Fix nil-pointer bug in tty.checkChange.

The logic is the same as Linux's __tty_check_change, but we forgot to check the
case that the tty does not have an associated process group.

PiperOrigin-RevId: 693119939

pkg/sentry/fsimpl/host/tty.go

@@ -334,11 +334,12 @@ func (t *TTYFileDescription) checkChange(ctx context.Context, sig linux.Signal)
 
 	tg := task.ThreadGroup()
 	pg := tg.ProcessGroup()
+	ttyTg := t.tty.ThreadGroup()
 
 	// If the session for the task is different than the session for the
 	// controlling TTY, then the change is allowed. Seems like a bad idea,
 	// but that's exactly what linux does.
-	if tg.Session() != t.tty.ThreadGroup().Session() {
+	if ttyTg == nil || tg.Session() != ttyTg.Session() {
 		return nil
 	}
 

runsc/boot/loader.go

@@ -1667,8 +1667,15 @@ func (l *Loader) signalForegrondProcessGroup(cid string, tgid kernel.ThreadID, s
 	if tty == nil {
 		return fmt.Errorf("no TTY attached")
 	}
-	pg, _ := tty.ThreadGroup().ForegroundProcessGroup(tty.TTY())
 	si := &linux.SignalInfo{Signo: signo}
+	ttyTg := tty.ThreadGroup()
+	if ttyTg == nil {
+		// No thread group has been set. Signal the original thread
+		// group.
+		log.Warningf("No thread group for container %q and PID %d. Sending signal directly to PID %d.", cid, tgid, tgid)
+		return l.k.SendExternalSignalThreadGroup(tg, si)
+	}
+	pg, _ := ttyTg.ForegroundProcessGroup(tty.TTY())
 	if pg == nil {
 		// No foreground process group has been set. Signal the
 		// original thread group.