From 9d2ee975ef9fe627bf0a6f01c1f69e8ef1d4f05d Mon Sep 17 00:00:00 2001 From: Roland Shoemaker Date: Mon, 20 Nov 2023 12:06:18 -0800 Subject: [PATCH] ssh: implement strict KEX protocol changes MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Implement the "strict KEX" protocol changes, as described in section 1.9 of the OpenSSH PROTOCOL file (as of OpenSSH version 9.6/9.6p1). Namely this makes the following changes: * Both the server and the client add an additional algorithm to the initial KEXINIT message, indicating support for the strict KEX mode. * When one side of the connection sees the strict KEX extension algorithm, the strict KEX mode is enabled for messages originating from the other side of the connection. If the sequence number for the side which requested the extension is not 1 (indicating that it has already received non-KEXINIT packets), the connection is terminated. * When strict kex mode is enabled, unexpected messages during the handshake are considered fatal. Additionally when a key change occurs (on the receipt of the NEWKEYS message) the message sequence numbers are reset. Thanks to Fabian Bäumer, Marcus Brinkmann, and Jörg Schwenk from Ruhr University Bochum for reporting this issue. Fixes CVE-2023-48795 Fixes golang/go#64784 Change-Id: I96b53afd2bd2fb94d2b6f2a46a5dacf325357604 Reviewed-on: https://go-review.googlesource.com/c/crypto/+/550715 Reviewed-by: Nicola Murino Reviewed-by: Tatiana Bradley TryBot-Result: Gopher Robot Run-TryBot: Roland Shoemaker Reviewed-by: Damien Neil LUCI-TryBot-Result: Go LUCI --- ssh/handshake.go | 56 +++++++- ssh/handshake_test.go | 309 ++++++++++++++++++++++++++++++++++++++++++ ssh/transport.go | 32 ++++- 3 files changed, 388 insertions(+), 9 deletions(-) diff --git a/ssh/handshake.go b/ssh/handshake.go index 49bbba7692..56cdc7c21c 100644 --- a/ssh/handshake.go +++ b/ssh/handshake.go @@ -35,6 +35,16 @@ type keyingTransport interface { // direction will be effected if a msgNewKeys message is sent // or received. prepareKeyChange(*algorithms, *kexResult) error + + // setStrictMode sets the strict KEX mode, notably triggering + // sequence number resets on sending or receiving msgNewKeys. + // If the sequence number is already > 1 when setStrictMode + // is called, an error is returned. + setStrictMode() error + + // setInitialKEXDone indicates to the transport that the initial key exchange + // was completed + setInitialKEXDone() } // handshakeTransport implements rekeying on top of a keyingTransport @@ -100,6 +110,10 @@ type handshakeTransport struct { // The session ID or nil if first kex did not complete yet. sessionID []byte + + // strictMode indicates if the other side of the handshake indicated + // that we should be following the strict KEX protocol restrictions. + strictMode bool } type pendingKex struct { @@ -209,7 +223,10 @@ func (t *handshakeTransport) readLoop() { close(t.incoming) break } - if p[0] == msgIgnore || p[0] == msgDebug { + // If this is the first kex, and strict KEX mode is enabled, + // we don't ignore any messages, as they may be used to manipulate + // the packet sequence numbers. + if !(t.sessionID == nil && t.strictMode) && (p[0] == msgIgnore || p[0] == msgDebug) { continue } t.incoming <- p @@ -441,6 +458,11 @@ func (t *handshakeTransport) readOnePacket(first bool) ([]byte, error) { return successPacket, nil } +const ( + kexStrictClient = "kex-strict-c-v00@openssh.com" + kexStrictServer = "kex-strict-s-v00@openssh.com" +) + // sendKexInit sends a key change message. func (t *handshakeTransport) sendKexInit() error { t.mu.Lock() @@ -454,7 +476,6 @@ func (t *handshakeTransport) sendKexInit() error { } msg := &kexInitMsg{ - KexAlgos: t.config.KeyExchanges, CiphersClientServer: t.config.Ciphers, CiphersServerClient: t.config.Ciphers, MACsClientServer: t.config.MACs, @@ -464,6 +485,13 @@ func (t *handshakeTransport) sendKexInit() error { } io.ReadFull(rand.Reader, msg.Cookie[:]) + // We mutate the KexAlgos slice, in order to add the kex-strict extension algorithm, + // and possibly to add the ext-info extension algorithm. Since the slice may be the + // user owned KeyExchanges, we create our own slice in order to avoid using user + // owned memory by mistake. + msg.KexAlgos = make([]string, 0, len(t.config.KeyExchanges)+2) // room for kex-strict and ext-info + msg.KexAlgos = append(msg.KexAlgos, t.config.KeyExchanges...) + isServer := len(t.hostKeys) > 0 if isServer { for _, k := range t.hostKeys { @@ -488,17 +516,24 @@ func (t *handshakeTransport) sendKexInit() error { msg.ServerHostKeyAlgos = append(msg.ServerHostKeyAlgos, keyFormat) } } + + if t.sessionID == nil { + msg.KexAlgos = append(msg.KexAlgos, kexStrictServer) + } } else { msg.ServerHostKeyAlgos = t.hostKeyAlgorithms // As a client we opt in to receiving SSH_MSG_EXT_INFO so we know what // algorithms the server supports for public key authentication. See RFC // 8308, Section 2.1. + // + // We also send the strict KEX mode extension algorithm, in order to opt + // into the strict KEX mode. if firstKeyExchange := t.sessionID == nil; firstKeyExchange { - msg.KexAlgos = make([]string, 0, len(t.config.KeyExchanges)+1) - msg.KexAlgos = append(msg.KexAlgos, t.config.KeyExchanges...) msg.KexAlgos = append(msg.KexAlgos, "ext-info-c") + msg.KexAlgos = append(msg.KexAlgos, kexStrictClient) } + } packet := Marshal(msg) @@ -604,6 +639,13 @@ func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error { return err } + if t.sessionID == nil && ((isClient && contains(serverInit.KexAlgos, kexStrictServer)) || (!isClient && contains(clientInit.KexAlgos, kexStrictClient))) { + t.strictMode = true + if err := t.conn.setStrictMode(); err != nil { + return err + } + } + // We don't send FirstKexFollows, but we handle receiving it. // // RFC 4253 section 7 defines the kex and the agreement method for @@ -679,6 +721,12 @@ func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error { return unexpectedMessageError(msgNewKeys, packet[0]) } + if firstKeyExchange { + // Indicates to the transport that the first key exchange is completed + // after receiving SSH_MSG_NEWKEYS. + t.conn.setInitialKEXDone() + } + return nil } diff --git a/ssh/handshake_test.go b/ssh/handshake_test.go index 65afc2059e..2bc607b649 100644 --- a/ssh/handshake_test.go +++ b/ssh/handshake_test.go @@ -395,6 +395,10 @@ func (n *errorKeyingTransport) readPacket() ([]byte, error) { return n.packetConn.readPacket() } +func (n *errorKeyingTransport) setStrictMode() error { return nil } + +func (n *errorKeyingTransport) setInitialKEXDone() {} + func TestHandshakeErrorHandlingRead(t *testing.T) { for i := 0; i < 20; i++ { testHandshakeErrorHandlingN(t, i, -1, false) @@ -710,3 +714,308 @@ func TestPickIncompatibleHostKeyAlgo(t *testing.T) { t.Fatal("incompatible signer returned") } } + +func TestStrictKEXResetSeqFirstKEX(t *testing.T) { + if runtime.GOOS == "plan9" { + t.Skip("see golang.org/issue/7237") + } + + checker := &syncChecker{ + waitCall: make(chan int, 10), + called: make(chan int, 10), + } + + checker.waitCall <- 1 + trC, trS, err := handshakePair(&ClientConfig{HostKeyCallback: checker.Check}, "addr", false) + if err != nil { + t.Fatalf("handshakePair: %v", err) + } + <-checker.called + + t.Cleanup(func() { + trC.Close() + trS.Close() + }) + + // Throw away the msgExtInfo packet sent during the handshake by the server + _, err = trC.readPacket() + if err != nil { + t.Fatalf("readPacket failed: %s", err) + } + + // close the handshake transports before checking the sequence number to + // avoid races. + trC.Close() + trS.Close() + + // check that the sequence number counters. We reset after msgNewKeys, but + // then the server immediately writes msgExtInfo, and we close the + // transports so we expect read 2, write 0 on the client and read 1, write 1 + // on the server. + if trC.conn.(*transport).reader.seqNum != 2 || trC.conn.(*transport).writer.seqNum != 0 || + trS.conn.(*transport).reader.seqNum != 1 || trS.conn.(*transport).writer.seqNum != 1 { + t.Errorf( + "unexpected sequence counters:\nclient: reader %d (expected 2), writer %d (expected 0)\nserver: reader %d (expected 1), writer %d (expected 1)", + trC.conn.(*transport).reader.seqNum, + trC.conn.(*transport).writer.seqNum, + trS.conn.(*transport).reader.seqNum, + trS.conn.(*transport).writer.seqNum, + ) + } +} + +func TestStrictKEXResetSeqSuccessiveKEX(t *testing.T) { + if runtime.GOOS == "plan9" { + t.Skip("see golang.org/issue/7237") + } + + checker := &syncChecker{ + waitCall: make(chan int, 10), + called: make(chan int, 10), + } + + checker.waitCall <- 1 + trC, trS, err := handshakePair(&ClientConfig{HostKeyCallback: checker.Check}, "addr", false) + if err != nil { + t.Fatalf("handshakePair: %v", err) + } + <-checker.called + + t.Cleanup(func() { + trC.Close() + trS.Close() + }) + + // Throw away the msgExtInfo packet sent during the handshake by the server + _, err = trC.readPacket() + if err != nil { + t.Fatalf("readPacket failed: %s", err) + } + + // write and read five packets on either side to bump the sequence numbers + for i := 0; i < 5; i++ { + if err := trC.writePacket([]byte{msgRequestSuccess}); err != nil { + t.Fatalf("writePacket failed: %s", err) + } + if _, err := trS.readPacket(); err != nil { + t.Fatalf("readPacket failed: %s", err) + } + if err := trS.writePacket([]byte{msgRequestSuccess}); err != nil { + t.Fatalf("writePacket failed: %s", err) + } + if _, err := trC.readPacket(); err != nil { + t.Fatalf("readPacket failed: %s", err) + } + } + + // Request a key exchange, which should cause the sequence numbers to reset + checker.waitCall <- 1 + trC.requestKeyExchange() + <-checker.called + + // write a packet on the client, and then read it, to verify the key change has actually happened, since + // the HostKeyCallback is called _during_ the handshake, so isn't actually indicative of the handshake + // finishing. + dummyPacket := []byte{99} + if err := trS.writePacket(dummyPacket); err != nil { + t.Fatalf("writePacket failed: %s", err) + } + if p, err := trC.readPacket(); err != nil { + t.Fatalf("readPacket failed: %s", err) + } else if !bytes.Equal(p, dummyPacket) { + t.Fatalf("unexpected packet: got %x, want %x", p, dummyPacket) + } + + // close the handshake transports before checking the sequence number to + // avoid races. + trC.Close() + trS.Close() + + if trC.conn.(*transport).reader.seqNum != 2 || trC.conn.(*transport).writer.seqNum != 0 || + trS.conn.(*transport).reader.seqNum != 1 || trS.conn.(*transport).writer.seqNum != 1 { + t.Errorf( + "unexpected sequence counters:\nclient: reader %d (expected 2), writer %d (expected 0)\nserver: reader %d (expected 1), writer %d (expected 1)", + trC.conn.(*transport).reader.seqNum, + trC.conn.(*transport).writer.seqNum, + trS.conn.(*transport).reader.seqNum, + trS.conn.(*transport).writer.seqNum, + ) + } +} + +func TestSeqNumIncrease(t *testing.T) { + if runtime.GOOS == "plan9" { + t.Skip("see golang.org/issue/7237") + } + + checker := &syncChecker{ + waitCall: make(chan int, 10), + called: make(chan int, 10), + } + + checker.waitCall <- 1 + trC, trS, err := handshakePair(&ClientConfig{HostKeyCallback: checker.Check}, "addr", false) + if err != nil { + t.Fatalf("handshakePair: %v", err) + } + <-checker.called + + t.Cleanup(func() { + trC.Close() + trS.Close() + }) + + // Throw away the msgExtInfo packet sent during the handshake by the server + _, err = trC.readPacket() + if err != nil { + t.Fatalf("readPacket failed: %s", err) + } + + // write and read five packets on either side to bump the sequence numbers + for i := 0; i < 5; i++ { + if err := trC.writePacket([]byte{msgRequestSuccess}); err != nil { + t.Fatalf("writePacket failed: %s", err) + } + if _, err := trS.readPacket(); err != nil { + t.Fatalf("readPacket failed: %s", err) + } + if err := trS.writePacket([]byte{msgRequestSuccess}); err != nil { + t.Fatalf("writePacket failed: %s", err) + } + if _, err := trC.readPacket(); err != nil { + t.Fatalf("readPacket failed: %s", err) + } + } + + // close the handshake transports before checking the sequence number to + // avoid races. + trC.Close() + trS.Close() + + if trC.conn.(*transport).reader.seqNum != 7 || trC.conn.(*transport).writer.seqNum != 5 || + trS.conn.(*transport).reader.seqNum != 6 || trS.conn.(*transport).writer.seqNum != 6 { + t.Errorf( + "unexpected sequence counters:\nclient: reader %d (expected 7), writer %d (expected 5)\nserver: reader %d (expected 6), writer %d (expected 6)", + trC.conn.(*transport).reader.seqNum, + trC.conn.(*transport).writer.seqNum, + trS.conn.(*transport).reader.seqNum, + trS.conn.(*transport).writer.seqNum, + ) + } +} + +func TestStrictKEXUnexpectedMsg(t *testing.T) { + if runtime.GOOS == "plan9" { + t.Skip("see golang.org/issue/7237") + } + + // Check that unexpected messages during the handshake cause failure + _, _, err := handshakePair(&ClientConfig{HostKeyCallback: func(hostname string, remote net.Addr, key PublicKey) error { return nil }}, "addr", true) + if err == nil { + t.Fatal("handshake should fail when there are unexpected messages during the handshake") + } + + trC, trS, err := handshakePair(&ClientConfig{HostKeyCallback: func(hostname string, remote net.Addr, key PublicKey) error { return nil }}, "addr", false) + if err != nil { + t.Fatalf("handshake failed: %s", err) + } + + // Check that ignore/debug pacekts are still ignored outside of the handshake + if err := trC.writePacket([]byte{msgIgnore}); err != nil { + t.Fatalf("writePacket failed: %s", err) + } + if err := trC.writePacket([]byte{msgDebug}); err != nil { + t.Fatalf("writePacket failed: %s", err) + } + dummyPacket := []byte{99} + if err := trC.writePacket(dummyPacket); err != nil { + t.Fatalf("writePacket failed: %s", err) + } + + if p, err := trS.readPacket(); err != nil { + t.Fatalf("readPacket failed: %s", err) + } else if !bytes.Equal(p, dummyPacket) { + t.Fatalf("unexpected packet: got %x, want %x", p, dummyPacket) + } +} + +func TestStrictKEXMixed(t *testing.T) { + // Test that we still support a mixed connection, where one side sends kex-strict but the other + // side doesn't. + + a, b, err := netPipe() + if err != nil { + t.Fatalf("netPipe failed: %s", err) + } + + var trC, trS keyingTransport + + trC = newTransport(a, rand.Reader, true) + trS = newTransport(b, rand.Reader, false) + trS = addNoiseTransport(trS) + + clientConf := &ClientConfig{HostKeyCallback: func(hostname string, remote net.Addr, key PublicKey) error { return nil }} + clientConf.SetDefaults() + + v := []byte("version") + client := newClientTransport(trC, v, v, clientConf, "addr", a.RemoteAddr()) + + serverConf := &ServerConfig{} + serverConf.AddHostKey(testSigners["ecdsa"]) + serverConf.AddHostKey(testSigners["rsa"]) + serverConf.SetDefaults() + + transport := newHandshakeTransport(trS, &serverConf.Config, []byte("version"), []byte("version")) + transport.hostKeys = serverConf.hostKeys + transport.publicKeyAuthAlgorithms = serverConf.PublicKeyAuthAlgorithms + + readOneFailure := make(chan error, 1) + go func() { + if _, err := transport.readOnePacket(true); err != nil { + readOneFailure <- err + } + }() + + // Basically sendKexInit, but without the kex-strict extension algorithm + msg := &kexInitMsg{ + KexAlgos: transport.config.KeyExchanges, + CiphersClientServer: transport.config.Ciphers, + CiphersServerClient: transport.config.Ciphers, + MACsClientServer: transport.config.MACs, + MACsServerClient: transport.config.MACs, + CompressionClientServer: supportedCompressions, + CompressionServerClient: supportedCompressions, + ServerHostKeyAlgos: []string{KeyAlgoRSASHA256, KeyAlgoRSASHA512, KeyAlgoRSA}, + } + packet := Marshal(msg) + // writePacket destroys the contents, so save a copy. + packetCopy := make([]byte, len(packet)) + copy(packetCopy, packet) + if err := transport.pushPacket(packetCopy); err != nil { + t.Fatalf("pushPacket: %s", err) + } + transport.sentInitMsg = msg + transport.sentInitPacket = packet + + if err := transport.getWriteError(); err != nil { + t.Fatalf("getWriteError failed: %s", err) + } + var request *pendingKex + select { + case err = <-readOneFailure: + t.Fatalf("server readOnePacket failed: %s", err) + case request = <-transport.startKex: + break + } + + // We expect the following calls to fail if the side which does not support + // kex-strict sends unexpected/ignored packets during the handshake, even if + // the other side does support kex-strict. + + if err := transport.enterKeyExchange(request.otherInit); err != nil { + t.Fatalf("enterKeyExchange failed: %s", err) + } + if err := client.waitSession(); err != nil { + t.Fatalf("client.waitSession: %v", err) + } +} diff --git a/ssh/transport.go b/ssh/transport.go index da015801ea..0424d2d37c 100644 --- a/ssh/transport.go +++ b/ssh/transport.go @@ -49,6 +49,9 @@ type transport struct { rand io.Reader isClient bool io.Closer + + strictMode bool + initialKEXDone bool } // packetCipher represents a combination of SSH encryption/MAC @@ -74,6 +77,18 @@ type connectionState struct { pendingKeyChange chan packetCipher } +func (t *transport) setStrictMode() error { + if t.reader.seqNum != 1 { + return errors.New("ssh: sequence number != 1 when strict KEX mode requested") + } + t.strictMode = true + return nil +} + +func (t *transport) setInitialKEXDone() { + t.initialKEXDone = true +} + // prepareKeyChange sets up key material for a keychange. The key changes in // both directions are triggered by reading and writing a msgNewKey packet // respectively. @@ -112,11 +127,12 @@ func (t *transport) printPacket(p []byte, write bool) { // Read and decrypt next packet. func (t *transport) readPacket() (p []byte, err error) { for { - p, err = t.reader.readPacket(t.bufReader) + p, err = t.reader.readPacket(t.bufReader, t.strictMode) if err != nil { break } - if len(p) == 0 || (p[0] != msgIgnore && p[0] != msgDebug) { + // in strict mode we pass through DEBUG and IGNORE packets only during the initial KEX + if len(p) == 0 || (t.strictMode && !t.initialKEXDone) || (p[0] != msgIgnore && p[0] != msgDebug) { break } } @@ -127,7 +143,7 @@ func (t *transport) readPacket() (p []byte, err error) { return p, err } -func (s *connectionState) readPacket(r *bufio.Reader) ([]byte, error) { +func (s *connectionState) readPacket(r *bufio.Reader, strictMode bool) ([]byte, error) { packet, err := s.packetCipher.readCipherPacket(s.seqNum, r) s.seqNum++ if err == nil && len(packet) == 0 { @@ -140,6 +156,9 @@ func (s *connectionState) readPacket(r *bufio.Reader) ([]byte, error) { select { case cipher := <-s.pendingKeyChange: s.packetCipher = cipher + if strictMode { + s.seqNum = 0 + } default: return nil, errors.New("ssh: got bogus newkeys message") } @@ -170,10 +189,10 @@ func (t *transport) writePacket(packet []byte) error { if debugTransport { t.printPacket(packet, true) } - return t.writer.writePacket(t.bufWriter, t.rand, packet) + return t.writer.writePacket(t.bufWriter, t.rand, packet, t.strictMode) } -func (s *connectionState) writePacket(w *bufio.Writer, rand io.Reader, packet []byte) error { +func (s *connectionState) writePacket(w *bufio.Writer, rand io.Reader, packet []byte, strictMode bool) error { changeKeys := len(packet) > 0 && packet[0] == msgNewKeys err := s.packetCipher.writeCipherPacket(s.seqNum, w, rand, packet) @@ -188,6 +207,9 @@ func (s *connectionState) writePacket(w *bufio.Writer, rand io.Reader, packet [] select { case cipher := <-s.pendingKeyChange: s.packetCipher = cipher + if strictMode { + s.seqNum = 0 + } default: panic("ssh: no key material for msgNewKeys") }