Some x86 CPUs have hidden backdoors that let you seize root by sending a command to an undocumented RISC core that manages the main CPU, security researcher Christopher Domas told the Black Hat conference here Thursday (Aug. 9).
The command — “.byte 0x0f, 0x3f” in Linux — “isn’t supposed to exist, doesn’t have a name, and gives you root right away,” Domas said, adding that he calls it “God Mode.”
The backdoor completely breaks the protection-ring model of operating-system security, in which the OS kernel runs in ring 0, device drivers run in rings 1 and 2, and user applications and interfaces (“userland”) run in ring 3, furthest from the kernel and with the least privileges. To put it simply, Domas’ God Mode takes you from the outermost to the innermost ring in four bytes.
“We have direct ring 3 to ring 0 hardware privilege escalation,” Domas said. “This has never been done.”
That’s because of the hidden RISC chip, which lives so far down on the bare metal that Domas half-joked that it ought to be thought of as a new, deeper ring of privilege, following the theory that hypervisors and chip-management systems can be considered ring -1 or ring -2.
“This is really ring -4,” he said. “It’s a secret, co-located core buried alongside the x86 chip. It has unrestricted access to the x86.”
The good news is that, as far as Domas knows, this backdoor exists only on VIA C3 Nehemiah chips made in 2003 and used in embedded systems and thin clients. The bad news is that it’s entirely possible that such hidden backdoors exist on many other chipsets.