A group of academic researchers on Tuesday published a paper describing the first side-channel attack targeting the scheduler queues of modern processors.
Over the past years, researchers have demonstrated several CPU side-channel attacks that could allow attackers to obtain potentially sensitive information from memory. Some of these attacks rely on measuring contention, which is the conflict between multiple threads trying to use the same resource.
Superscalar processors rely on scheduler queues to decide the schedule of the instructions being executed. Intel CPUs have a single scheduler queue, but chips made by Apple and AMD have separate queues for each execution unit.
AMD processors also implement simultaneous multithreading (SMT), where a CPU core is split into multiple logical cores or hardware threads that execute independent instruction streams.
Researchers from the Graz University of Technology, the Georgia Institute of Technology, and the Lamarr Security Research non-profit research center discovered that an attacker on the same hardware core as the victim but in a different SMT thread can measure scheduler contention to obtain sensitive data. The attack method has been dubbed SQUIP (Scheduler Queue Usage via Interference Probing).
“An attacker running on the same host and CPU core as you could spy on which types of instructions you are executing due to the split-scheduler design on AMD CPUs.” Daniel Gruss, one of the Graz University of Technology researchers involved in the SQUIP project, explained in simple terms.
While Apple also uses separate scheduler queues for its M1 processors — and likely also M2 — it has yet to introduce SMT, which means its current processors are not impacted. However, if future Apple CPUs start using SMT, they could also be vulnerable to SQUIP attacks.
The researchers demonstrated the practicality of the attack by creating a covert channel that they used to exfiltrate data from a co-located virtual machine and a co-located process. Their experiments showed that an attacker can recover a full RSA-4096 encryption key.
The researchers have proposed some hardware countermeasures that can be implemented in future CPUs, including the use of a single scheduler design, making schedulers symmetric, or isolating hardware threads more strictly in the scheduler queues. There are also some software mitigations that can be implemented by applications or the operating system.
AMD was informed about the issue in December 2021 and assigned it the CVE identifier CVE-2021-46778 and a severity rating of ‘medium’. The chip giant published an advisory on Tuesday, informing customers that Zen 1, Zen 2 and Zen 3 microarchitectures are impacted.
The list of affected products includes Ryzen, Athlon and EPYC processors for desktops, workstations, mobile devices, Chromebooks, and servers.
While Intel and Apple products are currently not impacted, they have been notified as well.
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