exp/ssh: allow for msgUserAuthBanner during authentication

The SSH spec allows for the server to send a banner message to the client at any point during the authentication process. Currently the ssh client auth types all assume that the first response from the server after issuing a userAuthRequestMsg will be one of a couple of possible authentication success/failure messages. This means that client authentication breaks if the ssh server being connected to has a banner message configured.

This changeset refactors the noneAuth, passwordAuth and publickeyAuth types' auth() function and allows for msgUserAuthBanner during authentication.

R=golang-dev, rsc, dave, agl
CC=golang-dev
https://golang.org/cl/5432065

src/pkg/exp/ssh/client_auth.go

@@ -79,19 +79,7 @@ func (n *noneAuth) auth(session []byte, user string, t *transport, rand io.Reade
 		return false, nil, err
 	}
 
-	packet, err := t.readPacket()
-	if err != nil {
-		return false, nil, err
-	}
-
-	switch packet[0] {
-	case msgUserAuthSuccess:
-		return true, nil, nil
-	case msgUserAuthFailure:
-		msg := decode(packet).(*userAuthFailureMsg)
-		return false, msg.Methods, nil
-	}
-	return false, nil, UnexpectedMessageError{msgUserAuthSuccess, packet[0]}
+	return handleAuthResponse(t)
 }
 
 func (n *noneAuth) method() string {
@@ -127,19 +115,7 @@ func (p *passwordAuth) auth(session []byte, user string, t *transport, rand io.R
 		return false, nil, err
 	}
 
-	packet, err := t.readPacket()
-	if err != nil {
-		return false, nil, err
-	}
-
-	switch packet[0] {
-	case msgUserAuthSuccess:
-		return true, nil, nil
-	case msgUserAuthFailure:
-		msg := decode(packet).(*userAuthFailureMsg)
-		return false, msg.Methods, nil
-	}
-	return false, nil, UnexpectedMessageError{msgUserAuthSuccess, packet[0]}
+	return handleAuthResponse(t)
 }
 
 func (p *passwordAuth) method() string {
@@ -173,8 +149,7 @@ type publickeyAuth struct {
 	ClientKeyring
 }
 
-func (p *publickeyAuth) auth(session []byte, user string, t *transport, rand io.Reader) (bool, []string, error) {
-	type publickeyAuthMsg struct {
+type publickeyAuthMsg struct {
 	User    string
 	Service string
 	Method  string
@@ -183,9 +158,11 @@ func (p *publickeyAuth) auth(session []byte, user string, t *transport, rand io.
 	HasSig   bool
 	Algoname string
 	Pubkey   string
-		// Sig is defined as []byte so marshal will exclude it during the query phase
+	// Sig is defined as []byte so marshal will exclude it during validateKey
 	Sig []byte `ssh:"rest"`
-	}
+}
+
+func (p *publickeyAuth) auth(session []byte, user string, t *transport, rand io.Reader) (bool, []string, error) {
 
 	// Authentication is performed in two stages. The first stage sends an
 	// enquiry to test if each key is acceptable to the remote. The second
@@ -204,33 +181,13 @@ func (p *publickeyAuth) auth(session []byte, user string, t *transport, rand io.
 			// no more keys in the keyring
 			break
 		}
-		pubkey := serializePublickey(key)
-		algoname := algoName(key)
-		msg := publickeyAuthMsg{
-			User:     user,
-			Service:  serviceSSH,
-			Method:   p.method(),
-			HasSig:   false,
-			Algoname: algoname,
-			Pubkey:   string(pubkey),
-		}
-		if err := t.writePacket(marshal(msgUserAuthRequest, msg)); err != nil {
-			return false, nil, err
-		}
-		packet, err := t.readPacket()
+
+		if ok, err := p.validateKey(key, user, t); ok {
+			validKeys[index] = key
+		} else {
 			if err != nil {
 				return false, nil, err
 			}
-		switch packet[0] {
-		case msgUserAuthPubKeyOk:
-			msg := decode(packet).(*userAuthPubKeyOkMsg)
-			if msg.Algo != algoname || msg.PubKey != string(pubkey) {
-				continue
-			}
-			validKeys[index] = key
-		case msgUserAuthFailure:
-		default:
-			return false, nil, UnexpectedMessageError{msgUserAuthSuccess, packet[0]}
 		}
 		index++
 	}
@@ -265,24 +222,61 @@ func (p *publickeyAuth) auth(session []byte, user string, t *transport, rand io.
 		if err := t.writePacket(p); err != nil {
 			return false, nil, err
 		}
-		packet, err := t.readPacket()
+		success, methods, err := handleAuthResponse(t)
 		if err != nil {
 			return false, nil, err
 		}
+		if success {
+			return success, methods, err
+		}
+	}
+	return false, methods, nil
+}
+
+// validateKey validates the key provided it is acceptable to the server.
+func (p *publickeyAuth) validateKey(key interface{}, user string, t *transport) (bool, error) {
+	pubkey := serializePublickey(key)
+	algoname := algoName(key)
+	msg := publickeyAuthMsg{
+		User:     user,
+		Service:  serviceSSH,
+		Method:   p.method(),
+		HasSig:   false,
+		Algoname: algoname,
+		Pubkey:   string(pubkey),
+	}
+	if err := t.writePacket(marshal(msgUserAuthRequest, msg)); err != nil {
+		return false, err
+	}
+
+	return p.confirmKeyAck(key, t)
+}
+
+func (p *publickeyAuth) confirmKeyAck(key interface{}, t *transport) (bool, error) {
+	pubkey := serializePublickey(key)
+	algoname := algoName(key)
+
+	for {
+		packet, err := t.readPacket()
+		if err != nil {
+			return false, err
+		}
 		switch packet[0] {
-		case msgUserAuthSuccess:
-			return true, nil, nil
+		case msgUserAuthBanner:
+			// TODO(gpaul): add callback to present the banner to the user
+		case msgUserAuthPubKeyOk:
+			msg := decode(packet).(*userAuthPubKeyOkMsg)
+			if msg.Algo != algoname || msg.PubKey != string(pubkey) {
+				return false, nil
+			}
+			return true, nil
 		case msgUserAuthFailure:
-			msg := decode(packet).(*userAuthFailureMsg)
-			methods = msg.Methods
-			continue
-		case msgDisconnect:
-			return false, nil, io.EOF
+			return false, nil
 		default:
-			return false, nil, UnexpectedMessageError{msgUserAuthSuccess, packet[0]}
+			return false, UnexpectedMessageError{msgUserAuthSuccess, packet[0]}
 		}
 	}
-	return false, methods, nil
+	panic("unreachable")
 }
 
 func (p *publickeyAuth) method() string {
@@ -293,3 +287,30 @@ func (p *publickeyAuth) method() string {
 func ClientAuthPublickey(impl ClientKeyring) ClientAuth {
 	return &publickeyAuth{impl}
 }
+
+// handleAuthResponse returns whether the preceding authentication request succeeded
+// along with a list of remaining authentication methods to try next and
+// an error if an unexpected response was received.
+func handleAuthResponse(t *transport) (bool, []string, error) {
+	for {
+		packet, err := t.readPacket()
+		if err != nil {
+			return false, nil, err
+		}
+
+		switch packet[0] {
+		case msgUserAuthBanner:
+			// TODO: add callback to present the banner to the user
+		case msgUserAuthFailure:
+			msg := decode(packet).(*userAuthFailureMsg)
+			return false, msg.Methods, nil
+		case msgUserAuthSuccess:
+			return true, nil, nil
+		case msgDisconnect:
+			return false, nil, io.EOF
+		default:
+			return false, nil, UnexpectedMessageError{msgUserAuthSuccess, packet[0]}
+		}
+	}
+	panic("unreachable")
+}