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

@@ -9,7 +9,7 @@ import (
 	"io"
 )
 
-// authenticate authenticates with the remote server. See RFC 4252. 
+// authenticate authenticates with the remote server. See RFC 4252.
 func (c *ClientConn) authenticate(session []byte) error {
 	// initiate user auth session
 	if err := c.writePacket(marshal(msgServiceRequest, serviceRequestMsg{serviceUserAuth})); err != nil {
@@ -24,7 +24,7 @@ func (c *ClientConn) authenticate(session []byte) error {
 		return err
 	}
 	// during the authentication phase the client first attempts the "none" method
-	// then any untried methods suggested by the server. 
+	// then any untried methods suggested by the server.
 	tried, remain := make(map[string]bool), make(map[string]bool)
 	for auth := ClientAuth(new(noneAuth)); auth != nil; {
 		ok, methods, err := auth.auth(session, c.config.User, c.transport, c.config.rand())
@@ -57,9 +57,9 @@ func (c *ClientConn) authenticate(session []byte) error {
 
 // A ClientAuth represents an instance of an RFC 4252 authentication method.
 type ClientAuth interface {
-	// auth authenticates user over transport t. 
+	// auth authenticates user over transport t.
 	// Returns true if authentication is successful.
-	// If authentication is not successful, a []string of alternative 
+	// If authentication is not successful, a []string of alternative
 	// method names is returned.
 	auth(session []byte, user string, t *transport, rand io.Reader) (bool, []string, error)
 
@@ -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 {
@@ -159,7 +135,7 @@ func ClientAuthPassword(impl ClientPassword) ClientAuth {
 
 // ClientKeyring implements access to a client key ring.
 type ClientKeyring interface {
-	// Key returns the i'th rsa.Publickey or dsa.Publickey, or nil if 
+	// Key returns the i'th rsa.Publickey or dsa.Publickey, or nil if
 	// no key exists at i.
 	Key(i int) (key interface{}, err error)
 
@@ -173,27 +149,28 @@ type publickeyAuth struct {
 	ClientKeyring
 }
 
+type publickeyAuthMsg struct {
+	User    string
+	Service string
+	Method  string
+	// HasSig indicates to the reciver packet that the auth request is signed and
+	// should be used for authentication of the request.
+	HasSig   bool
+	Algoname string
+	Pubkey   string
+	// 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) {
-	type publickeyAuthMsg struct {
-		User    string
-		Service string
-		Method  string
-		// HasSig indicates to the reciver packet that the auth request is signed and
-		// should be used for authentication of the request.
-		HasSig   bool
-		Algoname string
-		Pubkey   string
-		// Sig is defined as []byte so marshal will exclude it during the query phase
-		Sig []byte `ssh:"rest"`
-	}
 
 	// Authentication is performed in two stages. The first stage sends an
 	// enquiry to test if each key is acceptable to the remote. The second
-	// stage attempts to authenticate with the valid keys obtained in the 
+	// stage attempts to authenticate with the valid keys obtained in the
 	// first stage.
 
 	var index int
-	// a map of public keys to their index in the keyring 
+	// a map of public keys to their index in the keyring
 	validKeys := make(map[int]interface{})
 	for {
 		key, err := p.Key(index)
@@ -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 err != nil {
-			return false, nil, err
-		}
-		switch packet[0] {
-		case msgUserAuthPubKeyOk:
-			msg := decode(packet).(*userAuthPubKeyOkMsg)
-			if msg.Algo != algoname || msg.PubKey != string(pubkey) {
-				continue
-			}
+
+		if ok, err := p.validateKey(key, user, t); ok {
 			validKeys[index] = key
-		case msgUserAuthFailure:
-		default:
-			return false, nil, UnexpectedMessageError{msgUserAuthSuccess, packet[0]}
+		} else {
+			if err != nil {
+				return false, nil, err
+			}
 		}
 		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")
+}