safesocket: connect to the macOS network extension on darwin (as last resort)

(For cmd/tailscale CLI support on macOS)

Signed-off-by: Brad Fitzpatrick <brad@danga.com>

safesocket/unixsocket.go

@@ -7,16 +7,34 @@
 package safesocket
 
 import (
+	"bufio"
+	"bytes"
 	"fmt"
+	"io"
+	"log"
 	"net"
 	"os"
+	"os/exec"
 	"path/filepath"
+	"runtime"
+	"strings"
 )
 
 // TODO(apenwarr): handle magic cookie auth
 func connect(path string, port uint16) (net.Conn, error) {
+	if runtime.GOOS == "darwin" && path == "" && port == 0 {
+		return connectMacOSAppSandbox()
+	}
 	pipe, err := net.Dial("unix", path)
 	if err != nil {
+		if runtime.GOOS == "darwin" {
+			extConn, err := connectMacOSAppSandbox()
+			if err != nil {
+				log.Printf("safesocket: failed to connect to Tailscale IPNExtension: %v", err)
+			} else {
+				return extConn, nil
+			}
+		}
 		return nil, err
 	}
 	return pipe, err
@@ -48,3 +66,66 @@ func listen(path string, port uint16) (ln net.Listener, _ uint16, err error) {
 	os.Chmod(path, 0666)
 	return pipe, 0, err
 }
+
+// connectMacOSAppSandbox connects to the Tailscale Network Extension,
+// which is necessarily running within the macOS App Sandbox.  Our
+// little dance to connect a regular user binary to the sandboxed
+// nework extension is:
+//
+//   * the sandboxed IPNExtension picks a random localhost:0 TCP port
+//     to listen on
+//   * it also picks a random hex string that acts as an auth token
+//   * it then creates a file named "sameuserproof-$PORT-$TOKEN" and leaves
+//     that file descriptor open forever.
+//   * then we come along here, running as the same UID, but outside
+//     of the sandbox, and look for it. We can run lsof on our own processes,
+//     but other users on the system can't.
+//   * we parse out the localhost port number and the auth token
+//   * we connect to TCP localhost:$PORT
+//   * we send $TOKEN + "\n"
+//   * server verifies $TOKEN, sends "#IPN\n" if okay.
+//   * server is now protocol switched
+//   * we return the net.Conn and the caller speaks the normal protocol
+func connectMacOSAppSandbox() (net.Conn, error) {
+	out, err := exec.Command("lsof",
+		"-n", // numeric sockets; don't do DNS lookups, etc
+		"-a", // logical AND remaining options
+		fmt.Sprintf("-u%d", os.Getuid()), // process of same user only
+		"-c", "IPNExtension", // starting with IPNExtension
+		"-F", // machine-readable output
+	).Output()
+	if err != nil {
+		return nil, err
+	}
+	bs := bufio.NewScanner(bytes.NewReader(out))
+	subStr := []byte(".tailscale.ipn.macos/sameuserproof-")
+	for bs.Scan() {
+		line := bs.Bytes()
+		i := bytes.Index(line, subStr)
+		if i == -1 {
+			continue
+		}
+		f := strings.SplitN(string(line[i+len(subStr):]), "-", 2)
+		if len(f) != 2 {
+			continue
+		}
+		portStr, token := f[0], f[1]
+		c, err := net.Dial("tcp", "localhost:"+portStr)
+		if err != nil {
+			return nil, fmt.Errorf("error dialing IPNExtension: %w", err)
+		}
+		if _, err := io.WriteString(c, token+"\n"); err != nil {
+			return nil, fmt.Errorf("error writing auth token: %w", err)
+		}
+		buf := make([]byte, 5)
+		const authOK = "#IPN\n"
+		if _, err := io.ReadFull(c, buf); err != nil {
+			return nil, fmt.Errorf("error reading from IPNExtension post-auth: %w", err)
+		}
+		if string(buf) != authOK {
+			return nil, fmt.Errorf("invalid response reading from IPNExtension post-auth")
+		}
+		return c, nil
+	}
+	return nil, fmt.Errorf("failed to find Tailscale's IPNExtension process")
+}