main: try to wipe cryptocore's secret keys on unmount

Raise the bar for recovering keys from memory.

https://github.com/rfjakob/gocryptfs/issues/211

internal/cryptocore/cryptocore.go

@@ -8,11 +8,13 @@ import (
 	"crypto/sha512"
 	"fmt"
 	"log"
+	"runtime"
 
 	"github.com/rfjakob/eme"
 
 	"github.com/rfjakob/gocryptfs/internal/siv_aead"
 	"github.com/rfjakob/gocryptfs/internal/stupidgcm"
+	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
 // AEADTypeEnum indicates the type of AEAD backend in use.
@@ -129,3 +131,25 @@ func New(key []byte, aeadType AEADTypeEnum, IVBitLen int, useHKDF bool, forceDec
 		IVLen:       IVLen,
 	}
 }
+
+// Wipe tries to wipe secret keys from memory by overwriting them with zeros
+// and/or setting references to nil.
+//
+// This is not bulletproof due to possible GC copies, but
+// still raises to bar for extracting the key.
+func (c *CryptoCore) Wipe() {
+	if c.AEADBackend == BackendOpenSSL {
+		tlog.Debug.Print("CryptoCore.Wipe: Wiping stupidgcm key")
+		// We don't use "x, ok :=" because we *want* to crash loudly if the
+		// type assertion fails (it should never fail).
+		sgcm := c.AEADCipher.(*stupidgcm.StupidGCM)
+		sgcm.Wipe()
+	} else {
+		tlog.Debug.Print("CryptoCore.Wipe: niling stdlib refs")
+	}
+	// We have no access to the keys (or key-equivalents) stored inside the
+	// Go stdlib. Best we can is to nil the references and force a GC.
+	c.AEADCipher = nil
+	c.EMECipher = nil
+	runtime.GC()
+}

internal/stupidgcm/stupidgcm.go

@@ -24,32 +24,32 @@ const (
 )
 
 // stupidGCM implements the cipher.AEAD interface
-type stupidGCM struct {
+type StupidGCM struct {
 	key         []byte
 	forceDecode bool
 }
 
 // Verify that we satisfy the cipher.AEAD interface
-var _ cipher.AEAD = &stupidGCM{}
+var _ cipher.AEAD = &StupidGCM{}
 
 // New returns a new cipher.AEAD implementation..
 func New(key []byte, forceDecode bool) cipher.AEAD {
 	if len(key) != keyLen {
 		log.Panicf("Only %d-byte keys are supported", keyLen)
 	}
-	return &stupidGCM{key: key, forceDecode: forceDecode}
+	return &StupidGCM{key: key, forceDecode: forceDecode}
 }
 
-func (g *stupidGCM) NonceSize() int {
+func (g *StupidGCM) NonceSize() int {
 	return ivLen
 }
 
-func (g *stupidGCM) Overhead() int {
+func (g *StupidGCM) Overhead() int {
 	return tagLen
 }
 
 // Seal encrypts "in" using "iv" and "authData" and append the result to "dst"
-func (g *stupidGCM) Seal(dst, iv, in, authData []byte) []byte {
+func (g *StupidGCM) Seal(dst, iv, in, authData []byte) []byte {
 	if len(iv) != ivLen {
 		log.Panicf("Only %d-byte IVs are supported", ivLen)
 	}
@@ -136,7 +136,7 @@ func (g *stupidGCM) Seal(dst, iv, in, authData []byte) []byte {
 }
 
 // Open decrypts "in" using "iv" and "authData" and append the result to "dst"
-func (g *stupidGCM) Open(dst, iv, in, authData []byte) ([]byte, error) {
+func (g *StupidGCM) Open(dst, iv, in, authData []byte) ([]byte, error) {
 	if len(iv) != ivLen {
 		log.Panicf("Only %d-byte IVs are supported", ivLen)
 	}
@@ -231,12 +231,12 @@ func (g *stupidGCM) Open(dst, iv, in, authData []byte) ([]byte, error) {
 	return append(dst, buf...), nil
 }
 
-// Wipe wipes the AES key from memory by overwriting it with zeros and
+// Wipe tries to wipe the AES key from memory by overwriting it with zeros
-// setting the reference to nil.
+// and setting the reference to nil.
 //
 // This is not bulletproof due to possible GC copies, but
 // still raises to bar for extracting the key.
-func (g *stupidGCM) Wipe() {
+func (g *StupidGCM) Wipe() {
 	for i := range g.key {
 		g.key[i] = 0
 	}

internal/stupidgcm/without_openssl.go

@@ -9,7 +9,7 @@ import (
 	"github.com/rfjakob/gocryptfs/internal/exitcodes"
 )
 
-type stupidGCM struct{}
+type StupidGCM struct{}
 
 const (
 	// BuiltWithoutOpenssl indicates if openssl been disabled at compile-time
@@ -21,28 +21,32 @@ func errExit() {
 	os.Exit(exitcodes.OpenSSL)
 }
 
-func New(_ []byte, _ bool) *stupidGCM {
+func New(_ []byte, _ bool) *StupidGCM {
 	errExit()
 	// Never reached
-	return &stupidGCM{}
+	return &StupidGCM{}
 }
 
-func (g *stupidGCM) NonceSize() int {
+func (g *StupidGCM) NonceSize() int {
 	errExit()
 	return -1
 }
 
-func (g *stupidGCM) Overhead() int {
+func (g *StupidGCM) Overhead() int {
 	errExit()
 	return -1
 }
 
-func (g *stupidGCM) Seal(_, _, _, _ []byte) []byte {
+func (g *StupidGCM) Seal(_, _, _, _ []byte) []byte {
 	errExit()
 	return nil
 }
 
-func (g *stupidGCM) Open(_, _, _, _ []byte) ([]byte, error) {
+func (g *StupidGCM) Open(_, _, _, _ []byte) ([]byte, error) {
 	errExit()
 	return nil, nil
 }
+
+func (g *StupidGCM) Wipe() {
+	errExit()
+}

mount.go

@@ -123,7 +123,7 @@ func doMount(args *argContainer) int {
 	// We cannot use JSON for pretty-printing as the fields are unexported
 	tlog.Debug.Printf("cli args: %#v", args)
 	// Initialize FUSE server
-	srv := initFuseFrontend(masterkey, args, confFile)
+	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".
@@ -162,6 +162,8 @@ func doMount(args *argContainer) int {
 	debug.FreeOSMemory()
 	// Jump into server loop. Returns when it gets an umount request from the kernel.
 	srv.Serve()
+	// Try to wipe secrect keys from memory
+	wipeKeys()
 	return 0
 }
 
@@ -194,7 +196,7 @@ type ctlsockFs interface {
 
 // initFuseFrontend - initialize gocryptfs/fusefrontend
 // Calls os.Exit on errors
-func initFuseFrontend(masterkey []byte, args *argContainer, confFile *configfile.ConfFile) *fuse.Server {
+func initFuseFrontend(masterkey []byte, args *argContainer, confFile *configfile.ConfFile) (srv *fuse.Server, wipeKeys func()) {
 	// Reconciliate CLI and config file arguments into a fusefrontend.Args struct
 	// that is passed to the filesystem implementation
 	cryptoBackend := cryptocore.BackendGoGCM
@@ -361,7 +363,7 @@ func initFuseFrontend(masterkey []byte, args *argContainer, confFile *configfile
 	// directories with the requested permissions.
 	syscall.Umask(0000)
 
-	return srv
+	return srv, func() { cCore.Wipe() }
 }
 
 func handleSigint(srv *fuse.Server, mountpoint string) {