public class LoginFiltro : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        object usuarioLogado = filterContext.HttpContext.Session["Nome"];

        if (usuarioLogado == null)
        {
            filterContext.Result = new RedirectToRouteResult(
                      new RouteValueDictionary(
                          new { action = "Index", controller = "Login" }));
        }
    }
}

[LoginFiltro]
public class Login: Controller { }