cmd/tailscale/cli/configure-kube.go

@@ -52,18 +52,15 @@ See: https://tailscale.com/s/k8s-auth-proxy
 }
 
 // kubeconfigPath returns the path to the kubeconfig file for the current user.
-func kubeconfigPath() (string, error) {
+func kubeconfigPath() string {
 	if kubeconfig := os.Getenv("KUBECONFIG"); kubeconfig != "" {
-		if version.IsSandboxedMacOS() {
-			return "", errors.New("cannot read $KUBECONFIG on GUI builds of the macOS client: this requires the open-source tailscaled distribution")
-		}
 		var out string
 		for _, out = range filepath.SplitList(kubeconfig) {
 			if info, err := os.Stat(out); !os.IsNotExist(err) && !info.IsDir() {
 				break
 			}
 		}
-		return out, nil
+		return out
 	}
 
 	var dir string
@@ -77,7 +74,64 @@ func kubeconfigPath() (string, error) {
 	} else {
 		dir = homedir.HomeDir()
 	}
-	return filepath.Join(dir, ".kube", "config"), nil
+	return filepath.Join(dir, ".kube", "config")
+}
+
+// checkKubeconfigWritable returns nil if the kubeconfig at path can be written,
+// or an error explaining why it can't. A not-yet-created file or .kube
+// directory is fine as long as the nearest existing ancestor is writable.
+//
+// On sandboxed macOS builds, kubeconfigPath resolves path to the user's real
+// ~/.kube/config, which we can only write via the home-relative-path
+// entitlement. If that write would fail (e.g. because $KUBECONFIG points
+// somewhere the sandbox can't reach), we want to surface a clear error pointing
+// at the open-source tailscaled distribution rather than silently writing a
+// config the user's kubectl will never read into the sandbox container.
+func checkKubeconfigWritable(path string) error {
+	for try := path; ; try = filepath.Dir(try) {
+		if _, err := os.Stat(try); err == nil {
+			if err := isWritable(try); err != nil {
+				return kubeconfigAccessErr(path, err)
+			}
+			return nil
+		} else if !os.IsNotExist(err) {
+			return kubeconfigAccessErr(path, err)
+		}
+		if parent := filepath.Dir(try); parent == try {
+			return nil // reached the filesystem root
+		}
+	}
+}
+
+// isWritable reports whether path can be opened or created for writing. For a
+// directory it probes by creating and removing a temporary file.
+func isWritable(path string) error {
+	fi, err := os.Stat(path)
+	if err != nil {
+		return err
+	}
+	if fi.IsDir() {
+		f, err := os.CreateTemp(path, ".tailscale-kubeconfig-*")
+		if err != nil {
+			return err
+		}
+		f.Close()
+		return os.Remove(f.Name())
+	}
+	f, err := os.OpenFile(path, os.O_WRONLY, 0)
+	if err != nil {
+		return err
+	}
+	return f.Close()
+}
+
+// kubeconfigAccessErr wraps err with context about path, adding macOS sandbox
+// guidance when the process is sandboxed.
+func kubeconfigAccessErr(path string, err error) error {
+	if version.IsSandboxedMacOS() {
+		return fmt.Errorf("cannot write kubeconfig at %q: %w; GUI builds of the macOS client run in a sandbox and can only access files under your home directory, use the open-source tailscaled distribution for other locations", path, err)
+	}
+	return fmt.Errorf("cannot write kubeconfig at %q: %w", path, err)
 }
 
 func runConfigureKubeconfig(ctx context.Context, args []string) error {
@@ -106,8 +160,8 @@ func runConfigureKubeconfig(ctx context.Context, args []string) error {
 		return err
 	}
 	targetFQDN = strings.TrimSuffix(targetFQDN, ".")
-	var kubeconfig string
-	if kubeconfig, err = kubeconfigPath(); err != nil {
+	kubeconfig := kubeconfigPath()
+	if err := checkKubeconfigWritable(kubeconfig); err != nil {
 		return err
 	}
 	scheme := "https://"
@@ -215,13 +269,8 @@ func setKubeconfigForPeer(scheme, fqdn, filePath string) error {
 		if !os.IsNotExist(err) {
 			return err
 		}
-		if err := os.Mkdir(dir, 0755); err != nil {
-			if version.IsSandboxedMacOS() && errors.Is(err, os.ErrPermission) {
-				// macOS sandboxing prevents us from creating the .kube directory
-				// in the home directory.
-				return errors.New("unable to create .kube directory in home directory, please create it manually (e.g. mkdir ~/.kube")
-			}
-			return err
+		if err := os.MkdirAll(dir, 0755); err != nil {
+			return kubeconfigAccessErr(filePath, err)
 		}
 	}
 	b, err := os.ReadFile(filePath)

cmd/tailscale/cli/configure-kube_test.go

@@ -7,6 +7,9 @@ package cli
 import (
 	"bytes"
 	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 
@@ -247,6 +250,69 @@ users:
 	}
 }
 
+func TestCheckKubeconfigWritable(t *testing.T) {
+	t.Run("nonexistent-file-in-writable-dir", func(t *testing.T) {
+		dir := t.TempDir()
+		if err := checkKubeconfigWritable(filepath.Join(dir, "config")); err != nil {
+			t.Errorf("unexpected error: %v", err)
+		}
+	})
+
+	t.Run("nonexistent-file-and-dir-in-writable-parent", func(t *testing.T) {
+		dir := t.TempDir()
+		// The .kube directory does not exist yet, but its parent does and is
+		// writable, so this should be fine.
+		if err := checkKubeconfigWritable(filepath.Join(dir, ".kube", "config")); err != nil {
+			t.Errorf("unexpected error: %v", err)
+		}
+	})
+
+	t.Run("existing-writable-file", func(t *testing.T) {
+		dir := t.TempDir()
+		path := filepath.Join(dir, "config")
+		if err := os.WriteFile(path, []byte("apiVersion: v1\nkind: Config\n"), 0600); err != nil {
+			t.Fatal(err)
+		}
+		if err := checkKubeconfigWritable(path); err != nil {
+			t.Errorf("unexpected error: %v", err)
+		}
+	})
+
+	t.Run("unwritable-existing-file", func(t *testing.T) {
+		if runtime.GOOS == "windows" {
+			t.Skip("file mode permissions are not enforced the same way on Windows")
+		}
+		if os.Getuid() == 0 {
+			t.Skip("root bypasses file permission checks")
+		}
+		dir := t.TempDir()
+		path := filepath.Join(dir, "config")
+		if err := os.WriteFile(path, []byte("x"), 0400); err != nil {
+			t.Fatal(err)
+		}
+		if err := checkKubeconfigWritable(path); err == nil {
+			t.Error("expected error for read-only file, got nil")
+		}
+	})
+
+	t.Run("unwritable-dir", func(t *testing.T) {
+		if runtime.GOOS == "windows" {
+			t.Skip("directory mode permissions are not enforced the same way on Windows")
+		}
+		if os.Getuid() == 0 {
+			t.Skip("root bypasses directory permission checks")
+		}
+		dir := t.TempDir()
+		sub := filepath.Join(dir, "ro")
+		if err := os.Mkdir(sub, 0500); err != nil {
+			t.Fatal(err)
+		}
+		if err := checkKubeconfigWritable(filepath.Join(sub, "config")); err == nil {
+			t.Error("expected error for unwritable dir, got nil")
+		}
+	})
+}
+
 func TestGetInputs(t *testing.T) {
 	for _, arg := range []string{
 		"foo.tail-scale.ts.net",