libgocryptfs/internal/readpassword/read.go

// Package readpassword reads a password from the terminal of from stdin.
package readpassword

import (
	"bytes"
	"fmt"
	"io"
	"os"
	"os/exec"
	"strings"
	"sync"
	"time"

	"golang.org/x/crypto/ssh/terminal"

	"github.com/rfjakob/gocryptfs/internal/exitcodes"
	"github.com/rfjakob/gocryptfs/internal/tlog"
)

const (
	// 2kB limit like EncFS
	maxPasswordLen = 2048
)

// Once tries to get a password from the user, either from the terminal, extpass
// or stdin.
func Once(extpass string) []byte {
	if extpass != "" {
		return readPasswordExtpass(extpass)
	}
	if !terminal.IsTerminal(int(os.Stdin.Fd())) {
		return readPasswordStdin()
	}
	return readPasswordTerminal("Password: ")
}

// Twice is the same as Once but will prompt twice if we get the password from
// the terminal.
func Twice(extpass string) []byte {
	if extpass != "" {
		return readPasswordExtpass(extpass)
	}
	if !terminal.IsTerminal(int(os.Stdin.Fd())) {
		return readPasswordStdin()
	}
	p1 := readPasswordTerminal("Password: ")
	p2 := readPasswordTerminal("Repeat: ")
	if !bytes.Equal(p1, p2) {
		tlog.Fatal.Println("Passwords do not match")
		os.Exit(exitcodes.ReadPassword)
	}
	// Wipe the password duplicate from memory
	for i := range p2 {
		p2[i] = 0
	}
	return p1
}

// readPasswordTerminal reads a line from the terminal.
// Exits on read error or empty result.
func readPasswordTerminal(prompt string) []byte {
	fd := int(os.Stdin.Fd())
	fmt.Fprintf(os.Stderr, prompt)
	// terminal.ReadPassword removes the trailing newline
	p, err := terminal.ReadPassword(fd)
	if err != nil {
		tlog.Fatal.Printf("Could not read password from terminal: %v\n", err)
		os.Exit(exitcodes.ReadPassword)
	}
	fmt.Fprintf(os.Stderr, "\n")
	if len(p) == 0 {
		tlog.Fatal.Println("Password is empty")
		os.Exit(exitcodes.PasswordEmpty)
	}
	return p
}

// readPasswordStdin reads a line from stdin.
// It exits with a fatal error on read error or empty result.
func readPasswordStdin() []byte {
	tlog.Info.Println("Reading password from stdin")
	p := readLineUnbuffered(os.Stdin)
	if len(p) == 0 {
		tlog.Fatal.Println("Got empty password from stdin")
		os.Exit(exitcodes.ReadPassword)
	}
	return p
}

// readPasswordExtpass executes the "extpass" program and returns the first line
// of the output.
// Exits on read error or empty result.
func readPasswordExtpass(extpass string) []byte {
	tlog.Info.Println("Reading password from extpass program")
	var parts []string
	// The option "-passfile=FILE" gets transformed to
	// "-extpass="/bin/cat -- FILE". We don't want to split FILE on spaces,
	// so let's handle it manually.
	passfileCat := "/bin/cat -- "
	if strings.HasPrefix(extpass, passfileCat) {
		parts = []string{"/bin/cat", "--", extpass[len(passfileCat):]}
	} else {
		parts = strings.Split(extpass, " ")
	}
	cmd := exec.Command(parts[0], parts[1:]...)
	cmd.Stderr = os.Stderr
	pipe, err := cmd.StdoutPipe()
	if err != nil {
		tlog.Fatal.Printf("extpass pipe setup failed: %v", err)
		os.Exit(exitcodes.ReadPassword)
	}
	err = cmd.Start()
	if err != nil {
		tlog.Fatal.Printf("extpass cmd start failed: %v", err)
		os.Exit(exitcodes.ReadPassword)
	}
	p := readLineUnbuffered(pipe)
	pipe.Close()
	err = cmd.Wait()
	if err != nil {
		tlog.Fatal.Printf("extpass program returned an error: %v", err)
		os.Exit(exitcodes.ReadPassword)
	}
	if len(p) == 0 {
		tlog.Fatal.Println("extpass: password is empty")
		os.Exit(exitcodes.ReadPassword)
	}
	return p
}

// readLineUnbuffered reads single bytes from "r" util it gets "\n" or EOF.
// The returned string does NOT contain the trailing "\n".
func readLineUnbuffered(r io.Reader) (l []byte) {
	b := make([]byte, 1)
	for {
		if len(l) > maxPasswordLen {
			tlog.Fatal.Printf("fatal: maximum password length of %d bytes exceeded", maxPasswordLen)
			os.Exit(exitcodes.ReadPassword)
		}
		n, err := r.Read(b)
		if err == io.EOF {
			return l
		}
		if err != nil {
			tlog.Fatal.Printf("readLineUnbuffered: %v", err)
			os.Exit(exitcodes.ReadPassword)
		}
		if n == 0 {
			continue
		}
		if b[0] == '\n' {
			return l
		}
		l = append(l, b...)
	}
}

// CheckTrailingGarbage tries to read one byte from stdin and exits with a
// fatal error if the read returns any data.
// This is meant to be called after reading the password, when there is no more
// data expected. This helps to catch problems with third-party tools that
// interface with gocryptfs.
//
// This is tested via TestInitTrailingGarbage() in tests/cli/cli_test.go.
func CheckTrailingGarbage() {
	if terminal.IsTerminal(int(os.Stdin.Fd())) {
		// Be lenient when interacting with a human.
		return
	}
	var wg sync.WaitGroup
	wg.Add(1)
	go func() {
		b := make([]byte, 1)
		wg.Done()
		n, _ := os.Stdin.Read(b)
		if n > 0 {
			tlog.Fatal.Printf("Received trailing garbage after the password")
			os.Exit(exitcodes.ReadPassword)
		}
	}()
	// Wait for the goroutine to start up plus one millisecond for the read to
	// return. If there is data available, this SHOULD be plenty of time to
	// read one byte. However, I don't see a way to be sure.
	wg.Wait()
	time.Sleep(1 * time.Millisecond)
}