @@ func loadKubeConfig(kubeConfigPath string) (cluster k8sapi.Cluster, user k8sapi.
 	return
 }
 
+func namespaceFromServiceAccountJWT(s string) (string, error) {
+	// The service account token is just a JWT. Parse it as such.
+	parts := strings.Split(s, ".")
+	if len(parts) < 2 {
+		// It's extremely important we don't log the actual service account token.
+		return "", fmt.Errorf("malformed service account token: expected 3 parts got %d", len(parts))
+	}
+	payload, err := base64.RawURLEncoding.DecodeString(parts[1])
+	if err != nil {
+		return "", fmt.Errorf("malformed service account token: %v", err)
+	}
+	var data struct {
+		// The claim Kubernetes uses to identify which namespace a service account belongs to.
+		//
+		// See: https://github.com/kubernetes/kubernetes/blob/v1.4.3/pkg/serviceaccount/jwt.go#L42
+		Namespace string `json:"kubernetes.io/serviceaccount/namespace"`
+	}
+	if err := json.Unmarshal(payload, &data); err != nil {
+		return "", fmt.Errorf("malformed service account token: %v", err)
+	}
+	if data.Namespace == "" {
+		return "", errors.New(`jwt claim "kubernetes.io/serviceaccount/namespace" not found`)
+	}
+	return data.Namespace, nil
+}
+
 func inClusterConfig() (cluster k8sapi.Cluster, user k8sapi.AuthInfo, namespace string, err error) {
 	host, port := os.Getenv("KUBERNETES_SERVICE_HOST"), os.Getenv("KUBERNETES_SERVICE_PORT")
 	if len(host) == 0 || len(port) == 0 {
@@ -332,17 +358,20 @@ func inClusterConfig() (cluster k8sapi.Cluster, user k8sapi.AuthInfo, namespace
 		Server:               "https://" + host + ":" + port,
 		CertificateAuthority: "/var/run/secrets/kubernetes.io/serviceaccount/ca.crt",
 	}
-
-	if namespace = os.Getenv("KUBERNETES_POD_NAMESPACE"); namespace == "" {
-		err = fmt.Errorf("unable to load in-cluster configuration, KUBERNETES_POD_NAMESPACE must be defined")
-		return
-	}
-
 	token, err := ioutil.ReadFile("/var/run/secrets/kubernetes.io/serviceaccount/token")
 	if err != nil {
 		return
 	}
 	user = k8sapi.AuthInfo{Token: string(token)}
+
+	if namespace = os.Getenv("KUBERNETES_POD_NAMESPACE"); namespace == "" {
+		namespace, err = namespaceFromServiceAccountJWT(user.Token)
+		if err != nil {
+			err = fmt.Errorf("failed to inspect service account token: %v", err)
+			return
+		}
+	}
+
 	return
 }
 

storage/kubernetes/client_test.go

@@ -0,0 +1,60 @@
+package kubernetes
+
+import "testing"
+
+func TestNamespaceFromServiceAccountJWT(t *testing.T) {
+	namespace, err := namespaceFromServiceAccountJWT(serviceAccountToken)
+	if err != nil {
+		t.Fatal(err)
+	}
+	wantNamespace := "dex-test-namespace"
+	if namespace != wantNamespace {
+		t.Errorf("expected namespace %q got %q", wantNamespace, namespace)
+	}
+}
+
+var serviceAccountToken = "eyJhbGciOiJSUzI1NiIsInR5cCI6IkpXVCJ9.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.KViBpPwCiBwxDvAjYUUXoVvLVwqV011aLlYQpNtX12Bh8M-QAFch-3RWlo_SR00bcdFg_nZo9JKACYlF_jHMEsf__PaYms9r7vEaSg0jPfkqnL2WXZktzQRyLBr0n-bxeUrbwIWsKOAC0DfFB5nM8XoXljRmq8yAx8BAdmQp7MIFb4EOV9nYthhua6pjzYyaFSiDiYTjw7HtXOvoL8oepodJ3-37pUKS8vdBvnvUoqC4M1YAhkO5L36JF6KV_RfmG8GPEdNQfXotHcsR-3jKi1n8S5l7Xd-rhrGOhSGQizH3dORzo9GvBAhYeqbq1O-NLzm2EQUiMQayIUx7o4g3Kw"
+
+// The following program was used to generate the example token. Since we don't want to
+// import Kubernetes, just leave it as a comment.
+
+/*
+package main
+
+import (
+	"crypto/rand"
+	"crypto/rsa"
+	"fmt"
+	"log"
+
+	"k8s.io/kubernetes/pkg/api"
+	"k8s.io/kubernetes/pkg/serviceaccount"
+	"k8s.io/kubernetes/pkg/util/uuid"
+)
+
+func main() {
+	key, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		log.Fatal(err)
+	}
+	sa := api.ServiceAccount{
+		ObjectMeta: api.ObjectMeta{
+			Namespace: "dex-test-namespace",
+			Name:      "dotherobot",
+			UID:       uuid.NewUUID(),
+		},
+	}
+	secret := api.Secret{
+		ObjectMeta: api.ObjectMeta{
+			Namespace: "dex-test-namespace",
+			Name:      "dotherobot-secret",
+			UID:       uuid.NewUUID(),
+		},
+	}
+	token, err := serviceaccount.JWTTokenGenerator(key).GenerateToken(sa, secret)
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Println(token)
+}
+*/