1 // MsgSyncReq: copy files to the remote host
3 // SPDX-License-Identifier: GPL-3.0-or-later
4 // Copyright (C) 2021-2024 Simon Ruderich
27 "github.com/ianbruene/go-difflib/difflib"
28 "golang.org/x/sys/unix"
30 "ruderich.org/simon/safcm"
33 // NOTE: Don't use plain os.Open, os.OpenFile, os.Remove or any other function
34 // which uses absolute paths. These are vulnerable to symlink attacks. Always
35 // use *at syscalls. See below for details.
37 // openReadonlyFlags are flags for open* syscalls to safely read a file or
40 // O_NOFOLLOW prevents symlink attacks in the last path component
41 // O_NONBLOCK is necessary to prevent blocking on FIFOs
42 const openReadonlyFlags = unix.O_RDONLY | unix.O_NOFOLLOW | unix.O_NONBLOCK
44 func (s *Sync) syncFiles() error {
45 // To create random file names for symlinks
46 rand.Seed(time.Now().UnixNano())
48 // Sort for deterministic order and so parent directories are present
49 // when files in them are created
50 var files []*safcm.File
51 for _, x := range s.req.Files {
52 files = append(files, x)
54 sort.Slice(files, func(i, j int) bool {
55 return files[i].Path < files[j].Path
58 for _, x := range files {
60 err := s.syncFile(x, &changed)
62 return fmt.Errorf("%q: %v", x.Path, err)
72 func (s *Sync) syncFile(file *safcm.File, changed *bool) error {
73 // The general strategy is "update by rename": If any property of a
74 // file changes the new version will be written to a temporary file
75 // and then renamed "over" the original file. This is simple and
76 // prevents race conditions where the file is partially readable while
77 // changes to permissions or owner/group are applied. However, this
78 // strategy does not work for directories which must be removed first
79 // (was directory), must remove the existing file (will be directory)
80 // or must be directly modified (changed permissions or owner). In the
81 // first two cases the old path is removed. In the last the directory
82 // is modified (carefully) in place.
84 // The implementation is careful not to follow any symlinks to prevent
85 // possible race conditions which can be exploited and are especially
86 // dangerous when running with elevated privileges (which will most
87 // likely be the case). This includes not using absolute paths in
88 // syscalls to prevent symlink attacks when a directory is writable by
89 // other users (e.g. when syncing a file to /home/user/dir/file the
90 // user could create dir as symlink to another directory and file
91 // would be written there). To prevent this *at syscalls are used and
92 // all symlinks in the path are rejected. This still permits the user
93 // to move dir during the sync but only to places which are writable
94 // by the user which cannot be prevented.
96 err := s.fileResolveIds(file)
101 change := safcm.FileChange{
103 New: safcm.FileChangeInfo{
112 debugf := func(format string, a ...interface{}) {
113 s.log.Debugf("files: %q (%s): %s",
114 file.Path, file.OrigGroup, fmt.Sprintf(format, a...))
116 verbosef := func(format string, a ...interface{}) {
117 s.log.Verbosef("files: %q (%s): %s",
118 file.Path, file.OrigGroup, fmt.Sprintf(format, a...))
121 parentFd, baseName, err := OpenParentDirectoryNoSymlinks(file.Path)
123 if os.IsNotExist(err) && s.req.DryRun {
124 change.Created = true
125 debugf("will create (parent missing)")
127 debugf("dry-run, skipping changes")
128 s.resp.FileChanges = append(s.resp.FileChanges, change)
133 defer unix.Close(parentFd)
135 var oldStat unix.Stat_t
137 oldFh, err := OpenAtNoFollow(parentFd, baseName)
139 if err == unix.ELOOP || err == unix.EMLINK {
140 // ELOOP from symbolic link, this is fine
141 err := unix.Fstatat(parentFd, baseName, &oldStat,
142 unix.AT_SYMLINK_NOFOLLOW)
146 if oldStat.Mode&unix.S_IFMT != unix.S_IFLNK {
147 debugf("type changed from symlink, retrying")
150 } else if os.IsNotExist(err) {
151 change.Created = true
152 debugf("will create")
159 err := unix.Fstat(int(oldFh.Fd()), &oldStat)
166 var changeType, changePerm, changeUserOrGroup, changeData bool
168 // Manually convert to FileMode; from src/os/stat_linux.go in
169 // Go's sources (stat_*.go for other UNIX systems are
170 // identical, except for stat_darwin.go which has an extra
172 mode := fs.FileMode(oldStat.Mode & 0777)
173 switch oldStat.Mode & unix.S_IFMT {
175 mode |= fs.ModeDevice
177 mode |= fs.ModeDevice | fs.ModeCharDevice
181 mode |= fs.ModeNamedPipe
183 mode |= fs.ModeSymlink
187 mode |= fs.ModeSocket
188 // Guard against unknown file types (e.g. on darwin); not in
191 return fmt.Errorf("unexpected file mode %v",
192 oldStat.Mode&unix.S_IFMT)
194 if oldStat.Mode&unix.S_ISGID != 0 {
195 mode |= fs.ModeSetgid
197 if oldStat.Mode&unix.S_ISUID != 0 {
198 mode |= fs.ModeSetuid
200 if oldStat.Mode&unix.S_ISVTX != 0 {
201 mode |= fs.ModeSticky
204 // Compare permissions
205 change.Old.Mode = mode
206 if change.Old.Mode.Type() == fs.ModeSymlink {
207 // Some BSD systems permit changing permissions of
208 // symlinks but ignore them on traversal. To keep it
209 // simple we don't support that and always use 0777
210 // for symlink permissions (the value on GNU/Linux)
211 // when comparing. The actual permissions on the file
212 // system might be different on BSD systems.
214 // TODO: Add proper support for symlinks on BSD
215 change.Old.Mode |= 0777
217 if change.Old.Mode != file.Mode {
218 if change.Old.Mode.Type() != file.Mode.Type() {
220 debugf("type differs %s -> %s",
221 change.Old.Mode.Type(),
224 // Be careful with .Perm() which does not
225 // contain the setuid/setgid/sticky bits!
227 debugf("permission differs %s -> %s",
228 change.Old.Mode, file.Mode)
232 // Compare user/group
233 change.Old.Uid = int(oldStat.Uid)
234 change.Old.Gid = int(oldStat.Gid)
235 if change.Old.Uid != file.Uid || change.Old.Gid != file.Gid {
236 changeUserOrGroup = true
237 debugf("uid/gid differs %d/%d -> %d/%d",
238 change.Old.Uid, change.Old.Gid,
241 u, g, err := resolveIdsToNames(change.Old.Uid, change.Old.Gid)
242 // Errors are not relevant as this is only used to report the
243 // change. If the user/group no longer exits only the ids will
250 // Compare file content (if possible)
251 switch change.Old.Mode.Type() { //nolint:exhaustive
252 case 0: // regular file
253 x, err := io.ReadAll(oldFh)
255 return fmt.Errorf("reading old content: %v",
260 buf := make([]byte, unix.PathMax)
261 n, err := unix.Readlinkat(parentFd, baseName, buf)
263 return fmt.Errorf("reading old content: %v",
268 if !changeType && file.Mode.Type() != fs.ModeDir {
269 if !bytes.Equal(oldData, file.Data) {
271 debugf("content differs")
277 if !change.Created && !changeType &&
278 !changePerm && !changeUserOrGroup &&
285 // Don't show a diff with the full content for newly created files or
286 // on type changes. This is just noise for the user as the new file
287 // content is obvious. But we always want to see a diff when files are
288 // replaced because this destroys data.
289 if !change.Created &&
290 (change.Old.Mode.Type() == 0 ||
291 change.Old.Mode.Type() == fs.ModeSymlink) {
292 change.DataDiff, err = diffData(oldData, file.Data)
298 // Add change here so it is stored even when applying it fails. This
299 // way the user knows exactly what was attempted.
300 s.resp.FileChanges = append(s.resp.FileChanges, change)
309 debugf("dry-run, skipping changes")
313 // We cannot rename over directories and vice versa
314 if changeType && (change.Old.Mode.IsDir() || file.Mode.IsDir()) {
315 debugf("removing (due to type change)")
316 // In the past os.RemoveAll() was used here. However, this is
317 // difficult to implement manually with *at syscalls. To keep
318 // it simple only permit removing files and empty directories
319 // here. This also has the bonus of preventing data loss when
320 // (accidentally) replacing a directory tree with a file.
321 const msg = "will not replace non-empty directory, " +
322 "please remove manually"
323 err := unix.Unlinkat(parentFd, baseName, 0 /* flags */)
324 if err != nil && !os.IsNotExist(err) {
325 err2 := unix.Unlinkat(parentFd, baseName,
327 if err2 != nil && !os.IsNotExist(err2) {
328 // See src/os/file_unix.go in Go's sources
329 if err2 == unix.ENOTDIR {
331 } else if err2 == unix.ENOTEMPTY {
332 return fmt.Errorf(msg)
340 // Directory: create new directory, also type change to directory
341 if file.Mode.IsDir() && (change.Created || changeType) {
342 debugf("creating directory")
343 err := unix.Mkdirat(parentFd, baseName, 0700)
347 // We must be careful not to chmod arbitrary files. If the
348 // target directory is writable then it might have changed to
349 // a symlink at this point. There's no lchmod and fchmodat is
350 // incomplete so open the directory.
351 debugf("chmodding %s", file.Mode)
352 dh, err := OpenAtNoFollow(parentFd, baseName)
358 err = dh.Chmod(file.Mode)
362 // Less restrictive access is not relevant here because there
363 // are no files present yet.
364 debugf("chowning %d/%d", file.Uid, file.Gid)
365 err = dh.Chown(file.Uid, file.Gid)
371 // Directory: changed permission or user/group
372 if file.Mode.IsDir() {
373 // We don't know if the new permission or if the new
374 // user/group is more restrictive (e.g. root:root 0750 ->
375 // user:group 0700; applying group first gives group
376 // unexpected access). To prevent a short window where the
377 // access might be too lax we temporarily deny all access.
378 if changePerm && changeUserOrGroup {
379 // Only drop group and other permission because user
380 // has access anyway (either before or after the
381 // change). This also prevents temporary errors during
382 // the error when the user tries to access this
383 // directory (access for the group will fail though).
384 mode := change.Old.Mode & fs.ModePerm & 0700
385 // Retain setgid/sticky so that the behavior does not
386 // change when creating and removing files.
387 mode |= change.Old.Mode & fs.ModeSetgid
388 mode |= change.Old.Mode & fs.ModeSticky
389 debugf("chmodding %#o (temporary)", mode)
390 err := oldFh.Chmod(mode)
395 if changeUserOrGroup {
396 debugf("chowning %d/%d", file.Uid, file.Gid)
397 err := oldFh.Chown(file.Uid, file.Gid)
403 debugf("chmodding %s", file.Mode)
404 err := oldFh.Chmod(file.Mode)
412 dir := slashpath.Dir(file.Path) // only used in debug messages
413 // Create hidden file which should be ignored by most other tools and
414 // thus not affect anything during creation
415 tmpBase := "." + baseName
417 switch file.Mode.Type() {
418 case 0: // regular file
419 debugf("creating temporary file %q",
420 slashpath.Join(dir, tmpBase+"*"))
421 x, err := WriteTempAt(parentFd, tmpBase, file.Data,
422 file.Uid, file.Gid, file.Mode)
431 x := tmpBase + strconv.Itoa(rand.Int())
432 debugf("creating temporary symlink %q",
433 slashpath.Join(dir, x))
434 err := unix.Symlinkat(string(file.Data), parentFd, x)
436 if os.IsExist(err) && i < 10000 {
444 err = unix.Fchownat(parentFd, tmpBase, file.Uid, file.Gid,
445 unix.AT_SYMLINK_NOFOLLOW)
447 unix.Unlinkat(parentFd, tmpBase, 0 /* flags */) //nolint:errcheck
450 // Permissions are irrelevant for symlinks (on most systems)
453 panic(fmt.Sprintf("invalid file type %s", file.Mode))
456 debugf("renaming %q", slashpath.Join(dir, tmpBase))
457 err = unix.Renameat(parentFd, tmpBase, parentFd, baseName)
459 unix.Unlinkat(parentFd, tmpBase, 0 /* flags */) //nolint:errcheck
462 // To guarantee durability fsync must be called on a parent directory
463 // after adding, renaming or removing files therein.
465 // Calling sync on the files itself is not enough according to POSIX;
466 // man 2 fsync: "Calling fsync() does not necessarily ensure that the
467 // entry in the directory containing the file has also reached disk.
468 // For that an explicit fsync() on a file descriptor for the directory
470 err = unix.Fsync(parentFd)
478 func (s *Sync) fileResolveIds(file *safcm.File) error {
479 if file.User != "" && file.Uid != -1 {
480 return fmt.Errorf("cannot set both User (%q) and Uid (%d)",
483 if file.Group != "" && file.Gid != -1 {
484 return fmt.Errorf("cannot set both Group (%q) and Gid (%d)",
485 file.Group, file.Gid)
488 if file.User == "" && file.Uid == -1 {
489 file.User = s.defaultUser
492 x, err := user.Lookup(file.User)
496 id, err := strconv.Atoi(x.Uid)
503 if file.Group == "" && file.Gid == -1 {
504 file.Group = s.defaultGroup
506 if file.Group != "" {
507 x, err := user.LookupGroup(file.Group)
511 id, err := strconv.Atoi(x.Gid)
521 // OpenParentDirectoryNoSymlinks opens the dirname of path without following
522 // any symlinks and returns a file descriptor to it and the basename of path.
523 // To prevent symlink attacks in earlier path components when these are
524 // writable by other users it starts at the root (or current directory for
525 // relative paths) and uses openat (with O_NOFOLLOW) for each path component.
526 // If a symlink is encountered it returns an error. However, it's impossible
527 // to guarantee that the returned descriptor refers to the same location as
528 // given in path because users with write access can rename path components.
529 // But this is not required to prevent the mentioned attacks.
530 func OpenParentDirectoryNoSymlinks(path string) (int, string, error) {
531 // Slash separated paths are used for the configuration
532 parts := strings.Split(path, "/")
536 // Root: use root itself as base name because root is the
539 parts = []string{"/"}
540 } else if parts[0] == "" {
544 } else if path == "." {
545 // Current directory: open parent directory and use current
546 // directory name as base name
547 wd, err := os.Getwd()
549 return -1, "", fmt.Errorf(
550 "failed to get working directory: %w", err)
553 parts = []string{filepath.Base(wd)}
555 // Relative path: start at the current directory
562 dirFd, err := unix.Openat(unix.AT_FDCWD, dir,
563 openReadonlyFlags, 0 /* mode */)
567 // Walk path one directory at a time to ensure there are no symlinks
568 // in the path. This prevents users with write access to change the
569 // path to point to arbitrary locations. O_NOFOLLOW when opening the
570 // path is not enough as only the last path component is checked.
571 for i, name := range parts[:len(parts)-1] {
572 fd, err := unix.Openat(dirFd, name, openReadonlyFlags, 0)
575 if err == unix.ELOOP || err == unix.EMLINK {
576 x := filepath.Join(append([]string{dir},
578 return -1, "", fmt.Errorf(
579 "symlink not permitted in path: %q",
588 return dirFd, parts[len(parts)-1], nil
591 func resolveIdsToNames(uid, gid int) (string, string, error) {
592 u, err := user.LookupId(strconv.Itoa(uid))
596 g, err := user.LookupGroupId(strconv.Itoa(gid))
600 return u.Username, g.Name, nil
603 func diffData(oldData []byte, newData []byte) (string, error) {
604 oldBin := !strings.HasPrefix(http.DetectContentType(oldData), "text/")
605 newBin := !strings.HasPrefix(http.DetectContentType(newData), "text/")
606 if oldBin && newBin {
607 return fmt.Sprintf("Binary files differ (%d -> %d bytes), "+
608 "cannot show diff", len(oldData), len(newData)), nil
611 oldData = []byte(fmt.Sprintf("<binary content, %d bytes>\n",
615 newData = []byte(fmt.Sprintf("<binary content, %d bytes>\n",
619 // TODO: difflib shows empty context lines at the end of the file
620 // which should not be there
621 // TODO: difflib has issues with missing newlines in either side
622 result, err := difflib.GetUnifiedDiffString(difflib.LineDiffParams{
623 A: difflib.SplitLines(string(oldData)),
624 B: difflib.SplitLines(string(newData)),
633 func OpenFileNoSymlinks(path string) (*os.File, error) {
634 parentFd, baseName, err := OpenParentDirectoryNoSymlinks(path)
638 defer unix.Close(parentFd)
639 return OpenAtNoFollow(parentFd, baseName)
642 func OpenAtNoFollow(dirFd int, base string) (*os.File, error) {
643 fd, err := unix.Openat(dirFd, base, openReadonlyFlags, 0 /* mode */)
647 return os.NewFile(uintptr(fd), ""), nil
650 func WriteTempAt(dirFd int, base string, data []byte, uid, gid int,
651 mode fs.FileMode) (string, error) {
653 fh, tmpBase, err := createTempAt(dirFd, base)
658 _, err = fh.Write(data)
661 unix.Unlinkat(dirFd, tmpBase, 0 /* flags */) //nolint:errcheck
664 // createTempAt() creates the file with 0600
665 err = fh.Chown(uid, gid)
668 unix.Unlinkat(dirFd, tmpBase, 0) //nolint:errcheck
674 unix.Unlinkat(dirFd, tmpBase, 0) //nolint:errcheck
680 unix.Unlinkat(dirFd, tmpBase, 0) //nolint:errcheck
685 unix.Unlinkat(dirFd, tmpBase, 0) //nolint:errcheck
692 // createTempAt works similar to os.CreateTemp but uses unix.Openat to create
693 // the temporary file.
694 func createTempAt(dirFd int, base string) (*os.File, string, error) {
699 tmpBase = base + strconv.Itoa(rand.Int())
701 fd, err := unix.Openat(dirFd, tmpBase,
702 unix.O_RDWR|unix.O_CREAT|unix.O_EXCL, 0600)
704 if os.IsExist(err) && i < 10000 {
711 return os.NewFile(uintptr(fd), ""), tmpBase, nil