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SSH connection hand-over #38545

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267 changes: 267 additions & 0 deletions lib/resumption/handover.go
Original file line number Diff line number Diff line change
@@ -0,0 +1,267 @@
// Teleport
// Copyright (C) 2024 Gravitational, Inc.
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU Affero General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Affero General Public License for more details.
//
// You should have received a copy of the GNU Affero General Public License
// along with this program. If not, see <http://www.gnu.org/licenses/>.

package resumption

import (
"context"
"crypto/sha256"
"encoding/base64"
"errors"
"io"
"net"
"net/netip"
"os"
"path/filepath"
"strings"
"syscall"
"time"

"github.com/gravitational/trace"

"github.com/gravitational/teleport"
"github.com/gravitational/teleport/lib/multiplexer"
"github.com/gravitational/teleport/lib/utils"
)

const sockSuffix = ".sock"

func sockPath(dataDir string, token resumptionToken) string {
hash := sha256.Sum256(token[:])
// unix domain sockets are limited to 108 or 104 characters, so the full
// sha256 hash is a bit too much (64 bytes in hex or 44 in b64); truncating
// the hash to 128 bits still gives us more than enough headroom to just
// assume that we'll have no collisions (a probability of one in a
// quintillion with 26 billion concurrent connections)
return filepath.Join(dataDir, "handover", base64.RawURLEncoding.EncodeToString(hash[:16])+sockSuffix)
}

func sockDir(dataDir string) string {
return filepath.Join(dataDir, "handover")
}

var errNoDataDir error = &trace.NotFoundError{Message: "data dir not configured"}

func (r *SSHServerWrapper) attemptHandover(conn *multiplexer.Conn, token resumptionToken) {
handoverConn, err := r.dialHandover(token)
if err != nil {
if trace.IsNotFound(err) {
r.log.Debug("Resumable connection not found or already deleted.")
_, _ = conn.Write([]byte{notFoundServerExchangeTag})
return
}
r.log.WithError(err).Error("Error while connecting to handover socket.")
return
}
defer handoverConn.Close()

var remoteIP netip.Addr
if t, _ := conn.RemoteAddr().(*net.TCPAddr); t != nil {
remoteIP, _ = netip.AddrFromSlice(t.IP)
}
remoteIP16 := remoteIP.As16()
Comment on lines +71 to +75
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Suggested change
var remoteIP netip.Addr
if t, _ := conn.RemoteAddr().(*net.TCPAddr); t != nil {
remoteIP, _ = netip.AddrFromSlice(t.IP)
}
remoteIP16 := remoteIP.As16()
var remoteIP netip.Addr
if t, ok := conn.RemoteAddr().(*net.TCPAddr); ok {
remoteIP, ok = netip.AddrFromSlice(t.IP)
if !ok {
r.log.Error("Failed to convert TCPAddr IP to netip.Addr")
return
}
} else {
r.log.Warn("Remote address is not a *net.TCPAddr, handover failed")
return
}
remoteIP16 := remoteIP.As16()

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I'm comfortable leaving the remote IP slice as all zeroes and let the server decide if that's good enough or not, rather than prohibit it in the client side.


if _, err := handoverConn.Write(remoteIP16[:]); err != nil {
if !utils.IsOKNetworkError(err) {
r.log.WithError(err).Error("Error while forwarding remote address to handover socket.")
}
return
}

r.log.Debug("Forwarding resuming connection to handover socket.")
if err := utils.ProxyConn(context.Background(), conn, handoverConn); err != nil && !utils.IsOKNetworkError(err) {
r.log.WithError(err).Debug("Finished forwarding resuming connection to handover socket.")
}
}

func (r *SSHServerWrapper) dialHandover(token resumptionToken) (net.Conn, error) {
if r.dataDir == "" {
return nil, trace.Wrap(errNoDataDir)
}

c, err := net.DialTimeout("unix", sockPath(r.dataDir, token), time.Second)
if err != nil {
return nil, trace.ConvertSystemError(err)
}
return c, nil
}

func (r *SSHServerWrapper) startHandoverListener(ctx context.Context, token resumptionToken, entry *connEntry) error {
l, err := r.createHandoverListener(token)
if err != nil {
return trace.Wrap(err)
}

go r.runHandoverListener(l, entry)
context.AfterFunc(ctx, func() { _ = l.Close() })

return nil
}

func (r *SSHServerWrapper) createHandoverListener(token resumptionToken) (net.Listener, error) {
if r.dataDir == "" {
return nil, trace.Wrap(errNoDataDir)
}

_ = os.MkdirAll(sockDir(r.dataDir), teleport.PrivateDirMode)
l, err := net.Listen("unix", sockPath(r.dataDir, token))
if err != nil {
return nil, trace.ConvertSystemError(err)
}
return l, nil
}

func (r *SSHServerWrapper) runHandoverListener(l net.Listener, entry *connEntry) {
defer l.Close()

var tempDelay time.Duration
for {
// the logic for this Accept loop is copied from [net/http.Server]
c, err := l.Accept()
if err == nil {
tempDelay = 0
go r.handleHandoverConnection(c, entry)
continue
}

if tempErr, ok := err.(interface{ Temporary() bool }); !ok || !tempErr.Temporary() {
if !utils.IsOKNetworkError(err) {
r.log.WithError(err).Warn("Accept error in handover listener.")
}
return
}

tempDelay = max(5*time.Millisecond, min(2*tempDelay, time.Second))
r.log.WithError(err).WithField("delay", tempDelay).Warn("Temporary accept error in handover listener, continuing after delay.")
time.Sleep(tempDelay)
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}
}

func (r *SSHServerWrapper) handleHandoverConnection(conn net.Conn, entry *connEntry) {
defer conn.Close()

var remoteIP16 [16]byte
if _, err := io.ReadFull(conn, remoteIP16[:]); err != nil {
if !utils.IsOKNetworkError(err) {
r.log.WithError(err).Error("Error while reading remote address from handover socket.")
}
return
}
remoteIP := netip.AddrFrom16(remoteIP16).Unmap()

r.resumeConnection(entry, conn, remoteIP)
}

// HandoverCleanup deletes handover sockets that were left over from previous
// runs of Teleport that failed to clean up after themselves (because of an
// uncatchable signal or a system crash). It will exhaustively clean up the
// current left over sockets, so it's sufficient to call it once per process.
func (r *SSHServerWrapper) HandoverCleanup(ctx context.Context) error {
const cleanupDelay = time.Second
return trace.Wrap(r.handoverCleanup(ctx, cleanupDelay))
}

func (r *SSHServerWrapper) handoverCleanup(ctx context.Context, cleanupDelay time.Duration) error {
if r.dataDir == "" {
return nil
}

dir := sockDir(r.dataDir)
entries, err := os.ReadDir(dir)
if err != nil {
if errors.Is(err, os.ErrNotExist) {
return nil
}
return trace.ConvertSystemError(err)
}

paths := make([]string, 0, len(entries))
for _, entry := range entries {
if strings.HasSuffix(entry.Name(), sockSuffix) {
paths = append(paths, filepath.Join(dir, entry.Name()))
}
}

paths, firstErr := retainNonConnectableSockets(ctx, paths)

if len(paths) < 1 {
return trace.Wrap(firstErr)
}

// unix domain sockets exist on disk between bind() and listen() but
// connecting before listen() results in ECONNREFUSED, so we just wait a
// little bit before testing them again; the first check lets us be done
// with the check immediately in the happy case where there's no
// unconnectable sockets
r.log.WithField("sockets", len(paths)).Debug("Found some unconnectable handover sockets, waiting before checking them again.")

select {
case <-ctx.Done():
return trace.NewAggregate(firstErr, ctx.Err())
case <-time.After(cleanupDelay):
}

paths, secondErr := retainNonConnectableSockets(ctx, paths)

if len(paths) < 1 {
r.log.Debug("Found no unconnectable handover sockets after waiting.")
return trace.NewAggregate(firstErr, secondErr)
}

r.log.WithField("sockets", len(paths)).Info("Cleaning up some non-connectable handover sockets from old Teleport instances.")

errs := []error{firstErr, secondErr}
for _, path := range paths {
if err := os.Remove(path); err != nil && !errors.Is(err, os.ErrNotExist) {
if len(errs) < 10 {
errs = append(errs, trace.ConvertSystemError(err))
}
}
}

return trace.NewAggregate(errs...)
}

// retainNonConnectableSockets attempts to connect to the given UNIX domain
// sockets, returning all and only the ones that exist and that refuse the
// connection.
func retainNonConnectableSockets(ctx context.Context, paths []string) ([]string, error) {
var d net.Dialer
var errs []error
filtered := paths[:0]
for _, path := range paths {
c, err := d.DialContext(ctx, "unix", path)
if err == nil {
_ = c.Close()
continue
}

if errors.Is(err, os.ErrNotExist) {
continue
}

if errors.Is(err, syscall.ECONNREFUSED) {
filtered = append(filtered, path)
continue
}

if len(errs) < 10 {
errs = append(errs, trace.ConvertSystemError(err))
}
}

return filtered, trace.NewAggregate(errs...)
}
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