main: purge masterkey from memory as soon as possible

Remove the "Masterkey" field from fusefrontend.Args because it
should not be stored longer than neccessary. Instead pass the
masterkey as a separate argument to the filesystem initializers.

Then overwrite it with zeros immediately so we don't have
to wait for garbage collection.

Note that the crypto implementation still stores at least a
masterkey-derived value, so this change makes it harder, but not
impossible, to extract the encryption keys from memory.

Suggested at https://github.com/rfjakob/gocryptfs/issues/137

internal/cryptocore/cryptocore.go

@@ -72,7 +72,7 @@ func New(key []byte, aeadType AEADTypeEnum, IVBitLen int, useHKDF bool, forceDec
 		emeCipher = eme.New(emeBlockCipher)
 	}
 
-	// Initilize an AEAD cipher for file content encryption.
+	// Initialize an AEAD cipher for file content encryption.
 	var aeadCipher cipher.AEAD
 	if aeadType == BackendOpenSSL || aeadType == BackendGoGCM {
 		gcmKey := key
@@ -84,7 +84,13 @@ func New(key []byte, aeadType AEADTypeEnum, IVBitLen int, useHKDF bool, forceDec
 			if IVLen != 16 {
 				log.Panic("stupidgcm only supports 128-bit IVs")
 			}
-			aeadCipher = stupidgcm.New(gcmKey, forceDecode)
+			// stupidgcm does not create a private copy of the key, so things
+			// break when initFuseFrontend() overwrites it with zeros. Create
+			// a copy here. This is unneccessary when useHKDF == true, but
+			// does no harm.
+			var stupidgcmKey []byte
+			stupidgcmKey = append(stupidgcmKey, gcmKey...)
+			aeadCipher = stupidgcm.New(stupidgcmKey, forceDecode)
 		case BackendGoGCM:
 			goGcmBlockCipher, err := aes.NewCipher(gcmKey)
 			if err != nil {

internal/fusefrontend/args.go

@@ -7,7 +7,6 @@ import (
 
 // Args is a container for arguments that are passed from main() to fusefrontend
 type Args struct {
-	Masterkey []byte
 	// Cipherdir is the backing storage directory (absolute path).
 	// For reverse mode, Cipherdir actually contains *plaintext* files.
 	Cipherdir      string

internal/fusefrontend/fs.go

@@ -42,8 +42,8 @@ type FS struct {
 var _ pathfs.FileSystem = &FS{} // Verify that interface is implemented.
 
 // NewFS returns a new encrypted FUSE overlay filesystem.
-func NewFS(args Args) *FS {
-	cryptoCore := cryptocore.New(args.Masterkey, args.CryptoBackend, contentenc.DefaultIVBits, args.HKDF, args.ForceDecode)
+func NewFS(masterkey []byte, args Args) *FS {
+	cryptoCore := cryptocore.New(masterkey, args.CryptoBackend, contentenc.DefaultIVBits, args.HKDF, args.ForceDecode)
 	contentEnc := contentenc.New(cryptoCore, contentenc.DefaultBS, args.ForceDecode)
 	nameTransform := nametransform.New(cryptoCore.EMECipher, args.LongNames, args.Raw64)
 

internal/fusefrontend_reverse/rfs.go

@@ -40,12 +40,12 @@ var _ pathfs.FileSystem = &ReverseFS{}
 // NewFS returns an encrypted FUSE overlay filesystem.
 // In this case (reverse mode) the backing directory is plain-text and
 // ReverseFS provides an encrypted view.
-func NewFS(args fusefrontend.Args) *ReverseFS {
+func NewFS(masterkey []byte, args fusefrontend.Args) *ReverseFS {
 	if args.CryptoBackend != cryptocore.BackendAESSIV {
 		log.Panic("reverse mode must use AES-SIV, everything else is insecure")
 	}
 	initLongnameCache()
-	cryptoCore := cryptocore.New(args.Masterkey, args.CryptoBackend, contentenc.DefaultIVBits, args.HKDF, false)
+	cryptoCore := cryptocore.New(masterkey, args.CryptoBackend, contentenc.DefaultIVBits, args.HKDF, false)
 	contentEnc := contentenc.New(cryptoCore, contentenc.DefaultBS, false)
 	nameTransform := nametransform.New(cryptoCore.EMECipher, args.LongNames, args.Raw64)
 

mount.go

@@ -170,7 +170,7 @@ func setOpenFileLimit() {
 
 // initFuseFrontend - initialize gocryptfs/fusefrontend
 // Calls os.Exit on errors
-func initFuseFrontend(key []byte, args *argContainer, confFile *configfile.ConfFile) *fuse.Server {
+func initFuseFrontend(masterkey []byte, args *argContainer, confFile *configfile.ConfFile) *fuse.Server {
 	// Reconciliate CLI and config file arguments into a fusefrontend.Args struct
 	// that is passed to the filesystem implementation
 	cryptoBackend := cryptocore.BackendGoGCM
@@ -187,7 +187,6 @@ func initFuseFrontend(key []byte, args *argContainer, confFile *configfile.ConfF
 	}
 	frontendArgs := fusefrontend.Args{
 		Cipherdir:      args.cipherdir,
-		Masterkey:      key,
 		PlaintextNames: args.plaintextnames,
 		LongNames:      args.longnames,
 		CryptoBackend:  cryptoBackend,
@@ -222,14 +221,19 @@ func initFuseFrontend(key []byte, args *argContainer, confFile *configfile.ConfF
 	var finalFs pathfs.FileSystem
 	var ctlSockBackend ctlsock.Interface
 	if args.reverse {
-		fs := fusefrontend_reverse.NewFS(frontendArgs)
+		fs := fusefrontend_reverse.NewFS(masterkey, frontendArgs)
 		finalFs = fs
 		ctlSockBackend = fs
 	} else {
-		fs := fusefrontend.NewFS(frontendArgs)
+		fs := fusefrontend.NewFS(masterkey, frontendArgs)
 		finalFs = fs
 		ctlSockBackend = fs
 	}
+	// fusefrontend / fusefrontend_reverse have initialized their crypto with
+	// derived keys (HKDF), we can purge the master key from memory.
+	for i := range masterkey {
+		masterkey[i] = 0
+	}
 	// We have opened the socket early so that we cannot fail here after
 	// asking the user for the password
 	if args._ctlsockFd != nil {