From 5b5c7a0a5d73859f74d2151061593ba2f9f9cac7 Mon Sep 17 00:00:00 2001
From: Jakob Unterwurzacher
Date: Sun, 18 Feb 2018 12:42:22 +0100
Subject: main: overwrite keys and let them run out of scope

As soon as we don't need them anymore, overwrite
keys with zeros. Make sure they run out of scope
so we don't create a risk of inadvertedly using
all-zero keys for encryption.

https://github.com/rfjakob/gocryptfs/issues/211
---
 init_dir.go | 19 ++++++++++-------
 mount.go    | 69 +++++++++++++++++++++++++++++++++----------------------------
 2 files changed, 49 insertions(+), 39 deletions(-)

diff --git a/init_dir.go b/init_dir.go
index 791f7d1..b13f741 100644
--- a/init_dir.go
+++ b/init_dir.go
@@ -36,13 +36,18 @@ func initDir(args *argContainer) {
 	if args.extpass == "" {
 		tlog.Info.Printf("Choose a password for protecting your files.")
 	}
-	password := readpassword.Twice(args.extpass)
-	readpassword.CheckTrailingGarbage()
-	creator := tlog.ProgramName + " " + GitVersion
-	err = configfile.CreateConfFile(args.config, password, args.plaintextnames, args.scryptn, creator, args.aessiv, args.devrandom)
-	if err != nil {
-		tlog.Fatal.Println(err)
-		os.Exit(exitcodes.WriteConf)
+	{
+		creator := tlog.ProgramName + " " + GitVersion
+		password := readpassword.Twice(args.extpass)
+		readpassword.CheckTrailingGarbage()
+		err = configfile.CreateConfFile(args.config, password, args.plaintextnames, args.scryptn, creator, args.aessiv, args.devrandom)
+		if err != nil {
+			tlog.Fatal.Println(err)
+			os.Exit(exitcodes.WriteConf)
+		}
+		// Note: cannot overwrite password because in Go, strings are
+		// read-only byte slices.
+		// password runs out of scope here
 	}
 	// Forward mode with filename encryption enabled needs a gocryptfs.diriv
 	// in the root dir
diff --git a/mount.go b/mount.go
index 9b7410d..c19d684 100644
--- a/mount.go
+++ b/mount.go
@@ -93,37 +93,47 @@ func doMount(args *argContainer) int {
 			}
 		}()
 	}
-	// Get master key (may prompt for the password)
-	var masterkey []byte
 	var confFile *configfile.ConfFile
-	if args.masterkey != "" {
-		// "-masterkey"
-		masterkey = parseMasterKey(args.masterkey)
-	} else if args.zerokey {
-		// "-zerokey"
-		tlog.Info.Printf("Using all-zero dummy master key.")
-		tlog.Info.Printf(tlog.ColorYellow +
-			"ZEROKEY MODE PROVIDES NO SECURITY AT ALL AND SHOULD ONLY BE USED FOR TESTING." +
-			tlog.ColorReset)
-		masterkey = make([]byte, cryptocore.KeyLen)
-	} else {
-		// Load master key from config file
-		// Prompts the user for the password
-		masterkey, confFile, err = loadConfig(args)
-		if err != nil {
-			if args._ctlsockFd != nil {
-				// Close the socket file (which also deletes it)
-				args._ctlsockFd.Close()
+	var srv *fuse.Server
+	var wipeKeys func()
+	{
+		// Get master key (may prompt for the password)
+		var masterkey []byte
+		if args.masterkey != "" {
+			// "-masterkey"
+			masterkey = parseMasterKey(args.masterkey)
+		} else if args.zerokey {
+			// "-zerokey"
+			tlog.Info.Printf("Using all-zero dummy master key.")
+			tlog.Info.Printf(tlog.ColorYellow +
+				"ZEROKEY MODE PROVIDES NO SECURITY AT ALL AND SHOULD ONLY BE USED FOR TESTING." +
+				tlog.ColorReset)
+			masterkey = make([]byte, cryptocore.KeyLen)
+		} else {
+			// Load master key from config file
+			// Prompts the user for the password
+			masterkey, confFile, err = loadConfig(args)
+			if err != nil {
+				if args._ctlsockFd != nil {
+					// Close the socket file (which also deletes it)
+					args._ctlsockFd.Close()
+				}
+				exitcodes.Exit(err)
 			}
-			exitcodes.Exit(err)
+			readpassword.CheckTrailingGarbage()
+			printMasterKey(masterkey)
+		}
+		// We cannot use JSON for pretty-printing as the fields are unexported
+		tlog.Debug.Printf("cli args: %#v", args)
+		// Initialize FUSE server
+		srv, wipeKeys = initFuseFrontend(masterkey, args, confFile)
+		// fusefrontend / fusefrontend_reverse have initialized their crypto,
+		// we can purge the master key from memory.
+		for i := range masterkey {
+			masterkey[i] = 0
 		}
-		readpassword.CheckTrailingGarbage()
-		printMasterKey(masterkey)
+		// masterkey runs out of scope here
 	}
-	// We cannot use JSON for pretty-printing as the fields are unexported
-	tlog.Debug.Printf("cli args: %#v", args)
-	// Initialize FUSE server
-	srv, wipeKeys := initFuseFrontend(masterkey, args, confFile)
 	tlog.Info.Println(tlog.ColorGreen + "Filesystem mounted and ready." + tlog.ColorReset)
 	// We have been forked into the background, as evidenced by the set
 	// "notifypid".
@@ -267,11 +277,6 @@ func initFuseFrontend(masterkey []byte, args *argContainer, confFile *configfile
 	} else {
 		fs = fusefrontend.NewFS(frontendArgs, cEnc, nameTransform)
 	}
-	// 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 {
-- 
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