Hacking the Xbox

Note
Chapter 9 - Sneaking in the Back Door 149
Code compartmentalization based on hardware security
levels is a different technique from sand-boxing. Sandboxing
does not provide an adequate solution for situations
where a user program requires direction from or interaction
with secret or protected code or data. Lately,
new processor architectures have been proposed that can
solve this problem through the use of data tags that embed
a sort of security audit log. 4
Another source of back doors are the design bugs that exist in every
complex chip. It is common practice to ship chips with plenty of known
bugs, also known as errata. For example, the Intel i860 XP processor (first
released in 1991, not to be confused with the recently released i860 chipset
for the Pentium4 processor) shipped with a book of errata that was
comparable in size to the processor’s data sheet. Another example closer to
home is the bug in the nVidia MCPX’s address space decoder that made the
MIST Premature Unmap attack possible. Most of these errata have simple
work-arounds or have minor implications for the functionality of the chip
under nominal conditions. However, some errata, such as those dealing
with cache coherence, address decoding, and memory management can result
in major software security holes.
In the case of the Xbox, the business impact of a hardware back door is
probably small. Perhaps Microsoft loses some small fraction of game sales
revenue, but the losses due to piracy are dwarfed by the losses Microsoft
takes on hardware sales. Also, the Xbox is just a game console — grandma’s
bank account is not being tapped dry or credit card numbers stolen as a
result of security weaknesses in the Xbox. However, more than game
revenues will be at risk with the trusted PC. Unless the trusted PC architecture
is a fundamental change from legacy PCs, people will be blindly trusting
financial secrets and personal data security to untrustworthy hardware.
Like most things in life, the first step is education. The more we learn about
hardware security, even if it involves poking around a game console, the
better our security systems will be tomorrow. Now, on with the lesson . . .
4 http://www.ai.mit.edu/projects/aries/Documents/Memos/ARIES-15.pdf.
“A Minimal Trusted Computing Base for Dynamically Ensuring
Secure Information Flow,” by Tom Knight and Jeremy Brown.