tailscale/taildrop/taildrop.go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause

package taildrop

import (
	"errors"
	"hash/adler32"
	"os"
	"path"
	"path/filepath"
	"strconv"
	"strings"
	"sync/atomic"
	"unicode"
	"unicode/utf8"

	"tailscale.com/tstime"
	"tailscale.com/types/logger"
	"tailscale.com/util/multierr"
)

type Handler struct {
	Logf  logger.Logf
	Clock tstime.Clock

	RootDir string // empty means file receiving unavailable

	// DirectFileMode is whether we're writing files directly to a
	// download directory (as *.partial files), rather than making
	// the frontend retrieve it over localapi HTTP and write it
	// somewhere itself. This is used on the GUI macOS versions
	// and on Synology.
	// In DirectFileMode, the peerapi doesn't do the final rename
	// from "foo.jpg.partial" to "foo.jpg" unless
	// directFileDoFinalRename is set.
	DirectFileMode bool

	// DirectFileDoFinalRename is whether in directFileMode we
	// additionally move the *.direct file to its final name after
	// it's received.
	DirectFileDoFinalRename bool

	KnownEmpty atomic.Bool
}

var (
	errNilHandler = errors.New("handler unavailable; not listening")
	ErrNoTaildrop = errors.New("Taildrop disabled; no storage directory")
)

const (
	// PartialSuffix is the suffix appended to files while they're
	// still in the process of being transferred.
	PartialSuffix = ".partial"

	// deletedSuffix is the suffix for a deleted marker file
	// that's placed next to a file (without the suffix) that we
	// tried to delete, but Windows wouldn't let us. These are
	// only written on Windows (and in tests), but they're not
	// permitted to be uploaded directly on any platform, like
	// partial files.
	deletedSuffix = ".deleted"
)

// redacted is a fake path name we use in errors, to avoid
// accidentally logging actual filenames anywhere.
const redacted = "redacted"

func validFilenameRune(r rune) bool {
	switch r {
	case '/':
		return false
	case '\\', ':', '*', '"', '<', '>', '|':
		// Invalid stuff on Windows, but we reject them everywhere
		// for now.
		// TODO(bradfitz): figure out a better plan. We initially just
		// wrote things to disk URL path-escaped, but that's gross
		// when debugging, and just moves the problem to callers.
		// So now we put the UTF-8 filenames on disk directly as
		// sent.
		return false
	}
	return unicode.IsPrint(r)
}

func (s *Handler) DiskPath(baseName string) (fullPath string, ok bool) {
	if !utf8.ValidString(baseName) {
		return "", false
	}
	if strings.TrimSpace(baseName) != baseName {
		return "", false
	}
	if len(baseName) > 255 {
		return "", false
	}
	// TODO: validate unicode normalization form too? Varies by platform.
	clean := path.Clean(baseName)
	if clean != baseName ||
		clean == "." || clean == ".." ||
		strings.HasSuffix(clean, deletedSuffix) ||
		strings.HasSuffix(clean, PartialSuffix) {
		return "", false
	}
	for _, r := range baseName {
		if !validFilenameRune(r) {
			return "", false
		}
	}
	if !filepath.IsLocal(baseName) {
		return "", false
	}
	return filepath.Join(s.RootDir, baseName), true
}

type redactedErr struct {
	msg   string
	inner error
}

func (re *redactedErr) Error() string {
	return re.msg
}

func (re *redactedErr) Unwrap() error {
	return re.inner
}

func redactString(s string) string {
	hash := adler32.Checksum([]byte(s))

	var buf [len(redacted) + len(".12345678")]byte
	b := append(buf[:0], []byte(redacted)...)
	b = append(b, '.')
	b = strconv.AppendUint(b, uint64(hash), 16)
	return string(b)
}

func RedactErr(root error) error {
	// redactStrings is a list of sensitive strings that were redacted.
	// It is not sufficient to just snub out sensitive fields in Go errors
	// since some wrapper errors like fmt.Errorf pre-cache the error string,
	// which would unfortunately remain unaffected.
	var redactStrings []string

	// Redact sensitive fields in known Go error types.
	var unknownErrors int
	multierr.Range(root, func(err error) bool {
		switch err := err.(type) {
		case *os.PathError:
			redactStrings = append(redactStrings, err.Path)
			err.Path = redactString(err.Path)
		case *os.LinkError:
			redactStrings = append(redactStrings, err.New, err.Old)
			err.New = redactString(err.New)
			err.Old = redactString(err.Old)
		default:
			unknownErrors++
		}
		return true
	})

	// If there are no redacted strings or no unknown error types,
	// then we can return the possibly modified root error verbatim.
	// Otherwise, we must replace redacted strings from any wrappers.
	if len(redactStrings) == 0 || unknownErrors == 0 {
		return root
	}

	// Stringify and replace any paths that we found above, then return
	// the error wrapped in a type that uses the newly-redacted string
	// while also allowing Unwrap()-ing to the inner error type(s).
	s := root.Error()
	for _, toRedact := range redactStrings {
		s = strings.ReplaceAll(s, toRedact, redactString(toRedact))
	}
	return &redactedErr{msg: s, inner: root}
}