1 // MsgSyncReq: copy files to the remote host
3 // Copyright (C) 2021 Simon Ruderich
5 // This program is free software: you can redistribute it and/or modify
6 // it under the terms of the GNU General Public License as published by
7 // the Free Software Foundation, either version 3 of the License, or
8 // (at your option) any later version.
10 // This program is distributed in the hope that it will be useful,
11 // but WITHOUT ANY WARRANTY; without even the implied warranty of
12 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 // GNU General Public License for more details.
15 // You should have received a copy of the GNU General Public License
16 // along with this program. If not, see <http://www.gnu.org/licenses/>.
39 "github.com/ianbruene/go-difflib/difflib"
40 "golang.org/x/sys/unix"
42 "ruderich.org/simon/safcm"
45 // NOTE: Don't use plain os.Open, os.OpenFile, os.Remove or any other function
46 // which uses absolute paths. These are vulnerable to symlink attacks. Always
47 // use *at syscalls. See below for details.
49 // openReadonlyFlags are flags for open* syscalls to safely read a file or
52 // O_NOFOLLOW prevents symlink attacks
53 // O_NONBLOCK is necessary to prevent blocking on FIFOs
54 const openReadonlyFlags = unix.O_RDONLY | unix.O_NOFOLLOW | unix.O_NONBLOCK
56 func (s *Sync) syncFiles() error {
57 // To create random file names for symlinks
58 rand.Seed(time.Now().UnixNano())
60 // Sort for deterministic order and so parent directories are present
61 // when files in them are created
62 var files []*safcm.File
63 for _, x := range s.req.Files {
64 files = append(files, x)
66 sort.Slice(files, func(i, j int) bool {
67 return files[i].Path < files[j].Path
70 for _, x := range files {
72 err := s.syncFile(x, &changed)
74 return fmt.Errorf("%q: %v", x.Path, err)
84 func (s *Sync) syncFile(file *safcm.File, changed *bool) error {
85 // The general strategy is "update by rename": If any property of a
86 // file changes it will be written to a temporary file and then
87 // renamed "over" the original file. This is simple and prevents race
88 // conditions where the file is partially readable while changes to
89 // permissions or owner/group are applied. However, this strategy does
90 // not work for directories which must be removed first (was
91 // directory), must remove the existing file (will be directory) or
92 // must be directly modified (changed permissions or owner). In the
93 // first two cases the old path is removed. In the last the directory
94 // is modified (carefully) in place.
96 // The implementation is careful not to follow any symlinks to prevent
97 // possible race conditions which can be exploited and are especially
98 // dangerous when running with elevated privileges (which will most
99 // likely be the case). This includes not using absolute paths in
100 // syscalls to prevent symlink attacks when a directory is writable by
101 // other users (e.g. when syncing a file to /home/user/dir/file the
102 // user could create dir as symlink to another directory and file
103 // would be written there). To prevent this *at syscalls are used and
104 // all symlinks in the path are rejected. This still permits the user
105 // to move dir during the sync but only to places which are writable
106 // by the user which cannot be prevented.
108 err := s.fileResolveIds(file)
113 change := safcm.FileChange{
115 New: safcm.FileChangeInfo{
124 debugf := func(format string, a ...interface{}) {
125 s.log.Debugf("files: %q (%s): %s",
126 file.Path, file.OrigGroup, fmt.Sprintf(format, a...))
128 verbosef := func(format string, a ...interface{}) {
129 s.log.Verbosef("files: %q (%s): %s",
130 file.Path, file.OrigGroup, fmt.Sprintf(format, a...))
133 parentFd, baseName, err := OpenParentDirectoryNoSymlinks(file.Path)
137 defer unix.Close(parentFd)
139 var oldStat unix.Stat_t
141 oldFh, err := OpenAtNoFollow(parentFd, baseName)
143 if err == unix.ELOOP || err == unix.EMLINK {
144 // ELOOP from symbolic link, this is fine
145 err := unix.Fstatat(parentFd, baseName, &oldStat,
146 unix.AT_SYMLINK_NOFOLLOW)
150 if oldStat.Mode&unix.S_IFMT != unix.S_IFLNK {
151 debugf("type changed from symlink, retrying")
154 } else if os.IsNotExist(err) {
155 change.Created = true
156 debugf("will create")
163 err := unix.Fstat(int(oldFh.Fd()), &oldStat)
170 var changeType, changePerm, changeUserOrGroup, changeData bool
172 // Manually convert to FileMode; from src/os/stat_linux.go in
173 // Go's sources (stat_*.go for other UNIX systems are
174 // identical, except for stat_darwin.go which has an extra
176 mode := fs.FileMode(oldStat.Mode & 0777)
177 switch oldStat.Mode & unix.S_IFMT {
179 mode |= fs.ModeDevice
181 mode |= fs.ModeDevice | fs.ModeCharDevice
185 mode |= fs.ModeNamedPipe
187 mode |= fs.ModeSymlink
191 mode |= fs.ModeSocket
192 // Guard against unknown file types (e.g. on darwin); not in
195 return fmt.Errorf("unexpected file mode %v",
196 oldStat.Mode&unix.S_IFMT)
198 if oldStat.Mode&unix.S_ISGID != 0 {
199 mode |= fs.ModeSetgid
201 if oldStat.Mode&unix.S_ISUID != 0 {
202 mode |= fs.ModeSetuid
204 if oldStat.Mode&unix.S_ISVTX != 0 {
205 mode |= fs.ModeSticky
208 // Compare permissions
209 change.Old.Mode = mode
210 if change.Old.Mode.Type() == fs.ModeSymlink {
211 // Some BSD systems permit changing permissions of
212 // symlinks but ignore them on traversal. To keep it
213 // simple we don't support that and always use 0777
214 // for symlink permissions (the value on GNU/Linux).
216 // TODO: Add proper support for symlinks on BSD
217 change.Old.Mode |= 0777
219 if change.Old.Mode != file.Mode {
220 if change.Old.Mode.Type() != file.Mode.Type() {
222 debugf("type differs %s -> %s",
223 change.Old.Mode.Type(),
226 // Be careful with .Perm() which does not
227 // contain the setuid/setgid/sticky bits!
229 debugf("permission differs %s -> %s",
230 change.Old.Mode, file.Mode)
234 // Compare user/group
235 change.Old.Uid = int(oldStat.Uid)
236 change.Old.Gid = int(oldStat.Gid)
237 if change.Old.Uid != file.Uid || change.Old.Gid != file.Gid {
238 changeUserOrGroup = true
239 debugf("uid/gid differs %d/%d -> %d/%d",
240 change.Old.Uid, change.Old.Gid,
243 u, g, err := resolveIdsToNames(change.Old.Uid, change.Old.Gid)
244 // Errors are not relevant as this is only used to report the
245 // change. If the user/group no longer exits only the ids will
252 // Compare file content (if possible)
253 switch change.Old.Mode.Type() {
254 case 0: // regular file
255 x, err := io.ReadAll(oldFh)
257 return fmt.Errorf("reading old content: %v",
262 buf := make([]byte, unix.PathMax)
263 n, err := unix.Readlinkat(parentFd, baseName, buf)
265 return fmt.Errorf("reading old content: %v",
270 if !changeType && file.Mode.Type() != fs.ModeDir {
271 if !bytes.Equal(oldData, file.Data) {
273 debugf("content differs")
279 if !change.Created && !changeType &&
280 !changePerm && !changeUserOrGroup &&
287 // Don't show a diff with the full content for newly created files or
288 // on type changes. This is just noise for the user as the new file
289 // content is obvious. But we always want to see a diff when files are
290 // replaced because this destroys data.
291 if !change.Created &&
292 (change.Old.Mode.Type() == 0 ||
293 change.Old.Mode.Type() == fs.ModeSymlink) {
294 change.DataDiff, err = diffData(oldData, file.Data)
300 // Add change here so it is stored even when applying it fails. This
301 // way the user knows exactly what was attempted.
302 s.resp.FileChanges = append(s.resp.FileChanges, change)
311 debugf("dry-run, skipping changes")
315 // We cannot rename over directories and vice versa
316 if changeType && (change.Old.Mode.IsDir() || file.Mode.IsDir()) {
317 debugf("removing (due to type change)")
318 // In the past os.RemoveAll() was used here. However, this is
319 // difficult to implement manually with *at syscalls. To keep
320 // it simple only permit removing files and empty directories
321 // here. This also has the bonus of preventing data loss when
322 // (accidentally) replacing a directory tree with a file.
323 const msg = "will not replace non-empty directory, " +
324 "please remove manually"
325 err := unix.Unlinkat(parentFd, baseName, 0)
326 if err != nil && !os.IsNotExist(err) {
327 err2 := unix.Unlinkat(parentFd, baseName,
329 if err2 != nil && !os.IsNotExist(err2) {
330 // See src/os/file_unix.go in Go's sources
331 if err2 == unix.ENOTDIR {
333 } else if err2 == unix.ENOTEMPTY {
334 return fmt.Errorf(msg)
342 // Directory: create new directory, also type change to directory
343 if file.Mode.IsDir() && (change.Created || changeType) {
344 debugf("creating directory")
345 err := unix.Mkdirat(parentFd, baseName, 0700)
349 // We must be careful not to chmod arbitrary files. If the
350 // target directory is writable then it might have changed to
351 // a symlink at this point. There's no lchmod and fchmodat is
352 // incomplete so open the directory.
353 debugf("chmodding %s", file.Mode)
354 dh, err := OpenAtNoFollow(parentFd, baseName)
360 err = dh.Chmod(file.Mode)
364 // Less restrictive access is not relevant here because there
365 // are no files present yet.
366 debugf("chowning %d/%d", file.Uid, file.Gid)
367 err = dh.Chown(file.Uid, file.Gid)
373 // Directory: changed permission or user/group
374 if file.Mode.IsDir() {
375 // We don't know if the new permission or if the new
376 // user/group is more restrictive (e.g. root:root 0750 ->
377 // user:group 0700; applying group first gives group
378 // unexpected access). To prevent a short window where the
379 // access might be too lax we temporarily deny all access.
380 if changePerm && changeUserOrGroup {
381 // Only drop group and other permission because user
382 // has access anyway (either before or after the
383 // change). This also prevents temporary errors during
384 // the error when the user tries to access this
385 // directory (access for the group will fail though).
386 mode := change.Old.Mode & fs.ModePerm & 0700
387 // Retain setgid/sticky so that the behavior does not
388 // change when creating and removing files.
389 mode |= change.Old.Mode & fs.ModeSetgid
390 mode |= change.Old.Mode & fs.ModeSticky
391 debugf("chmodding %#o (temporary)", mode)
392 err := oldFh.Chmod(mode)
397 if changeUserOrGroup {
398 debugf("chowning %d/%d", file.Uid, file.Gid)
399 err := oldFh.Chown(file.Uid, file.Gid)
405 debugf("chmodding %s", file.Mode)
406 err := oldFh.Chmod(file.Mode)
414 dir := slashpath.Dir(file.Path) // only used in debug messages
415 // Create hidden file which should be ignored by most other tools and
416 // thus not affect anything during creation
417 tmpBase := "." + baseName
419 switch file.Mode.Type() {
420 case 0: // regular file
421 debugf("creating temporary file %q",
422 slashpath.Join(dir, tmpBase+"*"))
423 x, err := WriteTempAt(parentFd, tmpBase, file.Data,
424 file.Uid, file.Gid, file.Mode)
433 x := tmpBase + strconv.Itoa(rand.Int())
434 debugf("creating temporary symlink %q",
435 slashpath.Join(dir, x))
436 err := unix.Symlinkat(string(file.Data), parentFd, x)
438 if os.IsExist(err) && i < 10000 {
446 err = unix.Fchownat(parentFd, tmpBase, file.Uid, file.Gid,
447 unix.AT_SYMLINK_NOFOLLOW)
449 unix.Unlinkat(parentFd, tmpBase, 0)
452 // Permissions are irrelevant for symlinks (on most systems)
455 panic(fmt.Sprintf("invalid file type %s", file.Mode))
458 debugf("renaming %q", slashpath.Join(dir, tmpBase))
459 err = unix.Renameat(parentFd, tmpBase, parentFd, baseName)
461 unix.Unlinkat(parentFd, tmpBase, 0)
464 // To guarantee durability fsync must be called on a parent directory
465 // after adding, renaming or removing files therein.
467 // Calling sync on the files itself is not enough according to POSIX;
468 // man 2 fsync: "Calling fsync() does not necessarily ensure that the
469 // entry in the directory containing the file has also reached disk.
470 // For that an explicit fsync() on a file descriptor for the directory
472 err = unix.Fsync(parentFd)
480 func (s *Sync) fileResolveIds(file *safcm.File) error {
481 if file.User != "" && file.Uid != -1 {
482 return fmt.Errorf("cannot set both User (%q) and Uid (%d)",
485 if file.Group != "" && file.Gid != -1 {
486 return fmt.Errorf("cannot set both Group (%q) and Gid (%d)",
487 file.Group, file.Gid)
490 if file.User == "" && file.Uid == -1 {
491 file.User = s.defaultUser
494 x, err := user.Lookup(file.User)
498 id, err := strconv.Atoi(x.Uid)
505 if file.Group == "" && file.Gid == -1 {
506 file.Group = s.defaultGroup
508 if file.Group != "" {
509 x, err := user.LookupGroup(file.Group)
513 id, err := strconv.Atoi(x.Gid)
523 // OpenParentDirectoryNoSymlinks opens the dirname of path without following
524 // any symlinks and returns a file descriptor to it and the basename of path.
525 // To prevent symlink attacks in earlier path components when these are
526 // writable by other users it starts at the root (or current directory for
527 // relative paths) and uses openat (with O_NOFOLLOW) for each path component.
528 // If a symlink is encountered it returns an error. However, it's impossible
529 // to guarantee that the returned descriptor refers to the same location as
530 // given in path because users with write access can rename path components.
531 // But this is not required to prevent the mentioned attacks.
532 func OpenParentDirectoryNoSymlinks(path string) (int, string, error) {
533 // Slash separated paths are used for the configuration
534 parts := strings.Split(path, "/")
538 // Root: use root itself as base name because root is the
541 parts = []string{"/"}
542 } else if parts[0] == "" {
546 } else if path == "." {
547 // Current directory: open parent directory and use current
548 // directory name as base name
549 wd, err := os.Getwd()
551 return -1, "", fmt.Errorf(
552 "failed to get working directory: %w", err)
555 parts = []string{filepath.Base(wd)}
557 // Relative path: start at the current directory
564 dirFd, err := unix.Openat(unix.AT_FDCWD, dir, openReadonlyFlags, 0)
568 // Walk path one directory at a time to ensure there are no symlinks
569 // in the path. This prevents users with write access to change the
570 // path to point to arbitrary locations. O_NOFOLLOW when opening the
571 // path is not enough as only the last path component is checked.
572 for i, name := range parts[:len(parts)-1] {
573 fd, err := unix.Openat(dirFd, name, openReadonlyFlags, 0)
576 if err == unix.ELOOP || err == unix.EMLINK {
577 x := filepath.Join(append([]string{dir},
579 return -1, "", fmt.Errorf(
580 "symlink not permitted in path: %q",
589 return dirFd, parts[len(parts)-1], nil
592 func resolveIdsToNames(uid, gid int) (string, string, error) {
593 u, err := user.LookupId(strconv.Itoa(uid))
597 g, err := user.LookupGroupId(strconv.Itoa(gid))
601 return u.Username, g.Name, nil
604 func diffData(oldData []byte, newData []byte) (string, error) {
605 oldBin := !strings.HasPrefix(http.DetectContentType(oldData), "text/")
606 newBin := !strings.HasPrefix(http.DetectContentType(newData), "text/")
607 if oldBin && newBin {
608 return fmt.Sprintf("Binary files differ (%d -> %d bytes), "+
609 "cannot show diff", len(oldData), len(newData)), nil
612 oldData = []byte(fmt.Sprintf("<binary content, %d bytes>\n",
616 newData = []byte(fmt.Sprintf("<binary content, %d bytes>\n",
620 // TODO: difflib shows empty context lines at the end of the file
621 // which should not be there
622 // TODO: difflib has issues with missing newlines in either side
623 result, err := difflib.GetUnifiedDiffString(difflib.LineDiffParams{
624 A: difflib.SplitLines(string(oldData)),
625 B: difflib.SplitLines(string(newData)),
634 func OpenFileNoSymlinks(path string) (*os.File, error) {
635 parentFd, baseName, err := OpenParentDirectoryNoSymlinks(path)
639 defer unix.Close(parentFd)
640 return OpenAtNoFollow(parentFd, baseName)
643 func OpenAtNoFollow(dirFd int, base string) (*os.File, error) {
644 fd, err := unix.Openat(dirFd, base, openReadonlyFlags, 0)
648 return os.NewFile(uintptr(fd), ""), nil
651 func WriteTempAt(dirFd int, base string, data []byte, uid, gid int,
652 mode fs.FileMode) (string, error) {
654 fh, tmpBase, err := createTempAt(dirFd, base)
659 _, err = fh.Write(data)
662 unix.Unlinkat(dirFd, tmpBase, 0)
665 // createTempAt() creates the file with 0600
666 err = fh.Chown(uid, gid)
669 unix.Unlinkat(dirFd, tmpBase, 0)
675 unix.Unlinkat(dirFd, tmpBase, 0)
681 unix.Unlinkat(dirFd, tmpBase, 0)
686 unix.Unlinkat(dirFd, tmpBase, 0)
693 // createTempAt works similar to os.CreateTemp but uses unix.Openat to create
694 // the temporary file.
695 func createTempAt(dirFd int, base string) (*os.File, string, error) {
700 tmpBase = base + strconv.Itoa(rand.Int())
702 fd, err := unix.Openat(dirFd, tmpBase,
703 unix.O_RDWR|unix.O_CREAT|unix.O_EXCL, 0600)
705 if os.IsExist(err) && i < 10000 {
712 return os.NewFile(uintptr(fd), ""), tmpBase, nil