nametransform: harden name decryption against invalid input

This fixes a few issues I have found reviewing the code:

1) Limit the amount of data ReadLongName() will read. Previously,
you could send gocryptfs into out-of-memory by symlinking
gocryptfs.diriv to /dev/zero.

2) Handle the empty input case in unPad16() by returning an
error. Previously, it would panic with an out-of-bounds array
read. It is unclear to me if this could actually be triggered.

3) Reject empty names after base64-decoding in DecryptName().
An empty name crashes emeCipher.Decrypt().
It is unclear to me if B64.DecodeString() can actually return
a non-error empty result, but let's guard against it anyway.

1 files changed, 6 insertions, 2 deletions

diff --git a/internal/nametransform/names.go b/internal/nametransform/names.go
index c96f7ce..3447583 100644
--- a/internal/nametransform/names.go
+++ b/internal/nametransform/names.go
@@ -42,6 +42,10 @@ func (n *NameTransform) DecryptName(cipherName string, iv []byte) (string, error
 	if err != nil {
 		return "", err
 	}
+	if len(bin) == 0 {
+		tlog.Warn.Printf("DecryptName: empty input")
+		return "", syscall.EBADMSG
+	}
 	if len(bin)%aes.BlockSize != 0 {
 		tlog.Debug.Printf("DecryptName %q: decoded length %d is not a multiple of 16", cipherName, len(bin))
 		return "", syscall.EBADMSG
@@ -49,14 +53,14 @@ func (n *NameTransform) DecryptName(cipherName string, iv []byte) (string, error
 	bin = n.emeCipher.Decrypt(iv, bin)
 	bin, err = unPad16(bin)
 	if err != nil {
-		tlog.Debug.Printf("DecryptName: unPad16 error: %v", err)
+		tlog.Debug.Printf("DecryptName: unPad16 error detail: %v", err)
 		// unPad16 returns detailed errors including the position of the
 		// incorrect bytes. Kill the padding oracle by lumping everything into
 		// a generic error.
 		return "", syscall.EBADMSG
 	}
 	// A name can never contain a null byte or "/". Make sure we never return those
-	// to the user, even when we read a corrupted (or fuzzed) filesystem.
+	// to the kernel, even when we read a corrupted (or fuzzed) filesystem.
 	if bytes.Contains(bin, []byte{0}) || bytes.Contains(bin, []byte("/")) {
 		return "", syscall.EBADMSG
 	}