fix(hub): bound SSH handshake so a silent peer can't hang the updater

ssh.ClientConfig.Timeout only covers the TCP connect. A peer that accepts
the connection but never sends an SSH banner blocked ssh.NewClientConn
forever, wedging the system's updater goroutine and leaking the socket.
Set a deadline on the conn for the handshake and clear it afterward.

internal/hub/systems/ssh_timeout_test.go

@@ -3,13 +3,17 @@
 package systems
 
 import (
+	"crypto/ed25519"
+	"crypto/rand"
 	"errors"
+	"net"
 	"sync"
 	"testing"
 	"testing/synctest"
 	"time"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"golang.org/x/crypto/ssh"
 )
 
@@ -91,3 +95,120 @@ func TestCreateSessionDuringClose(t *testing.T) {
 		wg.Wait()
 	}
 }
+
+// TestDialSSHHandshakeTimeout covers a peer that accepts the TCP connection but
+// never sends an SSH banner. Without a handshake deadline the dial blocks the
+// updater forever (GHSA-h9jh-29rh-w464).
+func TestDialSSHHandshakeTimeout(t *testing.T) {
+	prev := sshHandshakeTimeout
+	sshHandshakeTimeout = 200 * time.Millisecond
+	t.Cleanup(func() { sshHandshakeTimeout = prev })
+
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+	defer ln.Close()
+
+	accepted := make(chan net.Conn, 1)
+	go func() {
+		conn, err := ln.Accept()
+		if err == nil {
+			accepted <- conn
+		}
+	}()
+
+	config := &ssh.ClientConfig{
+		User:            "u",
+		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
+		Timeout:         4 * time.Second,
+	}
+
+	done := make(chan error, 1)
+	go func() {
+		client, err := dialSSHWithKeepAlive("tcp", ln.Addr().String(), config)
+		if client != nil {
+			client.Close()
+		}
+		done <- err
+	}()
+
+	select {
+	case err := <-done:
+		assert.Error(t, err, "a silent peer must fail the handshake")
+	case <-time.After(5 * time.Second):
+		t.Fatal("dial blocked on a peer that never sends an SSH banner")
+	}
+
+	// the hub must close its side of the connection
+	conn := <-accepted
+	defer conn.Close()
+	_ = conn.SetReadDeadline(time.Now().Add(2 * time.Second))
+	_, err = conn.Read(make([]byte, 256))
+	for err == nil {
+		_, err = conn.Read(make([]byte, 256))
+	}
+	var netErr net.Error
+	assert.False(t, errors.As(err, &netErr) && netErr.Timeout(), "hub should close the connection, got %v", err)
+}
+
+// TestDialSSHClearsHandshakeDeadline ensures the handshake deadline does not
+// carry over to the established connection, which is reused for many updates.
+func TestDialSSHClearsHandshakeDeadline(t *testing.T) {
+	prev := sshHandshakeTimeout
+	sshHandshakeTimeout = 200 * time.Millisecond
+	t.Cleanup(func() { sshHandshakeTimeout = prev })
+
+	_, hostPriv, err := ed25519.GenerateKey(rand.Reader)
+	require.NoError(t, err)
+	hostSigner, err := ssh.NewSignerFromKey(hostPriv)
+	require.NoError(t, err)
+	_, clientPriv, err := ed25519.GenerateKey(rand.Reader)
+	require.NoError(t, err)
+	clientSigner, err := ssh.NewSignerFromKey(clientPriv)
+	require.NoError(t, err)
+
+	serverConfig := &ssh.ServerConfig{
+		PublicKeyCallback: func(ssh.ConnMetadata, ssh.PublicKey) (*ssh.Permissions, error) { return nil, nil },
+	}
+	serverConfig.AddHostKey(hostSigner)
+
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	require.NoError(t, err)
+	defer ln.Close()
+
+	go func() {
+		conn, err := ln.Accept()
+		if err != nil {
+			return
+		}
+		defer conn.Close()
+		_, chans, reqs, err := ssh.NewServerConn(conn, serverConfig)
+		if err != nil {
+			return
+		}
+		go ssh.DiscardRequests(reqs)
+		for newChan := range chans {
+			ch, chReqs, err := newChan.Accept()
+			if err != nil {
+				continue
+			}
+			go ssh.DiscardRequests(chReqs)
+			ch.Close()
+		}
+	}()
+
+	config := &ssh.ClientConfig{
+		User:            "u",
+		Auth:            []ssh.AuthMethod{ssh.PublicKeys(clientSigner)},
+		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
+		Timeout:         4 * time.Second,
+	}
+	client, err := dialSSHWithKeepAlive("tcp", ln.Addr().String(), config)
+	require.NoError(t, err)
+	defer client.Close()
+
+	// wait past the handshake deadline; the connection must still be usable
+	time.Sleep(3 * sshHandshakeTimeout)
+	session, err := client.NewSession()
+	require.NoError(t, err, "connection should outlive the handshake deadline")
+	session.Close()
+}