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handshake.go
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handshake.go
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// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package ssh
import (
"crypto/rand"
"errors"
"fmt"
"io"
"log"
"net"
"sync"
)
// debugHandshake, if set, prints messages sent and received. Key
// exchange messages are printed as if DH were used, so the debug
// messages are wrong when using ECDH.
const debugHandshake = false
// keyingTransport is a packet based transport that supports key
// changes. It need not be thread-safe. It should pass through
// msgNewKeys in both directions.
type keyingTransport interface {
packetConn
// prepareKeyChange sets up a key change. The key change for a
// direction will be effected if a msgNewKeys message is sent
// or received.
prepareKeyChange(*algorithms, *kexResult) error
}
// handshakeTransport implements rekeying on top of a keyingTransport
// and offers a thread-safe writePacket() interface.
type handshakeTransport struct {
conn keyingTransport
config *Config
serverVersion []byte
clientVersion []byte
// hostKeys is non-empty if we are the server. In that case,
// it contains all host keys that can be used to sign the
// connection.
hostKeys []Signer
// hostKeyAlgorithms is non-empty if we are the client. In that case,
// we accept these key types from the server as host key.
hostKeyAlgorithms []string
// On read error, incoming is closed, and readError is set.
incoming chan []byte
readError error
mu sync.Mutex
writeError error
sentInitPacket []byte
sentInitMsg *kexInitMsg
pendingPackets [][]byte // Used when a key exchange is in progress.
// If the read loop wants to schedule a kex, it pings this
// channel, and the write loop will send out a kex message.
requestKex chan struct{}
// If the other side requests or confirms a kex, its kexInit
// packet is sent here for the write loop to find it.
startKex chan *pendingKex
// data for host key checking
hostKeyCallback func(hostname string, remote net.Addr, key PublicKey) error
dialAddress string
remoteAddr net.Addr
readSinceKex uint64
writtenSinceKex uint64
// The session ID or nil if first kex did not complete yet.
sessionID []byte
}
type pendingKex struct {
otherInit []byte
done chan error
}
func newHandshakeTransport(conn keyingTransport, config *Config, clientVersion, serverVersion []byte) *handshakeTransport {
t := &handshakeTransport{
conn: conn,
serverVersion: serverVersion,
clientVersion: clientVersion,
incoming: make(chan []byte, 16),
requestKex: make(chan struct{}, 1),
startKex: make(chan *pendingKex, 1),
config: config,
}
return t
}
func newClientTransport(conn keyingTransport, clientVersion, serverVersion []byte, config *ClientConfig, dialAddr string, addr net.Addr) *handshakeTransport {
t := newHandshakeTransport(conn, &config.Config, clientVersion, serverVersion)
t.dialAddress = dialAddr
t.remoteAddr = addr
t.hostKeyCallback = config.HostKeyCallback
if config.HostKeyAlgorithms != nil {
t.hostKeyAlgorithms = config.HostKeyAlgorithms
} else {
t.hostKeyAlgorithms = supportedHostKeyAlgos
}
go t.readLoop()
go t.kexLoop()
return t
}
func newServerTransport(conn keyingTransport, clientVersion, serverVersion []byte, config *ServerConfig) *handshakeTransport {
t := newHandshakeTransport(conn, &config.Config, clientVersion, serverVersion)
t.hostKeys = config.hostKeys
go t.readLoop()
go t.kexLoop()
return t
}
func (t *handshakeTransport) getSessionID() []byte {
return t.sessionID
}
// waitSession waits for the session to be established. This should be
// the first thing to call after instantiating handshakeTransport.
func (t *handshakeTransport) waitSession() error {
p, err := t.readPacket()
if err != nil {
return err
}
if p[0] != msgNewKeys {
return fmt.Errorf("ssh: first packet should be msgNewKeys")
}
return nil
}
func (t *handshakeTransport) id() string {
if len(t.hostKeys) > 0 {
return "server"
}
return "client"
}
func (t *handshakeTransport) printPacket(p []byte, write bool) {
action := "got"
if write {
action = "sent"
}
if p[0] == msgChannelData || p[0] == msgChannelExtendedData {
log.Printf("%s %s data (packet %d bytes)", t.id(), action, len(p))
} else {
msg, err := decode(p)
log.Printf("%s %s %T %v (%v)", t.id(), action, msg, msg, err)
}
}
func (t *handshakeTransport) readPacket() ([]byte, error) {
p, ok := <-t.incoming
if !ok {
return nil, t.readError
}
return p, nil
}
func (t *handshakeTransport) readLoop() {
// We always start with the mandatory key exchange. We use
// the channel for simplicity, and this works if we can rely
// on the SSH package itself not doing anything else before
// waitSession has completed.
t.requestKeyExchange()
first := true
for {
p, err := t.readOnePacket(first)
first = false
if err != nil {
t.readError = err
close(t.incoming)
break
}
if p[0] == msgIgnore || p[0] == msgDebug {
continue
}
t.incoming <- p
}
// Stop writers too.
t.recordWriteError(t.readError)
// Unblock the writer should it wait for this.
close(t.startKex)
// Don't close t.requestKex; it's also written to from writePacket.
}
func (t *handshakeTransport) pushPacket(p []byte) error {
if debugHandshake {
t.printPacket(p, true)
}
return t.conn.writePacket(p)
}
func (t *handshakeTransport) getWriteError() error {
t.mu.Lock()
defer t.mu.Unlock()
return t.writeError
}
func (t *handshakeTransport) recordWriteError(err error) {
t.mu.Lock()
defer t.mu.Unlock()
if t.writeError == nil && err != nil {
t.writeError = err
}
}
func (t *handshakeTransport) requestKeyExchange() {
select {
case t.requestKex <- struct{}{}:
default:
// something already requested a kex, so do nothing.
}
}
func (t *handshakeTransport) kexLoop() {
write:
for t.getWriteError() == nil {
var request *pendingKex
var sent bool
for request == nil || !sent {
var ok bool
select {
case request, ok = <-t.startKex:
if !ok {
break write
}
case <-t.requestKex:
}
if !sent {
if err := t.sendKexInit(); err != nil {
t.recordWriteError(err)
break
}
sent = true
}
}
if err := t.getWriteError(); err != nil {
if request != nil {
request.done <- err
}
break
}
// We're not servicing t.requestKex, but that is OK:
// we never block on sending to t.requestKex.
// We're not servicing t.startKex, but the remote end
// has just sent us a kexInitMsg, so it can't send
// another key change request.
err := t.enterKeyExchange(request.otherInit)
t.mu.Lock()
t.writeError = err
t.sentInitPacket = nil
t.sentInitMsg = nil
t.writtenSinceKex = 0
request.done <- t.writeError
// kex finished. Push packets that we received while
// the kex was in progress. Don't look at t.startKex
// and don't increment writtenSinceKex: if we trigger
// another kex while we are still busy with the last
// one, things will become very confusing.
for _, p := range t.pendingPackets {
t.writeError = t.pushPacket(p)
if t.writeError != nil {
break
}
}
t.pendingPackets = t.pendingPackets[0:]
t.mu.Unlock()
}
// drain startKex channel. We don't service t.requestKex
// because nobody does blocking sends there.
go func() {
for init := range t.startKex {
init.done <- t.writeError
}
}()
// Unblock reader.
t.conn.Close()
}
func (t *handshakeTransport) readOnePacket(first bool) ([]byte, error) {
if t.readSinceKex > t.config.RekeyThreshold {
t.requestKeyExchange()
}
p, err := t.conn.readPacket()
if err != nil {
return nil, err
}
t.readSinceKex += uint64(len(p))
if debugHandshake {
t.printPacket(p, false)
}
if first && p[0] != msgKexInit {
return nil, fmt.Errorf("ssh: first packet should be msgKexInit")
}
if p[0] != msgKexInit {
return p, nil
}
firstKex := t.sessionID == nil
kex := pendingKex{
done: make(chan error, 1),
otherInit: p,
}
t.startKex <- &kex
err = <-kex.done
if debugHandshake {
log.Printf("%s exited key exchange (first %v), err %v", t.id(), firstKex, err)
}
if err != nil {
return nil, err
}
t.readSinceKex = 0
// By default, a key exchange is hidden from higher layers by
// translating it into msgIgnore.
successPacket := []byte{msgIgnore}
if firstKex {
// sendKexInit() for the first kex waits for
// msgNewKeys so the authentication process is
// guaranteed to happen over an encrypted transport.
successPacket = []byte{msgNewKeys}
}
return successPacket, nil
}
// sendKexInit sends a key change message.
func (t *handshakeTransport) sendKexInit() error {
t.mu.Lock()
defer t.mu.Unlock()
if t.sentInitMsg != nil {
// kexInits may be sent either in response to the other side,
// or because our side wants to initiate a key change, so we
// may have already sent a kexInit. In that case, don't send a
// second kexInit.
return nil
}
msg := &kexInitMsg{
KexAlgos: t.config.KeyExchanges,
CiphersClientServer: t.config.Ciphers,
CiphersServerClient: t.config.Ciphers,
MACsClientServer: t.config.MACs,
MACsServerClient: t.config.MACs,
CompressionClientServer: supportedCompressions,
CompressionServerClient: supportedCompressions,
}
io.ReadFull(rand.Reader, msg.Cookie[:])
if len(t.hostKeys) > 0 {
for _, k := range t.hostKeys {
msg.ServerHostKeyAlgos = append(
msg.ServerHostKeyAlgos, k.PublicKey().Type())
}
} else {
msg.ServerHostKeyAlgos = t.hostKeyAlgorithms
}
packet := Marshal(msg)
// writePacket destroys the contents, so save a copy.
packetCopy := make([]byte, len(packet))
copy(packetCopy, packet)
if err := t.pushPacket(packetCopy); err != nil {
return err
}
t.sentInitMsg = msg
t.sentInitPacket = packet
return nil
}
func (t *handshakeTransport) writePacket(p []byte) error {
switch p[0] {
case msgKexInit:
return errors.New("ssh: only handshakeTransport can send kexInit")
case msgNewKeys:
return errors.New("ssh: only handshakeTransport can send newKeys")
}
t.mu.Lock()
defer t.mu.Unlock()
if t.writeError != nil {
return t.writeError
}
if t.sentInitMsg != nil {
// Copy the packet so the writer can reuse the buffer.
cp := make([]byte, len(p))
copy(cp, p)
t.pendingPackets = append(t.pendingPackets, cp)
return nil
}
t.writtenSinceKex += uint64(len(p))
if t.writtenSinceKex > t.config.RekeyThreshold {
t.requestKeyExchange()
}
if err := t.pushPacket(p); err != nil {
t.writeError = err
}
return nil
}
func (t *handshakeTransport) Close() error {
return t.conn.Close()
}
func (t *handshakeTransport) enterKeyExchange(otherInitPacket []byte) error {
if debugHandshake {
log.Printf("%s entered key exchange", t.id())
}
otherInit := &kexInitMsg{}
if err := Unmarshal(otherInitPacket, otherInit); err != nil {
return err
}
magics := handshakeMagics{
clientVersion: t.clientVersion,
serverVersion: t.serverVersion,
clientKexInit: otherInitPacket,
serverKexInit: t.sentInitPacket,
}
clientInit := otherInit
serverInit := t.sentInitMsg
if len(t.hostKeys) == 0 {
clientInit, serverInit = serverInit, clientInit
magics.clientKexInit = t.sentInitPacket
magics.serverKexInit = otherInitPacket
}
algs, err := findAgreedAlgorithms(clientInit, serverInit)
if err != nil {
return err
}
// We don't send FirstKexFollows, but we handle receiving it.
//
// RFC 4253 section 7 defines the kex and the agreement method for
// first_kex_packet_follows. It states that the guessed packet
// should be ignored if the "kex algorithm and/or the host
// key algorithm is guessed wrong (server and client have
// different preferred algorithm), or if any of the other
// algorithms cannot be agreed upon". The other algorithms have
// already been checked above so the kex algorithm and host key
// algorithm are checked here.
if otherInit.FirstKexFollows && (clientInit.KexAlgos[0] != serverInit.KexAlgos[0] || clientInit.ServerHostKeyAlgos[0] != serverInit.ServerHostKeyAlgos[0]) {
// other side sent a kex message for the wrong algorithm,
// which we have to ignore.
if _, err := t.conn.readPacket(); err != nil {
return err
}
}
kex, ok := kexAlgoMap[algs.kex]
if !ok {
return fmt.Errorf("ssh: unexpected key exchange algorithm %v", algs.kex)
}
var result *kexResult
if len(t.hostKeys) > 0 {
result, err = t.server(kex, algs, &magics)
} else {
result, err = t.client(kex, algs, &magics)
}
if err != nil {
return err
}
if t.sessionID == nil {
t.sessionID = result.H
}
result.SessionID = t.sessionID
t.conn.prepareKeyChange(algs, result)
if err = t.conn.writePacket([]byte{msgNewKeys}); err != nil {
return err
}
if packet, err := t.conn.readPacket(); err != nil {
return err
} else if packet[0] != msgNewKeys {
return unexpectedMessageError(msgNewKeys, packet[0])
}
return nil
}
func (t *handshakeTransport) server(kex kexAlgorithm, algs *algorithms, magics *handshakeMagics) (*kexResult, error) {
var hostKey Signer
for _, k := range t.hostKeys {
if algs.hostKey == k.PublicKey().Type() {
hostKey = k
}
}
r, err := kex.Server(t.conn, t.config.Rand, magics, hostKey)
return r, err
}
func (t *handshakeTransport) client(kex kexAlgorithm, algs *algorithms, magics *handshakeMagics) (*kexResult, error) {
result, err := kex.Client(t.conn, t.config.Rand, magics)
if err != nil {
return nil, err
}
hostKey, err := ParsePublicKey(result.HostKey)
if err != nil {
return nil, err
}
if err := verifyHostKeySignature(hostKey, result); err != nil {
return nil, err
}
if t.hostKeyCallback != nil {
err = t.hostKeyCallback(t.dialAddress, t.remoteAddr, hostKey)
if err != nil {
return nil, err
}
}
return result, nil
}