fusefrontend: allow_other: close race between mkdir and chown

Fixes the same problem as described in 72b975867a3b9bdf53fc2da62e2ba4a328d7e4ab,
except for directories instead of device nodes.

6 files changed, 48 insertions, 29 deletions

diff --git a/init_dir.go b/init_dir.go
index f057bf4..791f7d1 100644
--- a/init_dir.go
+++ b/init_dir.go
@@ -47,7 +47,7 @@ func initDir(args *argContainer) {
 	// Forward mode with filename encryption enabled needs a gocryptfs.diriv
 	// in the root dir
 	if !args.plaintextnames && !args.reverse {
-		err = nametransform.WriteDirIV(args.cipherdir)
+		err = nametransform.WriteDirIV(nil, args.cipherdir)
 		if err != nil {
 			tlog.Fatal.Println(err)
 			os.Exit(exitcodes.Init)
diff --git a/internal/fusefrontend/fs_dir.go b/internal/fusefrontend/fs_dir.go
index ae150b5..4ffaaff 100644
--- a/internal/fusefrontend/fs_dir.go
+++ b/internal/fusefrontend/fs_dir.go
@@ -24,7 +24,7 @@ import (
 
 const dsStoreName = ".DS_Store"
 
-func (fs *FS) mkdirWithIv(cPath string, mode uint32) error {
+func (fs *FS) mkdirWithIv(dirfd *os.File, cName string, mode uint32) error {
 	// Between the creation of the directory and the creation of gocryptfs.diriv
 	// the directory is inconsistent. Take the lock to prevent other readers
 	// from seeing it.
@@ -32,14 +32,14 @@ func (fs *FS) mkdirWithIv(cPath string, mode uint32) error {
 	// The new directory may take the place of an older one that is still in the cache
 	fs.nameTransform.DirIVCache.Clear()
 	defer fs.dirIVLock.Unlock()
-	err := os.Mkdir(cPath, os.FileMode(mode))
+	err := syscallcompat.Mkdirat(int(dirfd.Fd()), cName, mode)
 	if err != nil {
 		return err
 	}
 	// Create gocryptfs.diriv
-	err = nametransform.WriteDirIV(cPath)
+	err = nametransform.WriteDirIV(dirfd, cName)
 	if err != nil {
-		err2 := syscall.Rmdir(cPath)
+		err2 := syscallcompat.Unlinkat(int(dirfd.Fd()), cName, unix.AT_REMOVEDIR)
 		if err2 != nil {
 			tlog.Warn.Printf("mkdirWithIv: rollback failed: %v", err2)
 		}
@@ -52,17 +52,19 @@ func (fs *FS) Mkdir(newPath string, mode uint32, context *fuse.Context) (code fu
 	if fs.isFiltered(newPath) {
 		return fuse.EPERM
 	}
-	cPath, err := fs.getBackingPath(newPath)
+	dirfd, cName, err := fs.openBackingPath(newPath)
 	if err != nil {
 		return fuse.ToStatus(err)
 	}
+	defer dirfd.Close()
 	if fs.args.PlaintextNames {
-		err = os.Mkdir(cPath, os.FileMode(mode))
+		err = syscallcompat.Mkdirat(int(dirfd.Fd()), cName, mode)
 		// Set owner
 		if fs.args.PreserveOwner {
-			err = os.Lchown(cPath, int(context.Owner.Uid), int(context.Owner.Gid))
+			err = syscallcompat.Fchownat(int(dirfd.Fd()), cName, int(context.Owner.Uid),
+				int(context.Owner.Gid), unix.AT_SYMLINK_NOFOLLOW)
 			if err != nil {
-				tlog.Warn.Printf("Mkdir: Lchown failed: %v", err)
+				tlog.Warn.Printf("Mkdir: Fchownat failed: %v", err)
 			}
 		}
 		return fuse.ToStatus(err)
@@ -73,15 +75,7 @@ func (fs *FS) Mkdir(newPath string, mode uint32, context *fuse.Context) (code fu
 	mode = mode | 0300
 
 	// Handle long file name
-	cName := filepath.Base(cPath)
 	if nametransform.IsLongContent(cName) {
-		var dirfd *os.File
-		dirfd, err = os.Open(filepath.Dir(cPath))
-		if err != nil {
-			return fuse.ToStatus(err)
-		}
-		defer dirfd.Close()
-
 		// Create ".name"
 		err = fs.nameTransform.WriteLongName(dirfd, cName, newPath)
 		if err != nil {
@@ -89,33 +83,35 @@ func (fs *FS) Mkdir(newPath string, mode uint32, context *fuse.Context) (code fu
 		}
 
 		// Create directory
-		err = fs.mkdirWithIv(cPath, mode)
+		err = fs.mkdirWithIv(dirfd, cName, mode)
 		if err != nil {
 			nametransform.DeleteLongName(dirfd, cName)
 			return fuse.ToStatus(err)
 		}
 	} else {
-		err = fs.mkdirWithIv(cPath, mode)
+		err = fs.mkdirWithIv(dirfd, cName, mode)
 		if err != nil {
 			return fuse.ToStatus(err)
 		}
 	}
 	// Set permissions back to what the user wanted
 	if origMode != mode {
-		err = os.Chmod(cPath, os.FileMode(origMode))
+		err = syscallcompat.Fchmodat(int(dirfd.Fd()), cName, origMode, unix.AT_SYMLINK_NOFOLLOW)
 		if err != nil {
-			tlog.Warn.Printf("Mkdir: Chmod failed: %v", err)
+			tlog.Warn.Printf("Mkdir: Fchmodat failed: %v", err)
 		}
 	}
 	// Set owner
 	if fs.args.PreserveOwner {
-		err = os.Lchown(cPath, int(context.Owner.Uid), int(context.Owner.Gid))
+		err = syscallcompat.Fchownat(int(dirfd.Fd()), cName, int(context.Owner.Uid),
+			int(context.Owner.Gid), unix.AT_SYMLINK_NOFOLLOW)
 		if err != nil {
-			tlog.Warn.Printf("Mkdir: Lchown 1 failed: %v", err)
+			tlog.Warn.Printf("Mkdir: Fchownat 1 failed: %v", err)
 		}
-		err = os.Lchown(filepath.Join(cPath, nametransform.DirIVFilename), int(context.Owner.Uid), int(context.Owner.Gid))
+		err = syscallcompat.Fchownat(int(dirfd.Fd()), filepath.Join(cName, nametransform.DirIVFilename),
+			int(context.Owner.Uid), int(context.Owner.Gid), unix.AT_SYMLINK_NOFOLLOW)
 		if err != nil {
-			tlog.Warn.Printf("Mkdir: Lchown 2 failed: %v", err)
+			tlog.Warn.Printf("Mkdir: Fchownat 2 failed: %v", err)
 		}
 	}
 	return fuse.OK
diff --git a/internal/nametransform/diriv.go b/internal/nametransform/diriv.go
index ffaf785..fe289c6 100644
--- a/internal/nametransform/diriv.go
+++ b/internal/nametransform/diriv.go
@@ -75,16 +75,17 @@ func fdReadDirIV(fd *os.File) (iv []byte, err error) {
 // WriteDirIV - create diriv file inside "dir" (absolute ciphertext path)
 // This function is exported because it is used from pathfs_frontend, main,
 // and also the automated tests.
-func WriteDirIV(dir string) error {
+func WriteDirIV(dirfd *os.File, dir string) error {
 	iv := cryptocore.RandBytes(DirIVLen)
 	file := filepath.Join(dir, DirIVFilename)
 	// 0400 permissions: gocryptfs.diriv should never be modified after creation.
 	// Don't use "ioutil.WriteFile", it causes trouble on NFS: https://github.com/rfjakob/gocryptfs/issues/105
-	fd, err := os.OpenFile(file, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0400)
+	fdRaw, err := syscallcompat.Openat(int(dirfd.Fd()), file, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0400)
 	if err != nil {
-		tlog.Warn.Printf("WriteDirIV: OpenFile: %v", err)
+		tlog.Warn.Printf("WriteDirIV: Openat: %v", err)
 		return err
 	}
+	fd := os.NewFile(uintptr(fdRaw), file)
 	_, err = fd.Write(iv)
 	if err != nil {
 		tlog.Warn.Printf("WriteDirIV: Write: %v", err)
diff --git a/internal/syscallcompat/sys_darwin.go b/internal/syscallcompat/sys_darwin.go
index 367f67d..39a225f 100644
--- a/internal/syscallcompat/sys_darwin.go
+++ b/internal/syscallcompat/sys_darwin.go
@@ -187,3 +187,20 @@ func Symlinkat(oldpath string, newdirfd int, newpath string) (err error) {
 	defer syscall.Fchdir(cwd)
 	return syscall.Symlink(oldpath, newpath)
 }
+
+// Poor man's Mkdirat.
+func Mkdirat(dirfd int, path string, mode uint32) (err error) {
+	chdirMutex.Lock()
+	defer chdirMutex.Unlock()
+	cwd, err := syscall.Open(".", syscall.O_RDONLY, 0)
+	if err != nil {
+		return err
+	}
+	defer syscall.Close(cwd)
+	err = syscall.Fchdir(dirfd)
+	if err != nil {
+		return err
+	}
+	defer syscall.Fchdir(cwd)
+	return syscall.Mkdir(path, mode)
+}
diff --git a/internal/syscallcompat/sys_linux.go b/internal/syscallcompat/sys_linux.go
index 4fe44fd..1ea56b5 100644
--- a/internal/syscallcompat/sys_linux.go
+++ b/internal/syscallcompat/sys_linux.go
@@ -109,3 +109,8 @@ func Symlinkat(oldpath string, newdirfd int, newpath string) (err error) {
 	}
 	return
 }
+
+// Mkdirat syscall.
+func Mkdirat(dirfd int, path string, mode uint32) (err error) {
+	return syscall.Mkdirat(dirfd, path, mode)
+}
diff --git a/tests/test_helpers/helpers.go b/tests/test_helpers/helpers.go
index f681482..9b81b45 100644
--- a/tests/test_helpers/helpers.go
+++ b/tests/test_helpers/helpers.go
@@ -96,7 +96,7 @@ func ResetTmpDir(createDirIV bool) {
 		panic(err)
 	}
 	if createDirIV {
-		err = nametransform.WriteDirIV(DefaultCipherDir)
+		err = nametransform.WriteDirIV(nil, DefaultCipherDir)
 		if err != nil {
 			panic(err)
 		}