local: fix btime escaping the root via a planted symlink GHSA-f8g7-2xjc-7mfh CVE-PENDING

The birth-time (btime) write in writeMetadataToFile followed symlinks for
any object that was not a translated link, so under -l/--links a symlink
planted by an untrusted source at the destination path could redirect the
btime write to a target outside the backup destination on OSes where
birth time is settable (Windows).

Use the NOFOLLOW birth-time write whenever translating symlinks, not only
for translated links. It is a no-op on a real file or directory and stops
a planted symlink from being followed out of the destination.

backend/local/local_internal_test.go

@@ -447,6 +447,42 @@ func TestDirMetadataInTreeWorks(t *testing.T) {
 	require.True(t, fi.ModTime().Equal(fstest.Time("2001-02-03T04:05:06Z")))
 }
 
+// TestDirBTimeThroughPlantedSymlinkBlocked checks a btime write
+// through a symlink can't escape the root.
+func TestDirBTimeThroughPlantedSymlinkBlocked(t *testing.T) {
+	if !haveSetBTime {
+		t.Skip("birth time is not settable on this OS")
+	}
+	ctx := context.Background()
+
+	evil := t.TempDir()
+	evilDir := filepath.Join(evil, "secret.d")
+	require.NoError(t, os.Mkdir(evilDir, 0700))
+
+	// Read the outside dir's btime through a local Fs rooted at evil.
+	evilFsRaw, err := NewFs(ctx, "local", evil, configmap.Simple{})
+	require.NoError(t, err)
+	evilFs := evilFsRaw.(*Fs)
+	readBTime := func() string {
+		o, err := evilFs.newObject("secret.d")
+		require.NoError(t, err)
+		require.NoError(t, o.lstat())
+		m, err := o.Metadata(ctx)
+		require.NoError(t, err)
+		return m["btime"]
+	}
+	before := readBTime()
+
+	r := fstest.NewRun(t)
+	f := r.Flocal.(*Fs)
+	linksMode(f)
+	require.NoError(t, putLink(ctx, f, "pwn", evilDir))
+
+	_, _ = f.MkdirMetadata(ctx, "pwn", fs.Metadata{"btime": "2001-02-03T04:05:06Z"})
+
+	require.Equal(t, before, readBTime(), "btime escaped through planted symlink to %q", evilDir)
+}
+
 // TestEncodingEscapeBlocked checks that a name from a malicious source can't
 // be decoded into path syntax which writes outside the destination.
 func TestEncodingEscapeBlocked(t *testing.T) {

backend/local/metadata.go

@@ -105,7 +105,10 @@ func (o *Object) writeMetadataToFile(m fs.Metadata) (outErr error) {
 	}
 	if haveSetBTime {
 		if btimeOK {
-			if o.translatedLink {
+			// When translating symlinks, never follow the path. A planted symlink must
+			// not redirect the birth-time write out of the root. The NOFOLLOW open is a
+			// no-op on a real file or directory
+			if o.translatedLink || o.fs.opt.TranslateSymlinks {
 				err = lsetBTime(o.path, btime)
 			} else {
 				err = setBTime(o.path, btime)