ssh: return unexpected msg error when server fails keyboard-interactive auth early

Seems the OpenSSH server running on windows fails keyboard-interactive
auth this way without sending any prompt to client. In such case the
golang ssh client should not retry keyboard-interactive auth when the
auth method is wrapped in a RetryableAuthMethod(). Rather the auth
method should be immediately marked as tried&failed and the client auth
process should move on to next available and acceptable auth method.

Fixes golang/go#67855

Change-Id: I6c64ae58ff8325774e37af716601b112f8833d8f
GitHub-Last-Rev: 7fafc4d1c8
GitHub-Pull-Request: golang/crypto#297
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/590956
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>

ssh/client_auth.go

@@ -555,6 +555,7 @@ func (cb KeyboardInteractiveChallenge) auth(session []byte, user string, c packe
 	}
 
 	gotMsgExtInfo := false
+	gotUserAuthInfoRequest := false
 	for {
 		packet, err := c.readPacket()
 		if err != nil {
@@ -585,6 +586,9 @@ func (cb KeyboardInteractiveChallenge) auth(session []byte, user string, c packe
 			if msg.PartialSuccess {
 				return authPartialSuccess, msg.Methods, nil
 			}
+			if !gotUserAuthInfoRequest {
+				return authFailure, msg.Methods, unexpectedMessageError(msgUserAuthInfoRequest, packet[0])
+			}
 			return authFailure, msg.Methods, nil
 		case msgUserAuthSuccess:
 			return authSuccess, nil, nil
@@ -596,6 +600,7 @@ func (cb KeyboardInteractiveChallenge) auth(session []byte, user string, c packe
 		if err := Unmarshal(packet, &msg); err != nil {
 			return authFailure, nil, err
 		}
+		gotUserAuthInfoRequest = true
 
 		// Manually unpack the prompt/echo pairs.
 		rest := msg.Prompts

ssh/client_auth_test.go

@@ -1293,3 +1293,92 @@ func TestCertAuthOpenSSHCompat(t *testing.T) {
 		t.Fatalf("unable to dial remote side: %s", err)
 	}
 }
+
+func TestKeyboardInteractiveAuthEarlyFail(t *testing.T) {
+	const maxAuthTries = 2
+
+	c1, c2, err := netPipe()
+	if err != nil {
+		t.Fatalf("netPipe: %v", err)
+	}
+	defer c1.Close()
+	defer c2.Close()
+
+	// Start testserver
+	serverConfig := &ServerConfig{
+		MaxAuthTries: maxAuthTries,
+		KeyboardInteractiveCallback: func(c ConnMetadata,
+			client KeyboardInteractiveChallenge) (*Permissions, error) {
+			// Fail keyboard-interactive authentication early before
+			// any prompt is sent to client.
+			return nil, errors.New("keyboard-interactive auth failed")
+		},
+		PasswordCallback: func(c ConnMetadata,
+			pass []byte) (*Permissions, error) {
+			if string(pass) == clientPassword {
+				return nil, nil
+			}
+			return nil, errors.New("password auth failed")
+		},
+	}
+	serverConfig.AddHostKey(testSigners["rsa"])
+
+	serverDone := make(chan struct{})
+	go func() {
+		defer func() { serverDone <- struct{}{} }()
+		conn, chans, reqs, err := NewServerConn(c2, serverConfig)
+		if err != nil {
+			return
+		}
+		_ = conn.Close()
+
+		discarderDone := make(chan struct{})
+		go func() {
+			defer func() { discarderDone <- struct{}{} }()
+			DiscardRequests(reqs)
+		}()
+		for newChannel := range chans {
+			newChannel.Reject(Prohibited,
+				"testserver not accepting requests")
+		}
+
+		<-discarderDone
+	}()
+
+	// Connect to testserver, expect KeyboardInteractive() to be not called,
+	// PasswordCallback() to be called and connection to succeed.
+	passwordCallbackCalled := false
+	clientConfig := &ClientConfig{
+		User: "testuser",
+		Auth: []AuthMethod{
+			RetryableAuthMethod(KeyboardInteractive(func(name,
+				instruction string, questions []string,
+				echos []bool) ([]string, error) {
+				t.Errorf("unexpected call to KeyboardInteractive()")
+				return []string{clientPassword}, nil
+			}), maxAuthTries),
+			RetryableAuthMethod(PasswordCallback(func() (secret string,
+				err error) {
+				t.Logf("PasswordCallback()")
+				passwordCallbackCalled = true
+				return clientPassword, nil
+			}), maxAuthTries),
+		},
+		HostKeyCallback: InsecureIgnoreHostKey(),
+	}
+
+	conn, _, _, err := NewClientConn(c1, "", clientConfig)
+	if err != nil {
+		t.Errorf("unexpected NewClientConn() error: %v", err)
+	}
+	if conn != nil {
+		conn.Close()
+	}
+
+	// Wait for server to finish.
+	<-serverDone
+
+	if !passwordCallbackCalled {
+		t.Errorf("expected PasswordCallback() to be called")
+	}
+}