When a room filled with hundreds of security professionals erupts into applause, it's notable. When that happens less than five minutes into a presentation, it's remarkable. But that's what transpired when security researcher Christopher Domas last week showed a room at Black Hat USA how to break the so-called ring-privilege model of modern CPU security.
In the hardware, different types of accounts are assigned to different "rings of privilege," with users at ring three and the system administrator at ring 0. Domas in his research hacked the ring with a string consisting of four hexadecimal characters. Such an attack could allow a program from a "regular" user to assume kernel-level control, executing at a higher privilege than most security software - and bypassing the vast majority of techniques used by anti-malware and hardware control systems today.
Domas, well-known in the security research community for his dissections of the X86 instruction set, titled his presentation "God Mode Unlocked: Hardware Backdoors in X86 CPUs." In talks at both Black Hat USA and DEF CON in Las Vegas, he not only proved that he had done just that, but he also shared the "how" with the world.
There are, luckily for the global IT security community, limitations to the research. The target was an older processor, with the C2 Mehemiah core, generally used in the embedded systems market. As a proof-of-concept, though, the research has profound implications for IT security.
The secret, Domas found, was making use of model-specific-registers (MSRs) - special CPU registers used in addition to the normal registers used in programming - to instruct the CPU to do things that its designers don't want it to do. And the secret isn't in the existence of MSRs — those are known. It's in the existence of so many MSRs, including many that the system designers and vendors don't include in any documentation.
Domas' research computer farm and methodology included multiple computers running specific instructions and reporting which ones returned fault conditions.
On the target CPUs, Domas found 1,300 MSRs. He said exploring all of those would have taken far too long, so he developed a method for understanding which were unique - and therefore not functional duplicates of one the other, more commonly used registers - based on how long it took to send and instruction and return a value.
Justification for this (and later) effort came from a series of patent filings Domas analyzed which hinted at a mysterious core to the x86 core in modern Intel-architecture CPUs. This DEC - a term Domas invented to describe a secondary core not generally known to software developers that is designed to enable functions also generally unknown to developers - shares portions of the instruction pipeline with the x86. But it's also its own entity with its own architecture.
Getting access to the DEC, Domas speculated, would require a global configuration register and a launch instruction — neither of which is documented. And there, his research got very real.
Domas reverse-engineered both the architecture and instruction set of the DEC. The latter, he said, involved 4,000 hours of compute time which generated 15 gigabytes of logs. When analyzed, the logs yielded the instruction for launching the DEC, completing tasks, and completely bypassing all of the protections of the ring-privilege model.
So a limited user account could execute code as the system administrator without being known or challenged. This would break virtually every anti-malware and device security system in use.
This very specific CPU vulnerability is unlikely to be used in widespread attacks against an enterprise because of the age and limited application of the CPUs involved. As Domas says, though, this is a proof-of-concept that may lead other researchers to seek similar vulnerabilities in more modern and widely used CPUs.
Domas has released his toolset in Project Rosenbridge on GitHub, and is actively seeking other researchers to add to and continue the work.
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