diff --git a/agent/agentssh/x11.go b/agent/agentssh/x11.go
index 05d9f866c16f6..28286a5da9305 100644
--- a/agent/agentssh/x11.go
+++ b/agent/agentssh/x11.go
@@ -83,16 +83,17 @@ func (*Server) x11Callback(_ ssh.Context, _ ssh.X11) bool {
// x11Handler is called when a session has requested X11 forwarding.
// It listens for X11 connections and forwards them to the client.
-func (x *x11Forwarder) x11Handler(ctx ssh.Context, sshSession ssh.Session) (displayNumber int, handled bool) {
+func (x *x11Forwarder) x11Handler(sshCtx ssh.Context, sshSession ssh.Session) (displayNumber int, handled bool) {
x11, hasX11 := sshSession.X11()
if !hasX11 {
return -1, false
}
- serverConn, valid := ctx.Value(ssh.ContextKeyConn).(*gossh.ServerConn)
+ serverConn, valid := sshCtx.Value(ssh.ContextKeyConn).(*gossh.ServerConn)
if !valid {
- x.logger.Warn(ctx, "failed to get server connection")
+ x.logger.Warn(sshCtx, "failed to get server connection")
return -1, false
}
+ ctx := slog.With(sshCtx, slog.F("session_id", fmt.Sprintf("%x", serverConn.SessionID())))
hostname, err := os.Hostname()
if err != nil {
@@ -127,6 +128,7 @@ func (x *x11Forwarder) x11Handler(ctx ssh.Context, sshSession ssh.Session) (disp
}()
go x.listenForConnections(ctx, x11session, serverConn, x11)
+ x.logger.Debug(ctx, "X11 forwarding started", slog.F("display", x11session.display))
return x11session.display, true
}
diff --git a/agent/agentssh/x11_test.go b/agent/agentssh/x11_test.go
index a680c088de703..c8e3dd45c1db5 100644
--- a/agent/agentssh/x11_test.go
+++ b/agent/agentssh/x11_test.go
@@ -5,6 +5,7 @@ import (
"bytes"
"encoding/hex"
"fmt"
+ "io"
"net"
"os"
"path/filepath"
@@ -127,3 +128,162 @@ func TestServer_X11(t *testing.T) {
_, err = fs.Stat(filepath.Join(home, ".Xauthority"))
require.NoError(t, err)
}
+
+func TestServer_X11_EvictionLRU(t *testing.T) {
+ t.Parallel()
+ if runtime.GOOS != "linux" {
+ t.Skip("X11 forwarding is only supported on Linux")
+ }
+
+ ctx := testutil.Context(t, testutil.WaitLong)
+ logger := testutil.Logger(t)
+ fs := afero.NewMemMapFs()
+
+ // Use in-process networking for X11 forwarding.
+ inproc := testutil.NewInProcNet()
+
+ cfg := &agentssh.Config{
+ X11Net: inproc,
+ }
+
+ s, err := agentssh.NewServer(ctx, logger, prometheus.NewRegistry(), fs, agentexec.DefaultExecer, cfg)
+ require.NoError(t, err)
+ defer s.Close()
+ err = s.UpdateHostSigner(42)
+ require.NoError(t, err)
+
+ ln, err := net.Listen("tcp", "127.0.0.1:0")
+ require.NoError(t, err)
+
+ done := testutil.Go(t, func() {
+ err := s.Serve(ln)
+ assert.Error(t, err)
+ })
+
+ c := sshClient(t, ln.Addr().String())
+
+ // Calculate how many simultaneous X11 sessions we can create given the
+ // configured port range.
+ startPort := agentssh.X11StartPort + agentssh.X11DefaultDisplayOffset
+ maxSessions := agentssh.X11MaxPort - startPort + 1
+ require.Greater(t, maxSessions, 0, "expected a positive maxSessions value")
+
+ // shellSession holds references to the session and its standard streams so
+ // that the test can keep them open (and optionally interact with them) for
+ // the lifetime of the test. If we don't start the Shell with pipes in place,
+ // the session will be torn down asynchronously during the test.
+ type shellSession struct {
+ sess *gossh.Session
+ stdin io.WriteCloser
+ stdout io.Reader
+ stderr io.Reader
+ // scanner is used to read the output of the session, line by line.
+ scanner *bufio.Scanner
+ }
+
+ sessions := make([]shellSession, 0, maxSessions)
+ for i := 0; i < maxSessions; i++ {
+ sess, err := c.NewSession()
+ require.NoError(t, err)
+
+ _, err = sess.SendRequest("x11-req", true, gossh.Marshal(ssh.X11{
+ AuthProtocol: "MIT-MAGIC-COOKIE-1",
+ AuthCookie: hex.EncodeToString([]byte(fmt.Sprintf("cookie%d", i))),
+ ScreenNumber: uint32(0),
+ }))
+ require.NoError(t, err)
+
+ stdin, err := sess.StdinPipe()
+ require.NoError(t, err)
+ stdout, err := sess.StdoutPipe()
+ require.NoError(t, err)
+ stderr, err := sess.StderrPipe()
+ require.NoError(t, err)
+ require.NoError(t, sess.Shell())
+
+ // The SSH server lazily starts the session. We need to write a command
+ // and read back to ensure the X11 forwarding is started.
+ scanner := bufio.NewScanner(stdout)
+ msg := fmt.Sprintf("ready-%d", i)
+ _, err = stdin.Write([]byte("echo " + msg + "\n"))
+ require.NoError(t, err)
+ // Read until we get the message (first token may be empty due to shell prompt)
+ for scanner.Scan() {
+ line := strings.TrimSpace(scanner.Text())
+ if strings.Contains(line, msg) {
+ break
+ }
+ }
+ require.NoError(t, scanner.Err())
+
+ sessions = append(sessions, shellSession{
+ sess: sess,
+ stdin: stdin,
+ stdout: stdout,
+ stderr: stderr,
+ scanner: scanner,
+ })
+ }
+
+ // Create one more session which should evict the first (LRU) session and
+ // therefore reuse the very first display/port.
+ extraSess, err := c.NewSession()
+ require.NoError(t, err)
+
+ _, err = extraSess.SendRequest("x11-req", true, gossh.Marshal(ssh.X11{
+ AuthProtocol: "MIT-MAGIC-COOKIE-1",
+ AuthCookie: hex.EncodeToString([]byte("extra")),
+ ScreenNumber: uint32(0),
+ }))
+ require.NoError(t, err)
+
+ // Ask the remote side for the DISPLAY value so we can extract the display
+ // number that was assigned to this session.
+ out, err := extraSess.Output("echo DISPLAY=$DISPLAY")
+ require.NoError(t, err)
+
+ // Example output line: "DISPLAY=localhost:10.0".
+ var newDisplayNumber int
+ {
+ sc := bufio.NewScanner(bytes.NewReader(out))
+ for sc.Scan() {
+ line := strings.TrimSpace(sc.Text())
+ if strings.HasPrefix(line, "DISPLAY=") {
+ parts := strings.SplitN(line, ":", 2)
+ require.Len(t, parts, 2)
+ displayPart := parts[1]
+ if strings.Contains(displayPart, ".") {
+ displayPart = strings.SplitN(displayPart, ".", 2)[0]
+ }
+ var convErr error
+ newDisplayNumber, convErr = strconv.Atoi(displayPart)
+ require.NoError(t, convErr)
+ break
+ }
+ }
+ require.NoError(t, sc.Err())
+ }
+
+ // The display number should have wrapped around to the starting value.
+ assert.Equal(t, agentssh.X11DefaultDisplayOffset, newDisplayNumber, "expected display number to be reused after eviction")
+
+ // validate that the first session was torn down.
+ _, err = sessions[0].stdin.Write([]byte("echo DISPLAY=$DISPLAY\n"))
+ require.ErrorIs(t, err, io.EOF)
+ err = sessions[0].sess.Wait()
+ require.Error(t, err)
+
+ // Cleanup.
+ for _, sh := range sessions[1:] {
+ err = sh.stdin.Close()
+ require.NoError(t, err)
+ err = sh.sess.Wait()
+ require.NoError(t, err)
+ }
+ err = extraSess.Close()
+ require.ErrorIs(t, err, io.EOF)
+
+ err = s.Close()
+ require.NoError(t, err)
+ _ = testutil.TryReceive(ctx, t, done)
+}
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