crypto/ssh: return unexpected msg error when server fails keyboard-in…

…teractive auth early

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

@@ -15,6 +15,7 @@ import (
 	"os"
 	"runtime"
 	"strings"
+	"sync"
 	"testing"
 )
 
@@ -1282,3 +1283,147 @@ func TestCertAuthOpenSSHCompat(t *testing.T) {
 		t.Fatalf("unable to dial remote side: %s", err)
 	}
 }
+
+func TestKeyboardInteractiveAuthEarlyFail(t *testing.T) {
+	const maxAuthTries = 2
+
+	// Start testserver
+	dst, err := func() (string, error) {
+		var serverAddr string
+		var serverErr error
+		var wg sync.WaitGroup
+		wg.Add(1)
+
+		go func() {
+			config := &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")
+				},
+			}
+
+			// Use a static hostkey.
+			// This key has been generated with following ssh-keygen command
+			// and used exclusively in this unit test:
+			// $ ssh-keygen -t RSA -b 2048 -f /tmp/static_hostkey \
+			//   -C "Static RSA hostkey for golang.org/x/crypto/ssh unit test"
+
+			const privKeyData = `-----BEGIN OPENSSH PRIVATE KEY-----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+-----END OPENSSH PRIVATE KEY-----`
+
+			private, err := ParsePrivateKey([]byte(privKeyData))
+			if err != nil {
+				serverErr = err
+				wg.Done()
+				return
+			}
+			config.AddHostKey(private)
+
+			listener, err := net.Listen("tcp", "127.0.0.1:")
+			if err != nil {
+				serverErr = err
+				wg.Done()
+				return
+			}
+			defer listener.Close()
+			serverAddr = listener.Addr().String()
+			wg.Done()
+
+			nConn, err := listener.Accept()
+			if err != nil {
+				return
+			}
+
+			conn, chans, reqs, err := NewServerConn(nConn, config)
+			if err != nil {
+				return
+			}
+			_ = conn.Close()
+
+			var connWg sync.WaitGroup
+			connWg.Add(1)
+			go func() {
+				defer connWg.Done()
+				DiscardRequests(reqs)
+			}()
+			for newChannel := range chans {
+				newChannel.Reject(Prohibited,
+					"testserver not accepting requests")
+			}
+			connWg.Wait()
+		}()
+
+		wg.Wait()
+		return serverAddr, serverErr
+	}()
+	if err != nil {
+		t.Fatalf("failed to start testserver: %v", err)
+	}
+
+	// Connect to testserver expect KeyboardInteractive() to be not called and
+	// PasswordCallback() to be called.
+	passwordCallbackCalled := false
+	cfg := &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, _ := Dial("tcp", dst, cfg)
+	if conn != nil {
+		conn.Close()
+	}
+
+	if !passwordCallbackCalled {
+		t.Errorf("expected PasswordCallback() to be called")
+	}
+}