For some background, Intel issues SINIT authenticated code modules (ACMs) which are used for the trusted boot process. According to this advisory, these modules are vulnerable to a privilege escalation attack which could compromise system management mode. That compromise is likely undetectable at the kernel level. Here's a background paper on a different attack on SINIT:
<a href="http://www.invisiblethingslab.com/resources/2011/Attacking_Intel_TXT_via_SINIT_hijacking.pdf" rel="nofollow">http://www.invisiblethingslab.com/resources/2011/Attacking_I...</a>