fusefrontend: add xattr support

At the moment, only for reverse mode.

https://github.com/rfjakob/gocryptfs/issues/217

4 files changed, 201 insertions, 22 deletions

diff --git a/internal/contentenc/content.go b/internal/contentenc/content.go
index e2a531c..c4ba7c9 100644
--- a/internal/contentenc/content.go
+++ b/internal/contentenc/content.go
@@ -131,10 +131,11 @@ func (be *ContentEnc) DecryptBlocks(ciphertext []byte, firstBlockNo uint64, file
 
 // concatAD concatenates the block number and the file ID to a byte blob
 // that can be passed to AES-GCM as associated data (AD).
-// Result is: aData = blockNo.bigEndian + fileID.
+// Result is: aData = [blockNo.bigEndian fileID].
 func concatAD(blockNo uint64, fileID []byte) (aData []byte) {
 	if fileID != nil && len(fileID) != headerIDLen {
-		// fileID is nil when decrypting the master key from the config file
+		// fileID is nil when decrypting the master key from the config file,
+		// and for symlinks and xattrs.
 		log.Panicf("wrong fileID length: %d", len(fileID))
 	}
 	const lenUint64 = 8
diff --git a/internal/fusefrontend/fs.go b/internal/fusefrontend/fs.go
index 00361e8..738f113 100644
--- a/internal/fusefrontend/fs.go
+++ b/internal/fusefrontend/fs.go
@@ -592,23 +592,3 @@ func (fs *FS) Access(path string, mode uint32, context *fuse.Context) (code fuse
 	}
 	return fuse.ToStatus(syscall.Access(cPath, mode))
 }
-
-// GetXAttr implements pathfs.Filesystem.
-func (fs *FS) GetXAttr(name string, attr string, context *fuse.Context) ([]byte, fuse.Status) {
-	return nil, fuse.ENOSYS
-}
-
-// SetXAttr implements pathfs.Filesystem.
-func (fs *FS) SetXAttr(name string, attr string, data []byte, flags int, context *fuse.Context) fuse.Status {
-	return fuse.ENOSYS
-}
-
-// ListXAttr implements pathfs.Filesystem.
-func (fs *FS) ListXAttr(name string, context *fuse.Context) ([]string, fuse.Status) {
-	return nil, fuse.ENOSYS
-}
-
-// RemoveXAttr implements pathfs.Filesystem.
-func (fs *FS) RemoveXAttr(name string, attr string, context *fuse.Context) fuse.Status {
-	return fuse.ENOSYS
-}
diff --git a/internal/fusefrontend/xattr.go b/internal/fusefrontend/xattr.go
new file mode 100644
index 0000000..357e889
--- /dev/null
+++ b/internal/fusefrontend/xattr.go
@@ -0,0 +1,157 @@
+// Package fusefrontend interfaces directly with the go-fuse library.
+package fusefrontend
+
+// FUSE operations on paths
+
+import (
+	"strings"
+	"syscall"
+
+	"github.com/hanwen/go-fuse/fuse"
+	xattr "github.com/rfjakob/pkg-xattr"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+// xattr names are encrypted like file names, but with a fixed IV.
+var xattrNameIV = []byte("xattr_name_iv_xx")
+
+// Only allow the "user" namespace, block "trusted" and "security", as
+// these may be interpreted by the system, and we don't want to cause
+// trouble with our encrypted garbage.
+var xattrUserPrefix = "user."
+
+// We store encrypted xattrs under this prefix plus the base64-encoded
+// encrypted original name.
+var xattrStorePrefix = "user.gocryptfs."
+
+// GetXAttr: read the value of extended attribute "attr".
+// Implements pathfs.Filesystem.
+func (fs *FS) GetXAttr(path string, attr string, context *fuse.Context) ([]byte, fuse.Status) {
+	if fs.isFiltered(path) {
+		return nil, fuse.EPERM
+	}
+	cAttr, err := fs.encryptXattrName(attr)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	cPath, err := fs.getBackingPath(path)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	cData64, err := xattr.Get(cPath, cAttr)
+	if err != nil {
+		return nil, unpackXattrErr(err)
+	}
+	// xattr data is decrypted like a symlink target
+	data, err := fs.decryptSymlinkTarget(string(cData64))
+	if err != nil {
+		tlog.Warn.Printf("GetXAttr: %v", err)
+		return nil, fuse.EIO
+	}
+	return []byte(data), fuse.OK
+}
+
+// SetXAttr implements pathfs.Filesystem.
+func (fs *FS) SetXAttr(path string, attr string, data []byte, flags int, context *fuse.Context) fuse.Status {
+	if fs.isFiltered(path) {
+		return fuse.EPERM
+	}
+	if flags != 0 {
+		return fuse.EPERM
+	}
+	cPath, err := fs.getBackingPath(path)
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	cAttr, err := fs.encryptXattrName(attr)
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	// xattr data is encrypted like a symlink target
+	cData64 := []byte(fs.encryptSymlinkTarget(string(data)))
+	return unpackXattrErr(xattr.Set(cPath, cAttr, cData64))
+}
+
+// RemoveXAttr implements pathfs.Filesystem.
+func (fs *FS) RemoveXAttr(path string, attr string, context *fuse.Context) fuse.Status {
+	if fs.isFiltered(path) {
+		return fuse.EPERM
+	}
+	cPath, err := fs.getBackingPath(path)
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	cAttr, err := fs.encryptXattrName(attr)
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	return unpackXattrErr(xattr.Remove(cPath, cAttr))
+}
+
+// ListXAttr implements pathfs.Filesystem.
+func (fs *FS) ListXAttr(path string, context *fuse.Context) ([]string, fuse.Status) {
+	if fs.isFiltered(path) {
+		return nil, fuse.EPERM
+	}
+	cPath, err := fs.getBackingPath(path)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	cNames, err := xattr.List(cPath)
+	if err != nil {
+		return nil, unpackXattrErr(err)
+	}
+	names := make([]string, 0, len(cNames))
+	for _, curName := range cNames {
+		if !strings.HasPrefix(curName, xattrStorePrefix) {
+			continue
+		}
+		name, err := fs.decryptXattrName(curName)
+		if err != nil {
+			tlog.Warn.Printf("ListXAttr: invalid xattr name %q: %v", curName, err)
+			continue
+		}
+		names = append(names, name)
+	}
+	return names, fuse.OK
+}
+
+// encryptXattrName transforms "user.foo" to "user.gocryptfs.a5sAd4XAa47f5as6dAf"
+func (fs *FS) encryptXattrName(attr string) (cAttr string, err error) {
+	// Reject anything that does not start with "user."
+	if !strings.HasPrefix(attr, xattrUserPrefix) {
+		return "", syscall.EPERM
+	}
+	// xattr names are encrypted like file names, but with a fixed IV.
+	cAttr = xattrStorePrefix + fs.nameTransform.EncryptName(attr, xattrNameIV)
+	return cAttr, nil
+}
+
+func (fs *FS) decryptXattrName(cAttr string) (attr string, err error) {
+	// Reject anything that does not start with "user.gocryptfs."
+	if !strings.HasPrefix(cAttr, xattrStorePrefix) {
+		return "", syscall.EINVAL
+	}
+	// Strip "user.gocryptfs." prefix
+	cAttr = cAttr[len(xattrStorePrefix):]
+	attr, err = fs.nameTransform.DecryptName(cAttr, xattrNameIV)
+	if err != nil {
+		return "", err
+	}
+	return attr, nil
+}
+
+// unpackXattrErr unpacks an error value that we got from xattr.Get/Set/etc
+// and converts it to a fuse status.
+func unpackXattrErr(err error) fuse.Status {
+	if err == nil {
+		return fuse.OK
+	}
+	err2, ok := err.(*xattr.Error)
+	if !ok {
+		tlog.Warn.Printf("unpackXattrErr: cannot unpack err=%v", err)
+		return fuse.EIO
+	}
+	return fuse.ToStatus(err2.Err)
+}
diff --git a/internal/fusefrontend/xattr_unit_test.go b/internal/fusefrontend/xattr_unit_test.go
new file mode 100644
index 0000000..ea5d3bb
--- /dev/null
+++ b/internal/fusefrontend/xattr_unit_test.go
@@ -0,0 +1,41 @@
+package fusefrontend
+
+// This file is named "xattr_unit_test.go" because there is also a
+// "xattr_integration_test.go" in the test/xattr package.
+
+import (
+	"syscall"
+	"testing"
+
+	"github.com/rfjakob/gocryptfs/internal/contentenc"
+	"github.com/rfjakob/gocryptfs/internal/cryptocore"
+	"github.com/rfjakob/gocryptfs/internal/nametransform"
+)
+
+func newTestFS() *FS {
+	// Init crypto backend
+	key := make([]byte, cryptocore.KeyLen)
+	cCore := cryptocore.New(key, cryptocore.BackendGoGCM, contentenc.DefaultIVBits, true, false)
+	cEnc := contentenc.New(cCore, contentenc.DefaultBS, false)
+	nameTransform := nametransform.New(cCore.EMECipher, true, true)
+	args := Args{}
+	return NewFS(args, cEnc, nameTransform)
+}
+
+func TestEncryptDecryptXattrName(t *testing.T) {
+	fs := newTestFS()
+	_, err := fs.encryptXattrName("xxxx")
+	if err != syscall.EPERM {
+		t.Fatalf("Names that don't start with 'user.' should fail")
+	}
+	attr1 := "user.foo123456789"
+	cAttr, err := fs.encryptXattrName(attr1)
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("cAttr=%v", cAttr)
+	attr2, err := fs.decryptXattrName(cAttr)
+	if attr1 != attr2 {
+		t.Fatalf("Decrypt mismatch: %v != %v", attr1, attr2)
+	}
+}