AMD won’t patch Sinkclose security bug on older Zen CPUs

Updated Some AMD processors dating back to 2006 have a security vulnerability that's a boon for particularly underhand malware and rogue insiders, though the chip designer is only patching models made since 2020.

The flaw was discovered by the folks at infosec services outfit IOActive, and tracked as CVE-2023-31315 aka SinkClose. It's rated 7.5 out of 10 in terms of CVSS severity, which makes it important rather than critical.

For processors that are getting a fix, this will be delivered in the form of a firmware update via BIOS makers, or a hot-loadable microcode update.

The bug allows malicious software and rogue privileged users with access to the operating system kernel to run code in System Management Mode (SMM), a highly privileged execution environment present in x86 processors from Intel and AMD. SinkClose is unique to AMD.

SMM sits below the kernel and hypervisor, as well as applications, in that the management mode has unrestricted access to and control of the machine. The software running above it, including the kernel, can't meddle with SMM. If you can get arbitrary code execution in SMM you can undermine the whole system, doing whatever malicious stuff you want out of sight of the operating system and its security tools.

The management mode (not to be confused with Intel's Management Engine, which is a whole other kettle of fish) is by design supposed to be largely invisible to the rest of the system; it's there to supplement the BIOS (UEFI), perform background tasks, control system power and cooling, and other things specific to that motherboard hardware and chipset. The kernel can ask to invoke SMM, but generally has no control over it nor visibility into it. The code running in SMM is provided by the BIOS during startup, before the operating system is run.

Thus if malware or a rogue privileged user can get into SMM, such as by exploiting Sinkclose on AMD machines, they can really take over the box, spy on it, steal data and meddle with it, and infect it in a persistent way, at the BIOS level, that will make it hard to detect and cleanup.

But to exploit SMM, you will need to be in kernel mode – you will need to have taken control of the machine pretty much anyway. That makes the vulnerability a lot less critical, though it could be used by a smart adversary to really ruin your month.

Certain special CPU registers define the operation of SMM, and once these are set by the BIOS, they are usually locked until the next reboot (when the BIOS will set them again) so that no other software can change them. IOActive found a way to bypass that lock so that the SMM's operation can be brought under an attacker's control.

Sinkclose, we're told, affects AMD CPUs dating back nearly 20 years.

The chip designer's advisory regarding Sinkclose only mentions CPUs released since 2017, and even then not all of them.

The list of CPUs that have the flaw, and for which mitigations or fixes have been made available, include Epyc processors from generations one through four – aka Naples, Milan, Rome, Genoa, Bergamo, and Siena.

On the desktop, only some Ryzen 3000-series products will be repaired, along with plenty of SKUs from the 4000, 5000, 7000, and 8000 ranges.

Some Ryzen 1000 and 2000 embedded CPUs desktop will be fixed. But desktop models in the Ryzen range won’t get patches. The Register understands AMD considers models it won’t patch as having exited support.

If you're wondering if that means AMD's Zen and Zen+ desktop CPUs aren't getting patched, the fact that the chipmaker says there is "no fix planned" for vulnerable Zen 2-based Ryzen 3000 desktop CPUs codenamed “Matisse” should clear up any doubts.

Earlier this year, AMD patched CVE-2023-20577 on Ryzen 3000 desktop CPUs; AMD's decision to not patch Sinkclose on the very same processors may mark the end of security updates for the processors.

The Register has asked AMD to clarify if Ryzen 1000 and 2000 desktop CPUs are vulnerable, and why it's decided to not patch Ryzen 3000 (and potentially older) chips. ®

Updated to add on August 20

AMD has decided that its Ryzen 3000-series processors are actually worth patching.