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tailscale/ssh/tailssh/tailssh.go

1431 lines
39 KiB
Go

// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
//go:build linux || (darwin && !ios)
// +build linux darwin,!ios
// Package tailssh is an SSH server integrated into Tailscale.
package tailssh
import (
"bytes"
"context"
"crypto/rand"
"encoding/base64"
"encoding/json"
"errors"
"fmt"
"io"
"net"
"net/http"
"net/netip"
"net/url"
"os"
"os/exec"
"os/user"
"path/filepath"
"runtime"
"strconv"
"strings"
"sync"
"time"
gossh "github.com/tailscale/golang-x-crypto/ssh"
"tailscale.com/envknob"
"tailscale.com/ipn/ipnlocal"
"tailscale.com/logtail/backoff"
"tailscale.com/net/tsaddr"
"tailscale.com/tailcfg"
"tailscale.com/tempfork/gliderlabs/ssh"
"tailscale.com/types/logger"
"tailscale.com/types/netmap"
"tailscale.com/util/clientmetric"
"tailscale.com/util/mak"
)
var (
sshVerboseLogging = envknob.RegisterBool("TS_DEBUG_SSH_VLOG")
)
// ipnLocalBackend is the subset of ipnlocal.LocalBackend that we use.
// It is used for testing.
type ipnLocalBackend interface {
GetSSH_HostKeys() ([]gossh.Signer, error)
ShouldRunSSH() bool
NetMap() *netmap.NetworkMap
WhoIs(ipp netip.AddrPort) (n *tailcfg.Node, u tailcfg.UserProfile, ok bool)
DoNoiseRequest(req *http.Request) (*http.Response, error)
TailscaleVarRoot() string
}
type server struct {
lb ipnLocalBackend
logf logger.Logf
tailscaledPath string
pubKeyHTTPClient *http.Client // or nil for http.DefaultClient
timeNow func() time.Time // or nil for time.Now
sessionWaitGroup sync.WaitGroup
// mu protects the following
mu sync.Mutex
activeConns map[*conn]bool // set; value is always true
fetchPublicKeysCache map[string]pubKeyCacheEntry // by https URL
shutdownCalled bool
}
func (srv *server) now() time.Time {
if srv != nil && srv.timeNow != nil {
return srv.timeNow()
}
return time.Now()
}
func init() {
ipnlocal.RegisterNewSSHServer(func(logf logger.Logf, lb *ipnlocal.LocalBackend) (ipnlocal.SSHServer, error) {
tsd, err := os.Executable()
if err != nil {
return nil, err
}
srv := &server{
lb: lb,
logf: logf,
tailscaledPath: tsd,
}
return srv, nil
})
}
// attachSessionToConnIfNotShutdown ensures that srv is not shutdown before
// attaching the session to the conn. This ensures that once Shutdown is called,
// new sessions are not allowed and existing ones are cleaned up.
// It reports whether ss was attached to the conn.
func (srv *server) attachSessionToConnIfNotShutdown(ss *sshSession) bool {
srv.mu.Lock()
defer srv.mu.Unlock()
if srv.shutdownCalled {
// Do not start any new sessions.
return false
}
ss.conn.attachSession(ss)
return true
}
func (srv *server) trackActiveConn(c *conn, add bool) {
srv.mu.Lock()
defer srv.mu.Unlock()
if add {
mak.Set(&srv.activeConns, c, true)
return
}
delete(srv.activeConns, c)
}
// HandleSSHConn handles a Tailscale SSH connection from c.
// This is the entry point for all SSH connections.
// When this returns, the connection is closed.
func (srv *server) HandleSSHConn(nc net.Conn) error {
metricIncomingConnections.Add(1)
c, err := srv.newConn()
if err != nil {
return err
}
srv.trackActiveConn(c, true) // add
defer srv.trackActiveConn(c, false) // remove
c.HandleConn(nc)
// Return nil to signal to netstack's interception that it doesn't need to
// log. If ss.HandleConn had problems, it can log itself (ideally on an
// sshSession.logf).
return nil
}
// Shutdown terminates all active sessions.
func (srv *server) Shutdown() {
srv.mu.Lock()
srv.shutdownCalled = true
for c := range srv.activeConns {
c.Close()
}
srv.mu.Unlock()
srv.sessionWaitGroup.Wait()
}
// OnPolicyChange terminates any active sessions that no longer match
// the SSH access policy.
func (srv *server) OnPolicyChange() {
srv.mu.Lock()
defer srv.mu.Unlock()
for c := range srv.activeConns {
if c.info == nil {
// c.info is nil when the connection hasn't been authenticated yet.
// In that case, the connection will be terminated when it is.
continue
}
go c.checkStillValid()
}
}
// conn represents a single SSH connection and its associated
// ssh.Server.
//
// During the lifecycle of a connection, the following are called in order:
// Setup and discover server info
// - ServerConfigCallback
//
// Do the user auth
// - NoClientAuthHandler
// - PublicKeyHandler (only if NoClientAuthHandler returns errPubKeyRequired)
//
// Once auth is done, the conn can be multiplexed with multiple sessions and
// channels concurrently. At which point any of the following can be called
// in any order.
// - c.handleSessionPostSSHAuth
// - c.mayForwardLocalPortTo followed by ssh.DirectTCPIPHandler
type conn struct {
*ssh.Server
srv *server
insecureSkipTailscaleAuth bool // used by tests.
// idH is the RFC4253 sec8 hash H. It is used to identify the connection,
// and is shared among all sessions. It should not be shared outside
// process. It is confusingly referred to as SessionID by the gliderlabs/ssh
// library.
idH string
connID string // ID that's shared with control
noPubKeyPolicyAuthError error // set by BannerCallback
action0 *tailcfg.SSHAction // set by doPolicyAuth; first matching action
currentAction *tailcfg.SSHAction // set by doPolicyAuth, updated by resolveNextAction
finalAction *tailcfg.SSHAction // set by doPolicyAuth or resolveNextAction
finalActionErr error // set by doPolicyAuth or resolveNextAction
info *sshConnInfo // set by setInfo
localUser *user.User // set by doPolicyAuth
userGroupIDs []string // set by doPolicyAuth
pubKey gossh.PublicKey // set by doPolicyAuth
// mu protects the following fields.
//
// srv.mu should be acquired prior to mu.
// It is safe to just acquire mu, but unsafe to
// acquire mu and then srv.mu.
mu sync.Mutex // protects the following
sessions []*sshSession
}
func (c *conn) logf(format string, args ...any) {
format = fmt.Sprintf("%v: %v", c.connID, format)
c.srv.logf(format, args...)
}
// isAuthorized walks through the action chain and returns nil if the connection
// is authorized. If the connection is not authorized, it returns
// gossh.ErrDenied. If the action chain resolution fails, it returns the
// resolution error.
func (c *conn) isAuthorized(ctx ssh.Context) error {
action := c.currentAction
for {
if action.Accept {
if c.pubKey != nil {
metricPublicKeyAccepts.Add(1)
}
return nil
}
if action.Reject || action.HoldAndDelegate == "" {
return gossh.ErrDenied
}
var err error
action, err = c.resolveNextAction(ctx)
if err != nil {
return err
}
if action.Message != "" {
if err := ctx.SendAuthBanner(action.Message); err != nil {
return err
}
}
}
}
// errPubKeyRequired is returned by NoClientAuthCallback to make the client
// resort to public-key auth; not user visible.
var errPubKeyRequired = errors.New("ssh publickey required")
// NoClientAuthCallback implements gossh.NoClientAuthCallback and is called by
// the ssh.Server when the client first connects with the "none"
// authentication method.
//
// It is responsible for continuing policy evaluation from BannerCallback (or
// starting it afresh). It returns an error if the policy evaluation fails, or
// if the decision is "reject"
//
// It either returns nil (accept) or errPubKeyRequired or gossh.ErrDenied
// (reject). The errors may be wrapped.
func (c *conn) NoClientAuthCallback(ctx ssh.Context) error {
if c.insecureSkipTailscaleAuth {
return nil
}
if err := c.doPolicyAuth(ctx, nil /* no pub key */); err != nil {
return err
}
return c.isAuthorized(ctx)
}
// PublicKeyHandler implements ssh.PublicKeyHandler is called by the
// ssh.Server when the client presents a public key.
func (c *conn) PublicKeyHandler(ctx ssh.Context, pubKey ssh.PublicKey) error {
if err := c.doPolicyAuth(ctx, pubKey); err != nil {
// TODO(maisem/bradfitz): surface the error here.
c.logf("rejecting SSH public key %s: %v", bytes.TrimSpace(gossh.MarshalAuthorizedKey(pubKey)), err)
return err
}
if err := c.isAuthorized(ctx); err != nil {
return err
}
c.logf("accepting SSH public key %s", bytes.TrimSpace(gossh.MarshalAuthorizedKey(pubKey)))
return nil
}
// doPolicyAuth verifies that conn can proceed with the specified (optional)
// pubKey. It returns nil if the matching policy action is Accept or
// HoldAndDelegate. If pubKey is nil, there was no policy match but there is a
// policy that might match a public key it returns errPubKeyRequired. Otherwise,
// it returns gossh.ErrDenied.
func (c *conn) doPolicyAuth(ctx ssh.Context, pubKey ssh.PublicKey) error {
if err := c.setInfo(ctx); err != nil {
c.logf("failed to get conninfo: %v", err)
return gossh.ErrDenied
}
a, localUser, err := c.evaluatePolicy(pubKey)
if err != nil {
if pubKey == nil && c.havePubKeyPolicy() {
return errPubKeyRequired
}
return fmt.Errorf("%w: %v", gossh.ErrDenied, err)
}
c.action0 = a
c.currentAction = a
c.pubKey = pubKey
if a.Message != "" {
if err := ctx.SendAuthBanner(a.Message); err != nil {
return fmt.Errorf("SendBanner: %w", err)
}
}
if a.Accept || a.HoldAndDelegate != "" {
if a.Accept {
c.finalAction = a
}
lu, err := user.Lookup(localUser)
if err != nil {
c.logf("failed to lookup %v: %v", localUser, err)
ctx.SendAuthBanner(fmt.Sprintf("failed to lookup %v\r\n", localUser))
return err
}
gids, err := lu.GroupIds()
if err != nil {
return err
}
c.userGroupIDs = gids
c.localUser = lu
return nil
}
if a.Reject {
c.finalAction = a
return gossh.ErrDenied
}
// Shouldn't get here, but:
return gossh.ErrDenied
}
// ServerConfig implements ssh.ServerConfigCallback.
func (c *conn) ServerConfig(ctx ssh.Context) *gossh.ServerConfig {
return &gossh.ServerConfig{
// OpenSSH presents this on failure as `Permission denied (tailscale).`
ImplicitAuthMethod: "tailscale",
NoClientAuth: true, // required for the NoClientAuthCallback to run
}
}
func (srv *server) newConn() (*conn, error) {
srv.mu.Lock()
if srv.shutdownCalled {
srv.mu.Unlock()
// Stop accepting new connections.
// Connections in the auth phase are handled in handleConnPostSSHAuth.
// Existing sessions are terminated by Shutdown.
return nil, gossh.ErrDenied
}
srv.mu.Unlock()
c := &conn{srv: srv}
now := srv.now()
c.connID = fmt.Sprintf("ssh-conn-%s-%02x", now.UTC().Format("20060102T150405"), randBytes(5))
c.Server = &ssh.Server{
Version: "Tailscale",
ServerConfigCallback: c.ServerConfig,
NoClientAuthHandler: c.NoClientAuthCallback,
PublicKeyHandler: c.PublicKeyHandler,
Handler: c.handleSessionPostSSHAuth,
LocalPortForwardingCallback: c.mayForwardLocalPortTo,
SubsystemHandlers: map[string]ssh.SubsystemHandler{
"sftp": c.handleSessionPostSSHAuth,
},
// Note: the direct-tcpip channel handler and LocalPortForwardingCallback
// only adds support for forwarding ports from the local machine.
// TODO(maisem/bradfitz): add remote port forwarding support.
ChannelHandlers: map[string]ssh.ChannelHandler{
"direct-tcpip": ssh.DirectTCPIPHandler,
},
RequestHandlers: map[string]ssh.RequestHandler{},
}
ss := c.Server
for k, v := range ssh.DefaultRequestHandlers {
ss.RequestHandlers[k] = v
}
for k, v := range ssh.DefaultChannelHandlers {
ss.ChannelHandlers[k] = v
}
for k, v := range ssh.DefaultSubsystemHandlers {
ss.SubsystemHandlers[k] = v
}
keys, err := srv.lb.GetSSH_HostKeys()
if err != nil {
return nil, err
}
for _, signer := range keys {
ss.AddHostKey(signer)
}
return c, nil
}
// mayForwardLocalPortTo reports whether the ctx should be allowed to port forward
// to the specified host and port.
// TODO(bradfitz/maisem): should we have more checks on host/port?
func (c *conn) mayForwardLocalPortTo(ctx ssh.Context, destinationHost string, destinationPort uint32) bool {
if c.finalAction != nil && c.finalAction.AllowLocalPortForwarding {
metricLocalPortForward.Add(1)
return true
}
return false
}
// havePubKeyPolicy reports whether any policy rule may provide access by means
// of a ssh.PublicKey.
func (c *conn) havePubKeyPolicy() bool {
if c.info == nil {
panic("havePubKeyPolicy called before setInfo")
}
// Is there any rule that looks like it'd require a public key for this
// sshUser?
pol, ok := c.sshPolicy()
if !ok {
return false
}
for _, r := range pol.Rules {
if c.ruleExpired(r) {
continue
}
if mapLocalUser(r.SSHUsers, c.info.sshUser) == "" {
continue
}
for _, p := range r.Principals {
if len(p.PubKeys) > 0 && c.principalMatchesTailscaleIdentity(p) {
return true
}
}
}
return false
}
// sshPolicy returns the SSHPolicy for current node.
// If there is no SSHPolicy in the netmap, it returns a debugPolicy
// if one is defined.
func (c *conn) sshPolicy() (_ *tailcfg.SSHPolicy, ok bool) {
lb := c.srv.lb
if !lb.ShouldRunSSH() {
return nil, false
}
nm := lb.NetMap()
if nm == nil {
return nil, false
}
if pol := nm.SSHPolicy; pol != nil && !envknob.SSHIgnoreTailnetPolicy() {
return pol, true
}
debugPolicyFile := envknob.SSHPolicyFile()
if debugPolicyFile != "" {
c.logf("reading debug SSH policy file: %v", debugPolicyFile)
f, err := os.ReadFile(debugPolicyFile)
if err != nil {
c.logf("error reading debug SSH policy file: %v", err)
return nil, false
}
p := new(tailcfg.SSHPolicy)
if err := json.Unmarshal(f, p); err != nil {
c.logf("invalid JSON in %v: %v", debugPolicyFile, err)
return nil, false
}
return p, true
}
return nil, false
}
func toIPPort(a net.Addr) (ipp netip.AddrPort) {
ta, ok := a.(*net.TCPAddr)
if !ok {
return
}
tanetaddr, ok := netip.AddrFromSlice(ta.IP)
if !ok {
return
}
return netip.AddrPortFrom(tanetaddr.Unmap(), uint16(ta.Port))
}
// connInfo returns a populated sshConnInfo from the provided arguments,
// validating only that they represent a known Tailscale identity.
func (c *conn) setInfo(ctx ssh.Context) error {
if c.info != nil {
return nil
}
ci := &sshConnInfo{
sshUser: ctx.User(),
src: toIPPort(ctx.RemoteAddr()),
dst: toIPPort(ctx.LocalAddr()),
}
if !tsaddr.IsTailscaleIP(ci.dst.Addr()) {
return fmt.Errorf("tailssh: rejecting non-Tailscale local address %v", ci.dst)
}
if !tsaddr.IsTailscaleIP(ci.src.Addr()) {
return fmt.Errorf("tailssh: rejecting non-Tailscale remote address %v", ci.src)
}
node, uprof, ok := c.srv.lb.WhoIs(ci.src)
if !ok {
return fmt.Errorf("unknown Tailscale identity from src %v", ci.src)
}
ci.node = node
ci.uprof = uprof
c.idH = ctx.SessionID()
c.info = ci
c.logf("handling conn: %v", ci.String())
return nil
}
// evaluatePolicy returns the SSHAction and localUser after evaluating
// the SSHPolicy for this conn. The pubKey may be nil for "none" auth.
func (c *conn) evaluatePolicy(pubKey gossh.PublicKey) (_ *tailcfg.SSHAction, localUser string, _ error) {
pol, ok := c.sshPolicy()
if !ok {
return nil, "", fmt.Errorf("tailssh: rejecting connection; no SSH policy")
}
a, localUser, ok := c.evalSSHPolicy(pol, pubKey)
if !ok {
return nil, "", fmt.Errorf("tailssh: rejecting connection; no matching policy")
}
return a, localUser, nil
}
// pubKeyCacheEntry is the cache value for an HTTPS URL of public keys (like
// "https://github.com/foo.keys")
type pubKeyCacheEntry struct {
lines []string
etag string // if sent by server
at time.Time
}
const (
pubKeyCacheDuration = time.Minute // how long to cache non-empty public keys
pubKeyCacheEmptyDuration = 15 * time.Second // how long to cache empty responses
)
func (srv *server) fetchPublicKeysURLCached(url string) (ce pubKeyCacheEntry, ok bool) {
srv.mu.Lock()
defer srv.mu.Unlock()
// Mostly don't care about the size of this cache. Clean rarely.
if m := srv.fetchPublicKeysCache; len(m) > 50 {
tooOld := srv.now().Add(pubKeyCacheDuration * 10)
for k, ce := range m {
if ce.at.Before(tooOld) {
delete(m, k)
}
}
}
ce, ok = srv.fetchPublicKeysCache[url]
if !ok {
return ce, false
}
maxAge := pubKeyCacheDuration
if len(ce.lines) == 0 {
maxAge = pubKeyCacheEmptyDuration
}
return ce, srv.now().Sub(ce.at) < maxAge
}
func (srv *server) pubKeyClient() *http.Client {
if srv.pubKeyHTTPClient != nil {
return srv.pubKeyHTTPClient
}
return http.DefaultClient
}
// fetchPublicKeysURL fetches the public keys from a URL. The strings are in the
// the typical public key "type base64-string [comment]" format seen at e.g.
// https://github.com/USER.keys
func (srv *server) fetchPublicKeysURL(url string) ([]string, error) {
if !strings.HasPrefix(url, "https://") {
return nil, errors.New("invalid URL scheme")
}
ce, ok := srv.fetchPublicKeysURLCached(url)
if ok {
return ce.lines, nil
}
ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
defer cancel()
req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
if err != nil {
return nil, err
}
if ce.etag != "" {
req.Header.Add("If-None-Match", ce.etag)
}
res, err := srv.pubKeyClient().Do(req)
if err != nil {
return nil, err
}
defer res.Body.Close()
var lines []string
var etag string
switch res.StatusCode {
default:
err = fmt.Errorf("unexpected status %v", res.Status)
srv.logf("fetching public keys from %s: %v", url, err)
case http.StatusNotModified:
lines = ce.lines
etag = ce.etag
case http.StatusOK:
var all []byte
all, err = io.ReadAll(io.LimitReader(res.Body, 4<<10))
if s := strings.TrimSpace(string(all)); s != "" {
lines = strings.Split(s, "\n")
}
etag = res.Header.Get("Etag")
}
srv.mu.Lock()
defer srv.mu.Unlock()
mak.Set(&srv.fetchPublicKeysCache, url, pubKeyCacheEntry{
at: srv.now(),
lines: lines,
etag: etag,
})
return lines, err
}
// handleSessionPostSSHAuth runs an SSH session after the SSH-level authentication,
// but not necessarily before all the Tailscale-level extra verification has
// completed. It also handles SFTP requests.
func (c *conn) handleSessionPostSSHAuth(s ssh.Session) {
// Do this check after auth, but before starting the session.
switch s.Subsystem() {
case "sftp", "":
metricSFTP.Add(1)
default:
fmt.Fprintf(s.Stderr(), "Unsupported subsystem %q\r\n", s.Subsystem())
s.Exit(1)
return
}
ss := c.newSSHSession(s)
ss.logf("handling new SSH connection from %v (%v) to ssh-user %q", c.info.uprof.LoginName, c.info.src.Addr(), c.localUser.Username)
ss.logf("access granted to %v as ssh-user %q", c.info.uprof.LoginName, c.localUser.Username)
ss.run()
}
// resolveNextAction starts at c.currentAction and makes it way through the
// action chain one step at a time. An action without a HoldAndDelegate is
// considered the final action. Once a final action is reached, this function
// will keep returning that action. It updates c.currentAction to the next
// action in the chain. When the final action is reached, it also sets
// c.finalAction to the final action.
func (c *conn) resolveNextAction(sctx ssh.Context) (action *tailcfg.SSHAction, err error) {
if c.finalAction != nil || c.finalActionErr != nil {
return c.finalAction, c.finalActionErr
}
defer func() {
if action != nil {
c.currentAction = action
if action.Accept || action.Reject {
c.finalAction = action
}
}
if err != nil {
c.finalActionErr = err
}
}()
ctx, cancel := context.WithCancel(sctx)
defer cancel()
// Loop processing/fetching Actions until one reaches a
// terminal state (Accept, Reject, or invalid Action), or
// until fetchSSHAction times out due to the context being
// done (client disconnect) or its 30 minute timeout passes.
// (Which is a long time for somebody to see login
// instructions and go to a URL to do something.)
action = c.currentAction
if action.Accept || action.Reject {
if action.Reject {
metricTerminalReject.Add(1)
} else {
metricTerminalAccept.Add(1)
}
return action, nil
}
url := action.HoldAndDelegate
if url == "" {
metricTerminalMalformed.Add(1)
return nil, errors.New("reached Action that lacked Accept, Reject, and HoldAndDelegate")
}
metricHolds.Add(1)
url = c.expandDelegateURLLocked(url)
nextAction, err := c.fetchSSHAction(ctx, url)
if err != nil {
metricTerminalFetchError.Add(1)
return nil, fmt.Errorf("fetching SSHAction from %s: %w", url, err)
}
return nextAction, nil
}
func (c *conn) expandDelegateURLLocked(actionURL string) string {
nm := c.srv.lb.NetMap()
ci := c.info
lu := c.localUser
var dstNodeID string
if nm != nil {
dstNodeID = fmt.Sprint(int64(nm.SelfNode.ID))
}
return strings.NewReplacer(
"$SRC_NODE_IP", url.QueryEscape(ci.src.Addr().String()),
"$SRC_NODE_ID", fmt.Sprint(int64(ci.node.ID)),
"$DST_NODE_IP", url.QueryEscape(ci.dst.Addr().String()),
"$DST_NODE_ID", dstNodeID,
"$SSH_USER", url.QueryEscape(ci.sshUser),
"$LOCAL_USER", url.QueryEscape(lu.Username),
).Replace(actionURL)
}
func (c *conn) expandPublicKeyURL(pubKeyURL string) string {
if !strings.Contains(pubKeyURL, "$") {
return pubKeyURL
}
loginName := c.info.uprof.LoginName
localPart, _, _ := strings.Cut(loginName, "@")
return strings.NewReplacer(
"$LOGINNAME_EMAIL", loginName,
"$LOGINNAME_LOCALPART", localPart,
).Replace(pubKeyURL)
}
// sshSession is an accepted Tailscale SSH session.
type sshSession struct {
ssh.Session
sharedID string // ID that's shared with control
logf logger.Logf
ctx *sshContext // implements context.Context
conn *conn
agentListener net.Listener // non-nil if agent-forwarding requested+allowed
// initialized by launchProcess:
cmd *exec.Cmd
stdin io.WriteCloser
stdout io.ReadCloser
stderr io.Reader // nil for pty sessions
ptyReq *ssh.Pty // non-nil for pty sessions
// We use this sync.Once to ensure that we only terminate the process once,
// either it exits itself or is terminated
exitOnce sync.Once
}
func (ss *sshSession) vlogf(format string, args ...interface{}) {
if sshVerboseLogging() {
ss.logf(format, args...)
}
}
func (c *conn) newSSHSession(s ssh.Session) *sshSession {
sharedID := fmt.Sprintf("sess-%s-%02x", c.srv.now().UTC().Format("20060102T150405"), randBytes(5))
c.logf("starting session: %v", sharedID)
return &sshSession{
Session: s,
sharedID: sharedID,
ctx: newSSHContext(s.Context()),
conn: c,
logf: logger.WithPrefix(c.srv.logf, "ssh-session("+sharedID+"): "),
}
}
// isStillValid reports whether the conn is still valid.
func (c *conn) isStillValid() bool {
a, localUser, err := c.evaluatePolicy(c.pubKey)
if err != nil {
return false
}
if !a.Accept && a.HoldAndDelegate == "" {
return false
}
return c.localUser.Username == localUser
}
// checkStillValid checks that the conn is still valid per the latest SSHPolicy.
// If not, it terminates all sessions associated with the conn.
func (c *conn) checkStillValid() {
if c.isStillValid() {
return
}
metricPolicyChangeKick.Add(1)
c.logf("session no longer valid per new SSH policy; closing")
c.mu.Lock()
defer c.mu.Unlock()
for _, s := range c.sessions {
s.ctx.CloseWithError(userVisibleError{
fmt.Sprintf("Access revoked.\r\n"),
context.Canceled,
})
}
}
func (c *conn) fetchSSHAction(ctx context.Context, url string) (*tailcfg.SSHAction, error) {
ctx, cancel := context.WithTimeout(ctx, 30*time.Minute)
defer cancel()
bo := backoff.NewBackoff("fetch-ssh-action", c.logf, 10*time.Second)
for {
if err := ctx.Err(); err != nil {
return nil, err
}
req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
if err != nil {
return nil, err
}
res, err := c.srv.lb.DoNoiseRequest(req)
if err != nil {
bo.BackOff(ctx, err)
continue
}
if res.StatusCode != 200 {
body, _ := io.ReadAll(res.Body)
res.Body.Close()
if len(body) > 1<<10 {
body = body[:1<<10]
}
c.logf("fetch of %v: %s, %s", url, res.Status, body)
bo.BackOff(ctx, fmt.Errorf("unexpected status: %v", res.Status))
continue
}
a := new(tailcfg.SSHAction)
err = json.NewDecoder(res.Body).Decode(a)
res.Body.Close()
if err != nil {
c.logf("invalid next SSHAction JSON from %v: %v", url, err)
bo.BackOff(ctx, err)
continue
}
return a, nil
}
}
// killProcessOnContextDone waits for ss.ctx to be done and kills the process,
// unless the process has already exited.
func (ss *sshSession) killProcessOnContextDone() {
<-ss.ctx.Done()
// Either the process has already exited, in which case this does nothing.
// Or, the process is still running in which case this will kill it.
ss.exitOnce.Do(func() {
err := ss.ctx.Err()
if serr, ok := err.(SSHTerminationError); ok {
msg := serr.SSHTerminationMessage()
if msg != "" {
io.WriteString(ss.Stderr(), "\r\n\r\n"+msg+"\r\n\r\n")
}
}
ss.logf("terminating SSH session from %v: %v", ss.conn.info.src.Addr(), err)
// We don't need to Process.Wait here, sshSession.run() does
// the waiting regardless of termination reason.
// TODO(maisem): should this be a SIGTERM followed by a SIGKILL?
ss.cmd.Process.Kill()
})
}
// attachSession registers ss as an active session.
func (c *conn) attachSession(ss *sshSession) {
c.srv.sessionWaitGroup.Add(1)
if ss.sharedID == "" {
panic("empty sharedID")
}
c.mu.Lock()
defer c.mu.Unlock()
c.sessions = append(c.sessions, ss)
}
// detachSession unregisters s from the list of active sessions.
func (c *conn) detachSession(ss *sshSession) {
defer c.srv.sessionWaitGroup.Done()
c.mu.Lock()
defer c.mu.Unlock()
for i, s := range c.sessions {
if s == ss {
c.sessions = append(c.sessions[:i], c.sessions[i+1:]...)
break
}
}
}
var errSessionDone = errors.New("session is done")
// handleSSHAgentForwarding starts a Unix socket listener and in the background
// forwards agent connections between the listener and the ssh.Session.
// On success, it assigns ss.agentListener.
func (ss *sshSession) handleSSHAgentForwarding(s ssh.Session, lu *user.User) error {
if !ssh.AgentRequested(ss) || !ss.conn.finalAction.AllowAgentForwarding {
return nil
}
ss.logf("ssh: agent forwarding requested")
ln, err := ssh.NewAgentListener()
if err != nil {
return err
}
defer func() {
if err != nil && ln != nil {
ln.Close()
}
}()
uid, err := strconv.ParseUint(lu.Uid, 10, 32)
if err != nil {
return err
}
gid, err := strconv.ParseUint(lu.Gid, 10, 32)
if err != nil {
return err
}
socket := ln.Addr().String()
dir := filepath.Dir(socket)
// Make sure the socket is accessible only by the user.
if err := os.Chmod(socket, 0600); err != nil {
return err
}
if err := os.Chown(socket, int(uid), int(gid)); err != nil {
return err
}
// Make sure the dir is also accessible.
if err := os.Chmod(dir, 0755); err != nil {
return err
}
go ssh.ForwardAgentConnections(ln, s)
ss.agentListener = ln
return nil
}
// recordSSH is a temporary dev knob to test the SSH recording
// functionality and support off-node streaming.
//
// TODO(bradfitz,maisem): move this to SSHPolicy.
var recordSSH = envknob.RegisterBool("TS_DEBUG_LOG_SSH")
// run is the entrypoint for a newly accepted SSH session.
//
// It handles ss once it's been accepted and determined
// that it should run.
func (ss *sshSession) run() {
metricActiveSessions.Add(1)
defer metricActiveSessions.Add(-1)
defer ss.ctx.CloseWithError(errSessionDone)
if attached := ss.conn.srv.attachSessionToConnIfNotShutdown(ss); !attached {
fmt.Fprintf(ss, "Tailscale SSH is shutting down\r\n")
ss.Exit(1)
return
}
defer ss.conn.detachSession(ss)
lu := ss.conn.localUser
logf := ss.logf
if ss.conn.finalAction.SessionDuration != 0 {
t := time.AfterFunc(ss.conn.finalAction.SessionDuration, func() {
ss.ctx.CloseWithError(userVisibleError{
fmt.Sprintf("Session timeout of %v elapsed.", ss.conn.finalAction.SessionDuration),
context.DeadlineExceeded,
})
})
defer t.Stop()
}
if euid := os.Geteuid(); euid != 0 {
if lu.Uid != fmt.Sprint(euid) {
ss.logf("can't switch to user %q from process euid %v", lu.Username, euid)
fmt.Fprintf(ss, "can't switch user\r\n")
ss.Exit(1)
return
}
}
// Take control of the PTY so that we can configure it below.
// See https://github.com/tailscale/tailscale/issues/4146
ss.DisablePTYEmulation()
var rec *recording // or nil if disabled
if ss.Subsystem() != "sftp" {
if err := ss.handleSSHAgentForwarding(ss, lu); err != nil {
ss.logf("agent forwarding failed: %v", err)
} else if ss.agentListener != nil {
// TODO(maisem/bradfitz): add a way to close all session resources
defer ss.agentListener.Close()
}
if ss.shouldRecord() {
var err error
rec, err = ss.startNewRecording()
if err != nil {
fmt.Fprintf(ss, "can't start new recording\r\n")
ss.logf("startNewRecording: %v", err)
ss.Exit(1)
return
}
defer rec.Close()
}
}
err := ss.launchProcess()
if err != nil {
logf("start failed: %v", err.Error())
ss.Exit(1)
return
}
go ss.killProcessOnContextDone()
go func() {
defer ss.stdin.Close()
if _, err := io.Copy(rec.writer("i", ss.stdin), ss); err != nil {
logf("stdin copy: %v", err)
ss.ctx.CloseWithError(err)
}
}()
go func() {
defer ss.stdout.Close()
_, err := io.Copy(rec.writer("o", ss), ss.stdout)
if err != nil && !errors.Is(err, io.EOF) {
logf("stdout copy: %v", err)
ss.ctx.CloseWithError(err)
} else {
ss.CloseWrite()
}
}()
// stderr is nil for ptys.
if ss.stderr != nil {
go func() {
_, err := io.Copy(ss.Stderr(), ss.stderr)
if err != nil {
logf("stderr copy: %v", err)
}
}()
}
err = ss.cmd.Wait()
// This will either make the SSH Termination goroutine be a no-op,
// or itself will be a no-op because the process was killed by the
// aforementioned goroutine.
ss.exitOnce.Do(func() {})
if err == nil {
ss.logf("Session complete")
ss.Exit(0)
return
}
if ee, ok := err.(*exec.ExitError); ok {
code := ee.ProcessState.ExitCode()
ss.logf("Wait: code=%v", code)
ss.Exit(code)
return
}
ss.logf("Wait: %v", err)
ss.Exit(1)
return
}
func (ss *sshSession) shouldRecord() bool {
// for now only record pty sessions
// TODO(bradfitz,maisem): make configurable on SSHPolicy and
// support recording non-pty stuff too.
_, _, isPtyReq := ss.Pty()
return recordSSH() && isPtyReq
}
type sshConnInfo struct {
// sshUser is the requested local SSH username ("root", "alice", etc).
sshUser string
// src is the Tailscale IP and port that the connection came from.
src netip.AddrPort
// dst is the Tailscale IP and port that the connection came for.
dst netip.AddrPort
// node is srcIP's node.
node *tailcfg.Node
// uprof is node's UserProfile.
uprof tailcfg.UserProfile
}
func (ci *sshConnInfo) String() string {
return fmt.Sprintf("%v->%v@%v", ci.src, ci.sshUser, ci.dst)
}
func (c *conn) ruleExpired(r *tailcfg.SSHRule) bool {
if r.RuleExpires == nil {
return false
}
return r.RuleExpires.Before(c.srv.now())
}
func (c *conn) evalSSHPolicy(pol *tailcfg.SSHPolicy, pubKey gossh.PublicKey) (a *tailcfg.SSHAction, localUser string, ok bool) {
for _, r := range pol.Rules {
if a, localUser, err := c.matchRule(r, pubKey); err == nil {
return a, localUser, true
}
}
return nil, "", false
}
// internal errors for testing; they don't escape to callers or logs.
var (
errNilRule = errors.New("nil rule")
errNilAction = errors.New("nil action")
errRuleExpired = errors.New("rule expired")
errPrincipalMatch = errors.New("principal didn't match")
errUserMatch = errors.New("user didn't match")
errInvalidConn = errors.New("invalid connection state")
)
func (c *conn) matchRule(r *tailcfg.SSHRule, pubKey gossh.PublicKey) (a *tailcfg.SSHAction, localUser string, err error) {
if c == nil {
return nil, "", errInvalidConn
}
if c.info == nil {
c.logf("invalid connection state")
return nil, "", errInvalidConn
}
if r == nil {
return nil, "", errNilRule
}
if r.Action == nil {
return nil, "", errNilAction
}
if c.ruleExpired(r) {
return nil, "", errRuleExpired
}
if !r.Action.Reject {
// For all but Reject rules, SSHUsers is required.
// If SSHUsers is nil or empty, mapLocalUser will return an
// empty string anyway.
localUser = mapLocalUser(r.SSHUsers, c.info.sshUser)
if localUser == "" {
return nil, "", errUserMatch
}
}
if ok, err := c.anyPrincipalMatches(r.Principals, pubKey); err != nil {
return nil, "", err
} else if !ok {
return nil, "", errPrincipalMatch
}
return r.Action, localUser, nil
}
func mapLocalUser(ruleSSHUsers map[string]string, reqSSHUser string) (localUser string) {
v, ok := ruleSSHUsers[reqSSHUser]
if !ok {
v = ruleSSHUsers["*"]
}
if v == "=" {
return reqSSHUser
}
return v
}
func (c *conn) anyPrincipalMatches(ps []*tailcfg.SSHPrincipal, pubKey gossh.PublicKey) (bool, error) {
for _, p := range ps {
if p == nil {
continue
}
if ok, err := c.principalMatches(p, pubKey); err != nil {
return false, err
} else if ok {
return true, nil
}
}
return false, nil
}
func (c *conn) principalMatches(p *tailcfg.SSHPrincipal, pubKey gossh.PublicKey) (bool, error) {
if !c.principalMatchesTailscaleIdentity(p) {
return false, nil
}
return c.principalMatchesPubKey(p, pubKey)
}
// principalMatchesTailscaleIdentity reports whether one of p's four fields
// that match the Tailscale identity match (Node, NodeIP, UserLogin, Any).
// This function does not consider PubKeys.
func (c *conn) principalMatchesTailscaleIdentity(p *tailcfg.SSHPrincipal) bool {
ci := c.info
if p.Any {
return true
}
if !p.Node.IsZero() && ci.node != nil && p.Node == ci.node.StableID {
return true
}
if p.NodeIP != "" {
if ip, _ := netip.ParseAddr(p.NodeIP); ip == ci.src.Addr() {
return true
}
}
if p.UserLogin != "" && ci.uprof.LoginName == p.UserLogin {
return true
}
return false
}
func (c *conn) principalMatchesPubKey(p *tailcfg.SSHPrincipal, clientPubKey gossh.PublicKey) (bool, error) {
if len(p.PubKeys) == 0 {
return true, nil
}
if clientPubKey == nil {
return false, nil
}
knownKeys := p.PubKeys
if len(knownKeys) == 1 && strings.HasPrefix(knownKeys[0], "https://") {
var err error
knownKeys, err = c.srv.fetchPublicKeysURL(c.expandPublicKeyURL(knownKeys[0]))
if err != nil {
return false, err
}
}
for _, knownKey := range knownKeys {
if pubKeyMatchesAuthorizedKey(clientPubKey, knownKey) {
return true, nil
}
}
return false, nil
}
func pubKeyMatchesAuthorizedKey(pubKey ssh.PublicKey, wantKey string) bool {
wantKeyType, rest, ok := strings.Cut(wantKey, " ")
if !ok {
return false
}
if pubKey.Type() != wantKeyType {
return false
}
wantKeyB64, _, _ := strings.Cut(rest, " ")
wantKeyData, _ := base64.StdEncoding.DecodeString(wantKeyB64)
return len(wantKeyData) > 0 && bytes.Equal(pubKey.Marshal(), wantKeyData)
}
func randBytes(n int) []byte {
b := make([]byte, n)
if _, err := rand.Read(b); err != nil {
panic(err)
}
return b
}
// startNewRecording starts a new SSH session recording.
//
// It writes an asciinema file to
// $TAILSCALE_VAR_ROOT/ssh-sessions/ssh-session-<unixtime>-*.cast.
func (ss *sshSession) startNewRecording() (*recording, error) {
var w ssh.Window
if ptyReq, _, isPtyReq := ss.Pty(); isPtyReq {
w = ptyReq.Window
}
term := envValFromList(ss.Environ(), "TERM")
if term == "" {
term = "xterm-256color" // something non-empty
}
now := time.Now()
rec := &recording{
ss: ss,
start: now,
}
varRoot := ss.conn.srv.lb.TailscaleVarRoot()
if varRoot == "" {
return nil, errors.New("no var root for recording storage")
}
dir := filepath.Join(varRoot, "ssh-sessions")
if err := os.MkdirAll(dir, 0700); err != nil {
return nil, err
}
f, err := os.CreateTemp(dir, fmt.Sprintf("ssh-session-%v-*.cast", now.UnixNano()))
if err != nil {
return nil, err
}
rec.out = f
// {"version": 2, "width": 221, "height": 84, "timestamp": 1647146075, "env": {"SHELL": "/bin/bash", "TERM": "screen"}}
type CastHeader struct {
Version int `json:"version"`
Width int `json:"width"`
Height int `json:"height"`
Timestamp int64 `json:"timestamp"`
Env map[string]string `json:"env"`
}
j, err := json.Marshal(CastHeader{
Version: 2,
Width: w.Width,
Height: w.Height,
Timestamp: now.Unix(),
Env: map[string]string{
"TERM": term,
// TODO(bradfitz): anything else important?
// including all seems noisey, but maybe we should
// for auditing. But first need to break
// launchProcess's startWithStdPipes and
// startWithPTY up so that they first return the cmd
// without starting it, and then a step that starts
// it. Then we can (1) make the cmd, (2) start the
// recording, (3) start the process.
},
})
if err != nil {
f.Close()
return nil, err
}
ss.logf("starting asciinema recording to %s", f.Name())
j = append(j, '\n')
if _, err := f.Write(j); err != nil {
f.Close()
return nil, err
}
return rec, nil
}
// recording is the state for an SSH session recording.
type recording struct {
ss *sshSession
start time.Time
mu sync.Mutex // guards writes to, close of out
out *os.File // nil if closed
}
func (r *recording) Close() error {
r.mu.Lock()
defer r.mu.Unlock()
if r.out == nil {
return nil
}
err := r.out.Close()
r.out = nil
return err
}
// writer returns an io.Writer around w that first records the write.
//
// The dir should be "i" for input or "o" for output.
//
// If r is nil, it returns w unchanged.
func (r *recording) writer(dir string, w io.Writer) io.Writer {
if r == nil {
return w
}
return &loggingWriter{r, dir, w}
}
// loggingWriter is an io.Writer wrapper that writes first an
// asciinema JSON cast format recording line, and then writes to w.
type loggingWriter struct {
r *recording
dir string // "i" or "o" (input or output)
w io.Writer // underlying Writer, after writing to r.out
}
func (w loggingWriter) Write(p []byte) (n int, err error) {
j, err := json.Marshal([]interface{}{
time.Since(w.r.start).Seconds(),
w.dir,
string(p),
})
if err != nil {
return 0, err
}
j = append(j, '\n')
if err := w.writeCastLine(j); err != nil {
return 0, err
}
return w.w.Write(p)
}
func (w loggingWriter) writeCastLine(j []byte) error {
w.r.mu.Lock()
defer w.r.mu.Unlock()
if w.r.out == nil {
return errors.New("logger closed")
}
_, err := w.r.out.Write(j)
if err != nil {
return fmt.Errorf("logger Write: %w", err)
}
return nil
}
func envValFromList(env []string, wantKey string) (v string) {
for _, kv := range env {
if thisKey, v, ok := strings.Cut(kv, "="); ok && envEq(thisKey, wantKey) {
return v
}
}
return ""
}
// envEq reports whether environment variable a == b for the current
// operating system.
func envEq(a, b string) bool {
if runtime.GOOS == "windows" {
return strings.EqualFold(a, b)
}
return a == b
}
var (
metricActiveSessions = clientmetric.NewGauge("ssh_active_sessions")
metricIncomingConnections = clientmetric.NewCounter("ssh_incoming_connections")
metricPublicKeyConnections = clientmetric.NewCounter("ssh_publickey_connections") // total
metricPublicKeyAccepts = clientmetric.NewCounter("ssh_publickey_accepts") // accepted subset of ssh_publickey_connections
metricTerminalAccept = clientmetric.NewCounter("ssh_terminalaction_accept")
metricTerminalReject = clientmetric.NewCounter("ssh_terminalaction_reject")
metricTerminalInterrupt = clientmetric.NewCounter("ssh_terminalaction_interrupt")
metricTerminalMalformed = clientmetric.NewCounter("ssh_terminalaction_malformed")
metricTerminalFetchError = clientmetric.NewCounter("ssh_terminalaction_fetch_error")
metricHolds = clientmetric.NewCounter("ssh_holds")
metricPolicyChangeKick = clientmetric.NewCounter("ssh_policy_change_kick")
metricSFTP = clientmetric.NewCounter("ssh_sftp_requests")
metricLocalPortForward = clientmetric.NewCounter("ssh_local_port_forward_requests")
)