Intel CPU flaw allows for hard-to-detect rootkit


— 2:33 PM on March 19, 2009

A pair of security researchers has released a paper and proof-of-concept code detailing a security vulnerability in Intel's processors. In their words, the vulnerability allows the use of CPU cache poisoning to "read or write into (otherwise protected) SMRAM memory."

SMRAM means System Management Mode memory, and according to NetworkWorld blogger Jamey Heary, a rootkit running there would be incredibly difficult to detect. Naturally, such a rootkit could also conceal any number of trojans, viruses, and miscellaneous malware apps running on an infected system. Here's a snip from Heary's blog post:

The heart-stopping thing about this particular exploit is that it hides itself in the SMM space. To put that into perspective, SMM is more privileged than a hypervisor is and it's not controllable by any Operating System. By design, the operating system cannot override or disable System Management Interupt (SMI) calls. In practice, the only way for you to know what is running in SMM space is to physically disassemble the firmware of your computer. So, given that an SMI takes precedence over any OS call, the OS cannot control or read SMM, and the only way to read SMM is to disassemble the system makes an SMM rootkit incredibly stealthy!

So, why release details about the vulnerability publicly instead of working quietly with Intel? Well, paper co-author Joanna Rutkowska says she and fellow researchers have already notified Intel. She also explains in another post on the Invisible Things Lab blog:

Interestingly, however, none of us was even close to being the first discoverer of the underlying problem that our attacks exploit. In fact, the first mention of the possible attack using caching for compromising SMM has been discussed in certain documents authored as early as the end of 2005 (!) by nobody else than... Intel's own employees.

Rutkowska believes someone else would surely discover and exploit the flaw if it remains unpatched, so going public and forcing Intel's hands is a good thing. Oh, and the proof-of-concept code isn't a ready-made rootkit—it's "totally harmless," she claims.

   
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