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// Copyright 2011 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 (
"encoding/binary"
"errors"
"fmt"
"io"
"log"
"sync"
)
const (
minPacketLength = 9
// channelMaxPacket contains the maximum number of bytes that will be
// sent in a single packet. As per RFC 4253, section 6.1, 32k is also
// the minimum.
channelMaxPacket = 1 << 15
// We follow OpenSSH here.
channelWindowSize = 64 * channelMaxPacket
)
// NewChannel represents an incoming request to a channel. It must either be
// accepted for use by calling Accept, or rejected by calling Reject.
type NewChannel interface {
// Accept accepts the channel creation request. It returns the Channel
// and a Go channel containing SSH requests. The Go channel must be
// serviced otherwise the Channel will hang.
Accept() (Channel, <-chan *Request, error)
// Reject rejects the channel creation request. After calling
// this, no other methods on the Channel may be called.
Reject(reason RejectionReason, message string) error
// ChannelType returns the type of the channel, as supplied by the
// client.
ChannelType() string
// ExtraData returns the arbitrary payload for this channel, as supplied
// by the client. This data is specific to the channel type.
ExtraData() []byte
}
// A Channel is an ordered, reliable, flow-controlled, duplex stream
// that is multiplexed over an SSH connection.
type Channel interface {
// Read reads up to len(data) bytes from the channel.
Read(data []byte) (int, error)
// Write writes len(data) bytes to the channel.
Write(data []byte) (int, error)
// Close signals end of channel use. No data may be sent after this
// call.
Close() error
// CloseWrite signals the end of sending in-band
// data. Requests may still be sent, and the other side may
// still send data
CloseWrite() error
// SendRequest sends a channel request. If wantReply is true,
// it will wait for a reply and return the result as a
// boolean, otherwise the return value will be false. Channel
// requests are out-of-band messages so they may be sent even
// if the data stream is closed or blocked by flow control.
// If the channel is closed before a reply is returned, io.EOF
// is returned.
SendRequest(name string, wantReply bool, payload []byte) (bool, error)
// Stderr returns an io.ReadWriter that writes to this channel
// with the extended data type set to stderr. Stderr may
// safely be read and written from a different goroutine than
// Read and Write respectively.
Stderr() io.ReadWriter
}
// Request is a request sent outside of the normal stream of
// data. Requests can either be specific to an SSH channel, or they
// can be global.
type Request struct {
Type string
WantReply bool
Payload []byte
ch *channel
mux *mux
}
// Reply sends a response to a request. It must be called for all requests
// where WantReply is true and is a no-op otherwise. The payload argument is
// ignored for replies to channel-specific requests.
func (r *Request) Reply(ok bool, payload []byte) error {
if !r.WantReply {
return nil
}
if r.ch == nil {
return r.mux.ackRequest(ok, payload)
}
return r.ch.ackRequest(ok)
}
// RejectionReason is an enumeration used when rejecting channel creation
// requests. See RFC 4254, section 5.1.
type RejectionReason uint32
const (
Prohibited RejectionReason = iota + 1
ConnectionFailed
UnknownChannelType
ResourceShortage
)
// String converts the rejection reason to human readable form.
func (r RejectionReason) String() string {
switch r {
case Prohibited:
return "administratively prohibited"
case ConnectionFailed:
return "connect failed"
case UnknownChannelType:
return "unknown channel type"
case ResourceShortage:
return "resource shortage"
}
return fmt.Sprintf("unknown reason %d", int(r))
}
func min(a uint32, b int) uint32 {
if a < uint32(b) {
return a
}
return uint32(b)
}
type channelDirection uint8
const (
channelInbound channelDirection = iota
channelOutbound
)
// channel is an implementation of the Channel interface that works
// with the mux class.
type channel struct {
// R/O after creation
chanType string
extraData []byte
localId, remoteId uint32
// maxIncomingPayload and maxRemotePayload are the maximum
// payload sizes of normal and extended data packets for
// receiving and sending, respectively. The wire packet will
// be 9 or 13 bytes larger (excluding encryption overhead).
maxIncomingPayload uint32
maxRemotePayload uint32
mux *mux
// decided is set to true if an accept or reject message has been sent
// (for outbound channels) or received (for inbound channels).
decided bool
// direction contains either channelOutbound, for channels created
// locally, or channelInbound, for channels created by the peer.
direction channelDirection
// Pending internal channel messages.
msg chan interface{}
// Since requests have no ID, there can be only one request
// with WantReply=true outstanding. This lock is held by a
// goroutine that has such an outgoing request pending.
sentRequestMu sync.Mutex
incomingRequests chan *Request
sentEOF bool
// thread-safe data
remoteWin window
pending *buffer
extPending *buffer
// windowMu protects myWindow, the flow-control window.
windowMu sync.Mutex
myWindow uint32
// writeMu serializes calls to mux.conn.writePacket() and
// protects sentClose and packetPool. This mutex must be
// different from windowMu, as writePacket can block if there
// is a key exchange pending.
writeMu sync.Mutex
sentClose bool
// packetPool has a buffer for each extended channel ID to
// save allocations during writes.
packetPool map[uint32][]byte
}
// writePacket sends a packet. If the packet is a channel close, it updates
// sentClose. This method takes the lock c.writeMu.
func (ch *channel) writePacket(packet []byte) error {
ch.writeMu.Lock()
if ch.sentClose {
ch.writeMu.Unlock()
return io.EOF
}
ch.sentClose = (packet[0] == msgChannelClose)
err := ch.mux.conn.writePacket(packet)
ch.writeMu.Unlock()
return err
}
func (ch *channel) sendMessage(msg interface{}) error {
if debugMux {
log.Printf("send(%d): %#v", ch.mux.chanList.offset, msg)
}
p := Marshal(msg)
binary.BigEndian.PutUint32(p[1:], ch.remoteId)
return ch.writePacket(p)
}
// WriteExtended writes data to a specific extended stream. These streams are
// used, for example, for stderr.
func (ch *channel) WriteExtended(data []byte, extendedCode uint32) (n int, err error) {
if ch.sentEOF {
return 0, io.EOF
}
// 1 byte message type, 4 bytes remoteId, 4 bytes data length
opCode := byte(msgChannelData)
headerLength := uint32(9)
if extendedCode > 0 {
headerLength += 4
opCode = msgChannelExtendedData
}
ch.writeMu.Lock()
packet := ch.packetPool[extendedCode]
// We don't remove the buffer from packetPool, so
// WriteExtended calls from different goroutines will be
// flagged as errors by the race detector.
ch.writeMu.Unlock()
for len(data) > 0 {
space := min(ch.maxRemotePayload, len(data))
if space, err = ch.remoteWin.reserve(space); err != nil {
return n, err
}
if want := headerLength + space; uint32(cap(packet)) < want {
packet = make([]byte, want)
} else {
packet = packet[:want]
}
todo := data[:space]
packet[0] = opCode
binary.BigEndian.PutUint32(packet[1:], ch.remoteId)
if extendedCode > 0 {
binary.BigEndian.PutUint32(packet[5:], uint32(extendedCode))
}
binary.BigEndian.PutUint32(packet[headerLength-4:], uint32(len(todo)))
copy(packet[headerLength:], todo)
if err = ch.writePacket(packet); err != nil {
return n, err
}
n += len(todo)
data = data[len(todo):]
}
ch.writeMu.Lock()
ch.packetPool[extendedCode] = packet
ch.writeMu.Unlock()
return n, err
}
func (ch *channel) handleData(packet []byte) error {
headerLen := 9
isExtendedData := packet[0] == msgChannelExtendedData
if isExtendedData {
headerLen = 13
}
if len(packet) < headerLen {
// malformed data packet
return parseError(packet[0])
}
var extended uint32
if isExtendedData {
extended = binary.BigEndian.Uint32(packet[5:])
}
length := binary.BigEndian.Uint32(packet[headerLen-4 : headerLen])
if length == 0 {
return nil
}
if length > ch.maxIncomingPayload {
// TODO(hanwen): should send Disconnect?
return errors.New("ssh: incoming packet exceeds maximum payload size")
}
data := packet[headerLen:]
if length != uint32(len(data)) {
return errors.New("ssh: wrong packet length")
}
ch.windowMu.Lock()
if ch.myWindow < length {
ch.windowMu.Unlock()
// TODO(hanwen): should send Disconnect with reason?
return errors.New("ssh: remote side wrote too much")
}
ch.myWindow -= length
ch.windowMu.Unlock()
if extended == 1 {
ch.extPending.write(data)
} else if extended > 0 {
// discard other extended data.
} else {
ch.pending.write(data)
}
return nil
}
func (c *channel) adjustWindow(n uint32) error {
c.windowMu.Lock()
// Since myWindow is managed on our side, and can never exceed
// the initial window setting, we don't worry about overflow.
c.myWindow += uint32(n)
c.windowMu.Unlock()
return c.sendMessage(windowAdjustMsg{
AdditionalBytes: uint32(n),
})
}
func (c *channel) ReadExtended(data []byte, extended uint32) (n int, err error) {
switch extended {
case 1:
n, err = c.extPending.Read(data)
case 0:
n, err = c.pending.Read(data)
default:
return 0, fmt.Errorf("ssh: extended code %d unimplemented", extended)
}
if n > 0 {
err = c.adjustWindow(uint32(n))
// sendWindowAdjust can return io.EOF if the remote
// peer has closed the connection, however we want to
// defer forwarding io.EOF to the caller of Read until
// the buffer has been drained.
if n > 0 && err == io.EOF {
err = nil
}
}
return n, err
}
func (c *channel) close() {
c.pending.eof()
c.extPending.eof()
close(c.msg)
close(c.incomingRequests)
c.writeMu.Lock()
// This is not necessary for a normal channel teardown, but if
// there was another error, it is.
c.sentClose = true
c.writeMu.Unlock()
// Unblock writers.
c.remoteWin.close()
}
// responseMessageReceived is called when a success or failure message is
// received on a channel to check that such a message is reasonable for the
// given channel.
func (ch *channel) responseMessageReceived() error {
if ch.direction == channelInbound {
return errors.New("ssh: channel response message received on inbound channel")
}
if ch.decided {
return errors.New("ssh: duplicate response received for channel")
}
ch.decided = true
return nil
}
func (ch *channel) handlePacket(packet []byte) error {
switch packet[0] {
case msgChannelData, msgChannelExtendedData:
return ch.handleData(packet)
case msgChannelClose:
ch.sendMessage(channelCloseMsg{PeersID: ch.remoteId})
ch.mux.chanList.remove(ch.localId)
ch.close()
return nil
case msgChannelEOF:
// RFC 4254 is mute on how EOF affects dataExt messages but
// it is logical to signal EOF at the same time.
ch.extPending.eof()
ch.pending.eof()
return nil
}
decoded, err := decode(packet)
if err != nil {
return err
}
switch msg := decoded.(type) {
case *channelOpenFailureMsg:
if err := ch.responseMessageReceived(); err != nil {
return err
}
ch.mux.chanList.remove(msg.PeersID)
ch.msg <- msg
case *channelOpenConfirmMsg:
if err := ch.responseMessageReceived(); err != nil {
return err
}
if msg.MaxPacketSize < minPacketLength || msg.MaxPacketSize > 1<<31 {
return fmt.Errorf("ssh: invalid MaxPacketSize %d from peer", msg.MaxPacketSize)
}
ch.remoteId = msg.MyID
ch.maxRemotePayload = msg.MaxPacketSize
ch.remoteWin.add(msg.MyWindow)
ch.msg <- msg
case *windowAdjustMsg:
if !ch.remoteWin.add(msg.AdditionalBytes) {
return fmt.Errorf("ssh: invalid window update for %d bytes", msg.AdditionalBytes)
}
case *channelRequestMsg:
req := Request{
Type: msg.Request,
WantReply: msg.WantReply,
Payload: msg.RequestSpecificData,
ch: ch,
}
ch.incomingRequests <- &req
default:
ch.msg <- msg
}
return nil
}
func (m *mux) newChannel(chanType string, direction channelDirection, extraData []byte) *channel {
ch := &channel{
remoteWin: window{Cond: newCond()},
myWindow: channelWindowSize,
pending: newBuffer(),
extPending: newBuffer(),
direction: direction,
incomingRequests: make(chan *Request, chanSize),
msg: make(chan interface{}, chanSize),
chanType: chanType,
extraData: extraData,
mux: m,
packetPool: make(map[uint32][]byte),
}
ch.localId = m.chanList.add(ch)
return ch
}
var errUndecided = errors.New("ssh: must Accept or Reject channel")
var errDecidedAlready = errors.New("ssh: can call Accept or Reject only once")
type extChannel struct {
code uint32
ch *channel
}
func (e *extChannel) Write(data []byte) (n int, err error) {
return e.ch.WriteExtended(data, e.code)
}
func (e *extChannel) Read(data []byte) (n int, err error) {
return e.ch.ReadExtended(data, e.code)
}
func (ch *channel) Accept() (Channel, <-chan *Request, error) {
if ch.decided {
return nil, nil, errDecidedAlready
}
ch.maxIncomingPayload = channelMaxPacket
confirm := channelOpenConfirmMsg{
PeersID: ch.remoteId,
MyID: ch.localId,
MyWindow: ch.myWindow,
MaxPacketSize: ch.maxIncomingPayload,
}
ch.decided = true
if err := ch.sendMessage(confirm); err != nil {
return nil, nil, err
}
return ch, ch.incomingRequests, nil
}
func (ch *channel) Reject(reason RejectionReason, message string) error {
if ch.decided {
return errDecidedAlready
}
reject := channelOpenFailureMsg{
PeersID: ch.remoteId,
Reason: reason,
Message: message,
Language: "en",
}
ch.decided = true
return ch.sendMessage(reject)
}
func (ch *channel) Read(data []byte) (int, error) {
if !ch.decided {
return 0, errUndecided
}
return ch.ReadExtended(data, 0)
}
func (ch *channel) Write(data []byte) (int, error) {
if !ch.decided {
return 0, errUndecided
}
return ch.WriteExtended(data, 0)
}
func (ch *channel) CloseWrite() error {
if !ch.decided {
return errUndecided
}
ch.sentEOF = true
return ch.sendMessage(channelEOFMsg{
PeersID: ch.remoteId})
}
func (ch *channel) Close() error {
if !ch.decided {
return errUndecided
}
return ch.sendMessage(channelCloseMsg{
PeersID: ch.remoteId})
}
// Extended returns an io.ReadWriter that sends and receives data on the given,
// SSH extended stream. Such streams are used, for example, for stderr.
func (ch *channel) Extended(code uint32) io.ReadWriter {
if !ch.decided {
return nil
}
return &extChannel{code, ch}
}
func (ch *channel) Stderr() io.ReadWriter {
return ch.Extended(1)
}
func (ch *channel) SendRequest(name string, wantReply bool, payload []byte) (bool, error) {
if !ch.decided {
return false, errUndecided
}
if wantReply {
ch.sentRequestMu.Lock()
defer ch.sentRequestMu.Unlock()
}
msg := channelRequestMsg{
PeersID: ch.remoteId,
Request: name,
WantReply: wantReply,
RequestSpecificData: payload,
}
if err := ch.sendMessage(msg); err != nil {
return false, err
}
if wantReply {
m, ok := (<-ch.msg)
if !ok {
return false, io.EOF
}
switch m.(type) {
case *channelRequestFailureMsg:
return false, nil
case *channelRequestSuccessMsg:
return true, nil
default:
return false, fmt.Errorf("ssh: unexpected response to channel request: %#v", m)
}
}
return false, nil
}
// ackRequest either sends an ack or nack to the channel request.
func (ch *channel) ackRequest(ok bool) error {
if !ch.decided {
return errUndecided
}
var msg interface{}
if !ok {
msg = channelRequestFailureMsg{
PeersID: ch.remoteId,
}
} else {
msg = channelRequestSuccessMsg{
PeersID: ch.remoteId,
}
}
return ch.sendMessage(msg)
}
func (ch *channel) ChannelType() string {
return ch.chanType
}
func (ch *channel) ExtraData() []byte {
return ch.extraData
}