ssh: fail client auth immediately on receiving disconnect message

Fixes golang/go#66991

Change-Id: I60dd8a807578f162fda0e49bcd6fbf289d444396
GitHub-Last-Rev: f88329d35712873d0d7e3b39b9b11e7bfbc28e71
GitHub-Pull-Request: golang/crypto#293
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/581075
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Auto-Submit: Nicola Murino <nicola.murino@gmail.com>
Reviewed-by: Nicola Murino <nicola.murino@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

ssh/client_auth.go

@@ -71,6 +71,10 @@ func (c *connection) clientAuthenticate(config *ClientConfig) error {
 	for auth := AuthMethod(new(noneAuth)); auth != nil; {
 		ok, methods, err := auth.auth(sessionID, config.User, c.transport, config.Rand, extensions)
 		if err != nil {
+			// On disconnect, return error immediately
+			if _, ok := err.(*disconnectMsg); ok {
+				return err
+			}
 			// We return the error later if there is no other method left to
 			// try.
 			ok = authFailure

ssh/test/session_test.go

@@ -468,3 +468,72 @@ func TestClientAuthAlgorithms(t *testing.T) {
 		})
 	}
 }
+
+func TestClientAuthDisconnect(t *testing.T) {
+	// Use a static key that is not accepted by server.
+	// 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_key \
+	//   -C "Static RSA key for golang.org/x/crypto/ssh unit test"
+
+	const privKeyData = `-----BEGIN OPENSSH PRIVATE KEY-----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+-----END OPENSSH PRIVATE KEY-----`
+
+	signer, err := ssh.ParsePrivateKey([]byte(privKeyData))
+	if err != nil {
+		t.Fatalf("failed to create signer from key: %v", err)
+	}
+
+	// Start server with MaxAuthTries 1 and publickey and password auth
+	// enabled
+	server := newServerForConfig(t, "MaxAuthTries", map[string]string{})
+
+	// Connect to server, expect failure, that PublicKeysCallback is called
+	// and that PasswordCallback is not called.
+	publicKeysCallbackCalled := false
+	config := clientConfig()
+	config.Auth = []ssh.AuthMethod{
+		ssh.PublicKeysCallback(func() ([]ssh.Signer, error) {
+			publicKeysCallbackCalled = true
+			return []ssh.Signer{signer}, nil
+		}),
+		ssh.PasswordCallback(func() (string, error) {
+			t.Errorf("unexpected call to PasswordCallback()")
+			return "notaverygoodpassword", nil
+		}),
+	}
+	client, err := server.TryDial(config)
+	if err == nil {
+		t.Errorf("expected TryDial() to fail")
+		_ = client.Close()
+	}
+	if !publicKeysCallbackCalled {
+		t.Errorf("expected PublicKeysCallback() to be called")
+	}
+}

ssh/test/test_unix_test.go

@@ -58,12 +58,17 @@ UsePAM yes
 PasswordAuthentication yes
 ChallengeResponseAuthentication yes
 AuthenticationMethods {{.AuthMethods}}
+`
+	maxAuthTriesSshdConfigTail = `
+PasswordAuthentication yes
+MaxAuthTries 1
 `
 )
 
 var configTmpl = map[string]*template.Template{
-	"default":   template.Must(template.New("").Parse(defaultSshdConfig)),
-	"MultiAuth": template.Must(template.New("").Parse(defaultSshdConfig + multiAuthSshdConfigTail))}
+	"default":      template.Must(template.New("").Parse(defaultSshdConfig)),
+	"MultiAuth":    template.Must(template.New("").Parse(defaultSshdConfig + multiAuthSshdConfigTail)),
+	"MaxAuthTries": template.Must(template.New("").Parse(defaultSshdConfig + maxAuthTriesSshdConfigTail))}
 
 type server struct {
 	t          *testing.T