>I am assuming Malware is running under a legitimate Linux kernel (with no bugs) messing with Windows partition with full access right. So something like DISABLE_INTEGRITY_CHECKS in boot.ini no longer works on Windows 8? Also root can no longer use CertMgr to add custom certificate for custom signed drivers?
Nope. Neither DDISABLE_INTEGRITY_CHECKS nor installing your own certificates will work if secure boot is enabled.
Drivers have to be signed by MS's certificate to be installable.
>My point is that, with the complexity of NT, by having root access to those bits and bytes the attack surface is so tremendous there is probably no need to have an unsigned Linux kernel
There will be vulnerabilities, of course. But MS took care to close all the obvious loopholes.