Move daemon code to dedicated package

daemon/1shared.go

@@ -0,0 +1,9 @@
+package daemon
+
+import (
+	"os"
+	"path/filepath"
+)
+
+var binPath, _ = os.Executable()                                 // store binary path
+var socketPath = "/tmp/" + filepath.Base(binPath) + "-rcwd.sock" // store UNIX socket path

daemon/2server.go

@@ -0,0 +1,133 @@
+package daemon
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"errors"
+	"fmt"
+	"io"
+	"log"
+	"net"
+	"net/rpc"
+	"os"
+	"syscall"
+	"time"
+)
+
+var daemonHash []byte
+
+// RCWService provides an RPC method.
+type RCWService struct{}
+
+// Run should be called to start an RPC server.
+func Run() {
+	// store the hash of the daemon binary
+	daemonHash = getFileHash(binPath)
+
+	// remove the socket file if it already exists
+	if _, err := os.Stat(socketPath); err == nil {
+		if err := os.Remove(socketPath); err != nil {
+			log.Fatalf("Failed to remove existing socket: %v", err)
+		}
+	}
+
+	// register RCWService with the RPC package
+	if err := rpc.Register(&RCWService{}); err != nil {
+		log.Fatalf("Error registering RPC service: %v", err)
+	}
+
+	// listen on the Unix domain socket
+	listener, err := net.Listen("unix", socketPath)
+	if err != nil {
+		log.Fatalf("Failed to listen on UNIX socket %s: %v", socketPath, err)
+	}
+	defer listener.Close()
+	log.Printf("RPC daemon listening on unix://%s", socketPath)
+
+	// Accept connections (timeout after 3 minutes of inactivity)
+	for {
+		listener.(*net.UnixListener).SetDeadline(time.Now().Add(3 * time.Minute))
+
+		conn, err := listener.Accept()
+		if err != nil {
+			if err.(net.Error).Timeout() {
+				log.Println("Three minutes have passed without any connections. Exiting...")
+				os.Exit(0)
+			}
+			log.Printf("Accept error: %v", err)
+			continue
+		}
+		// use a goroutine to check the client's identity
+		go handleConn(conn)
+	}
+}
+
+// GetPass is the RPC method.
+// For now (as a test/example), it returns "hello" if the input is "hi".
+func (h *RCWService) GetPass(request string, reply *string) error {
+	if request == "hi" {
+		*reply = "hello"
+		return nil
+	}
+	return errors.New("unexpected input, expected \"hi\"")
+}
+
+// handleConn verifies the identity of the client.
+// It uses the file descriptor of the connection to get the PID of the client,
+// which is then used to get the path of the client's executable and calculate its hash.
+// The passphrase is only returned if the client's executable hash matches the daemon's hash.
+// This ensures that only the binary the daemon is embedded in can retrieve the passphrase.
+func handleConn(conn net.Conn) {
+	// ensure access to the underlying file descriptor
+	unixConn, ok := conn.(*net.UnixConn)
+	if !ok {
+		log.Printf("Connection is not a Unix domain socket")
+		conn.Close()
+		return
+	}
+	// obtain SyscallConn for direct access to the file descriptor
+	rawConn, err := unixConn.SyscallConn()
+	if err != nil {
+		log.Printf("SyscallConn error: %v", err)
+		conn.Close()
+		return
+	}
+
+	var callingPID int32
+	_ = rawConn.Control(func(fd uintptr) {
+		// use syscall.GetsockoptUcred to fetch credentials
+		ucred, err := syscall.GetsockoptUcred(int(fd), syscall.SOL_SOCKET, syscall.SO_PEERCRED)
+		if err != nil {
+			log.Printf("Error getting peer credentials: %v", err)
+			return
+		}
+		callingPID = ucred.Pid
+	})
+
+	// check if the RPC call is coming from an identical binary
+	callingBinPath := pidToPath(callingPID)
+	if bytes.Equal(getFileHash(callingBinPath), daemonHash) {
+		// valid client; hand off the connection to the RPC server
+		rpc.ServeConn(conn)
+	} else {
+		// invalid client; close the connection w/o a response,
+		// log the client's path, and kill the daemon
+		conn.Close()
+		log.Printf("Request received from invalid client: %s", callingBinPath) // TODO log to file
+		os.Exit(2)
+	}
+}
+
+// pidToPath returns the path of the executable that has the given PID.
+func pidToPath(pid int32) string {
+	path, _ := os.Readlink(fmt.Sprintf("/proc/%d/exe", pid))
+	return path
+}
+
+// getFileHash returns the SHA256 hash of the file at the given path.
+func getFileHash(path string) []byte {
+	file, _ := os.Open(path)
+	hash := sha256.New()
+	io.Copy(hash, file)
+	return hash.Sum(nil)
+}

daemon/3client.go

@@ -0,0 +1,31 @@
+package daemon
+
+import (
+	"log"
+	"net"
+	"net/rpc"
+)
+
+// Call connects to the RPC server and requests the passphrase.
+func Call() string {
+	// connect to the UNIX domain socket
+	conn, err := net.Dial("unix", socketPath)
+	if err != nil {
+		log.Fatalf("Dial error: %v", err)
+	}
+	defer conn.Close()
+
+	// create an RPC client using the connection
+	client := rpc.NewClient(conn)
+	defer client.Close()
+
+	// request the passphrase from the RPC server
+	var reply string
+	err = client.Call("RCWService.GetPass", "hi", &reply)
+	if err != nil {
+		log.Fatalf("Error calling RCWService.GetPass: %v", err)
+	}
+
+	// return the passphrase
+	return reply
+}