fusefrontend: make DecryptPath() symlink-safe

DecryptPath is now symlink-safe through the use of *at()
functions.

internal/fusefrontend/ctlsock_interface.go

@@ -4,9 +4,11 @@ import (
 	"fmt"
 	"path"
 	"strings"
+	"syscall"
 
 	"github.com/rfjakob/gocryptfs/internal/ctlsock"
 	"github.com/rfjakob/gocryptfs/internal/nametransform"
+	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
 )
 
 var _ ctlsock.Interface = &FS{} // Verify that interface is implemented.
@@ -17,22 +19,31 @@ func (fs *FS) EncryptPath(plainPath string) (string, error) {
 }
 
 // DecryptPath implements ctlsock.Backend
-func (fs *FS) DecryptPath(cipherPath string) (string, error) {
+func (fs *FS) DecryptPath(cipherPath string) (plainPath string, err error) {
+	dirfd, err := syscall.Open(fs.args.Cipherdir, syscall.O_RDONLY, 0)
+	if err != nil {
+		return "", err
+	}
+	defer syscall.Close(dirfd)
+	return fs.decryptPathAt(dirfd, cipherPath)
+}
+
+// decryptPathAt decrypts a ciphertext path relative to dirfd.
+func (fs *FS) decryptPathAt(dirfd int, cipherPath string) (plainPath string, err error) {
 	if fs.args.PlaintextNames || cipherPath == "" {
 		return cipherPath, nil
 	}
-	plainPath := ""
 	parts := strings.Split(cipherPath, "/")
-	wd := fs.args.Cipherdir
+	wd := dirfd
-	for _, part := range parts {
+	for i, part := range parts {
-		dirIV, err := nametransform.ReadDirIV(wd)
+		dirIV, err := nametransform.ReadDirIVAt(wd)
 		if err != nil {
 			fmt.Printf("ReadDirIV: %v\n", err)
 			return "", err
 		}
 		longPart := part
 		if nametransform.IsLongContent(part) {
-			longPart, err = nametransform.ReadLongName(wd + "/" + part)
+			longPart, err = nametransform.ReadLongNameAt(wd, part)
 			if err != nil {
 				fmt.Printf("ReadLongName: %v\n", err)
 				return "", err
@@ -44,7 +55,21 @@ func (fs *FS) DecryptPath(cipherPath string) (string, error) {
 			return "", err
 		}
 		plainPath = path.Join(plainPath, name)
-		wd = path.Join(wd, part)
+		// Last path component? We are done.
+		if i == len(parts)-1 {
+			break
+		}
+		// Descend into next directory
+		oldWd := wd
+		wd, err = syscallcompat.Openat(wd, part, syscall.O_NOFOLLOW, 0)
+		if err != nil {
+			return "", err
+		}
+		// Unless we are in the first iteration, where dirfd is our wd, close
+		// the old working directory.
+		if i > 0 {
+			syscall.Close(oldWd)
+		}
 	}
 	return plainPath, nil
 }

internal/fusefrontend/fs_dir.go

@@ -324,7 +324,7 @@ func (fs *FS) OpenDir(dirName string, context *fuse.Context) ([]fuse.DirEntry, f
 			isLong = nametransform.NameType(cName)
 		}
 		if isLong == nametransform.LongNameContent {
-			cNameLong, err := nametransform.ReadLongName(filepath.Join(cDirAbsPath, cName))
+			cNameLong, err := nametransform.ReadLongNameAt(fd, cName)
 			if err != nil {
 				tlog.Warn.Printf("OpenDir %q: invalid entry %q: Could not read .name: %v",
 					cDirName, cName, err)

internal/nametransform/longnames.go

@@ -64,18 +64,18 @@ func IsLongContent(cName string) bool {
 }
 
 // ReadLongName - read "$path.name"
-func ReadLongName(path string) (string, error) {
+func ReadLongNameAt(dirfd int, cName string) (string, error) {
-	path += LongNameSuffix
+	cName += LongNameSuffix
-	fd, err := os.Open(path)
+	fd, err := syscallcompat.Openat(dirfd, cName, syscall.O_NOFOLLOW, 0)
 	if err != nil {
 		return "", err
 	}
-	defer fd.Close()
+	defer syscall.Close(fd)
 	// 256 (=255 padded to 16) bytes base64-encoded take 344 bytes: "AAAAAAA...AAA=="
 	lim := 344
 	// Allocate a bigger buffer so we see whether the file is too big
 	buf := make([]byte, lim+1)
-	n, err := fd.ReadAt(buf, 0)
+	n, err := syscall.Pread(fd, buf, 0)
 	if err != nil && err != io.EOF {
 		return "", err
 	}
@@ -88,7 +88,9 @@ func ReadLongName(path string) (string, error) {
 	return string(buf[0:n]), nil
 }
 
-// DeleteLongName deletes "hashName.name".
+// DeleteLongName deletes "hashName.name" in the directory openend at "dirfd".
+//
+// This function is symlink-safe through the use of Unlinkat().
 func DeleteLongName(dirfd int, hashName string) error {
 	err := syscallcompat.Unlinkat(dirfd, hashName+LongNameSuffix, 0)
 	if err != nil {
@@ -99,7 +101,9 @@ func DeleteLongName(dirfd int, hashName string) error {
 
 // WriteLongName encrypts plainName and writes it into "hashName.name".
 // For the convenience of the caller, plainName may also be a path and will be
-// converted internally.
+// Base()named internally.
+//
+// This function is symlink-safe through the use of Openat().
 func (n *NameTransform) WriteLongName(dirfd int, hashName string, plainName string) (err error) {
 	plainName = filepath.Base(plainName)