Seamless SSO with CAS

Question

I have Spring MVC app with Spring Security set up to use CAS. Also I have Liferay Portal 5.2.3 configured to use CAS.

1. I authenticate from app_1
2. Open Liferay Portal (app_2)
3. It does not check authentication immediately (I see sign in link)
4. If I click Sign in then CAS verifies that user is authenticated and I'm automatically signed in.

How to create link in app_1 that would suggest app_2 to validate authentication...

I have spring filter chain

<filter>
 <filter-name>springSecurityFilterChain</filter-name>
 <filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
</filter>

And

<security:http entry-point-ref="casAuthenticationEntryPoint" auto-config="true" >
    <security:intercept-url pattern="/*/secure/**" access="ROLE_USER" />
    <security:custom-filter position="CAS_FILTER" ref="casAuthenticationFilter" />
    <security:anonymous enabled="false"/>
</security:http>

If I point all urls from app_1 to secure paths in app_2 then authentication is required (user is redirected to login page) so this doesn't work.