fusefrontend_reverse: Do not mix up cache information for different directories

Fixes https://github.com/rfjakob/gocryptfs/issues/168

Steps to reproduce the problem:

* Create a regular reverse mount point
* Create files with the same very long name in multiple directories - so far
  everything works as expected, and it will appear with a different name each
  time, for example, gocryptfs.longname.A in directory A and
  gocryptfs.longname.B in directory B
* Try to access a path with A/gocryptfs.longname.B or B/gocryptfs.longname.A -
  this should fail, but it actually works.

The problem is that the longname cache only uses the path as key and not the
dir or divIV. Assume an attacker can directly interact with a reverse mount and
knows the relation longname path -> unencoded path in one directory, it allows
to test if the same unencoded filename appears in any other directory.

internal/fusefrontend_reverse/reverse_longnames.go

@@ -51,7 +51,7 @@ func initLongnameCache() {
 // findLongnameParent converts "gocryptfs.longname.XYZ" to the plaintext name
 func (rfs *ReverseFS) findLongnameParent(dir string, dirIV []byte, longname string) (plaintextName string, err error) {
 	longnameCacheLock.Lock()
-	hit := longnameParentCache[longname]
+	hit := longnameParentCache[dir + "/" + longname]
 	longnameCacheLock.Unlock()
 	if hit != "" {
 		return hit, nil
@@ -79,7 +79,7 @@ func (rfs *ReverseFS) findLongnameParent(dir string, dirIV []byte, longname stri
 			log.Panic("logic error or wrong shortNameMax constant?")
 		}
 		hName := rfs.nameTransform.HashLongName(cName)
-		longnameParentCache[hName] = plaintextName
+		longnameParentCache[dir + "/" + hName] = plaintextName
 		if longname == hName {
 			hit = plaintextName
 		}

tests/reverse/ctlsock_test.go

@@ -35,10 +35,11 @@ func TestCtlSockPathOps(t *testing.T) {
 	test_helpers.MountOrFatal(t, "ctlsock_reverse_test_fs", mnt, "-reverse", "-extpass", "echo test", "-ctlsock="+sock)
 	defer test_helpers.UnmountPanic(mnt)
 	var req ctlsock.RequestStruct
+	var response ctlsock.ResponseStruct
 	for i, tc := range ctlSockTestCases {
 		// Decrypt
 		req = ctlsock.RequestStruct{DecryptPath: tc[0]}
-		response := test_helpers.QueryCtlSock(t, sock, req)
+		response = test_helpers.QueryCtlSock(t, sock, req)
 		if response.ErrNo != 0 {
 			t.Errorf("Testcase %d Decrypt: %q ErrNo=%d ErrText=%s", i, tc[0], response.ErrNo, response.ErrText)
 		} else if response.Result != tc[1] {
@@ -53,6 +54,18 @@ func TestCtlSockPathOps(t *testing.T) {
 			t.Errorf("Testcase %d Encrypt: Want %q got %q", i, tc[1], response.Result)
 		}
 	}
+	// At this point the longname parent cache should be populated.
+	// Check that we do not mix up information for different directories.
+	req = ctlsock.RequestStruct{DecryptPath: "gocryptfs.longname.y6rxCn6Id8hIZL2t_STpdLZpu-aE2HpprJR25xD60mk="}
+	response = test_helpers.QueryCtlSock(t, sock, req)
+	if response.ErrNo != -1 {
+		t.Errorf("File should not exist: ErrNo=%d ErrText=%s", response.ErrNo, response.ErrText)
+	}
+	req = ctlsock.RequestStruct{DecryptPath: "v6puXntoQOk7Mhl8zJ4Idg==/gocryptfs.longname.ZQCAoi5li3xvDZRO8McBV0L_kzJc4IcAOEzuW-2S1Y4="}
+	response = test_helpers.QueryCtlSock(t, sock, req)
+	if response.ErrNo != -1 {
+		t.Errorf("File should not exist: ErrNo=%d ErrText=%s", response.ErrNo, response.ErrText)
+	}
 }
 
 // We should not panic when somebody feeds requests that make no sense