AMD System Management Unit (SMU) mailbox interface: A malicious user can manipulate SMU mailbox entries and reach

Impact

A malicious user can manipulate SMU mailbox entries and reach arbitrary code execution in the System Management Unit. The SMU is the microcontroller that owns voltage, clock and thermal control for the package. Code execution there is not just another privilege boundary - it is PHYSICAL control of the part: undervolting to induce computational faults (the technique behind fault-injection attacks on secure enclaves), forcing thermal or power states that throttle or hard-shut-down a node, and doing it in a way the OS reports as a normal thermal event. On a dense GPU rack that is a denial-of-service lever against neighbours and potentially a hardware-damage lever. Sibling issues CVE-2021-26329 and CVE-2021-26330 are the overflow variants in the same interface.

Who can reach it

Local attacker able to reach the SMU mailbox - typically ring 0 on the host, or a bare-metal tenant.

What to do

AGESA / BIOS update per AMD-SB-1021 from the OEM, flash plus reboot. Independently, restrict tenant access to power and thermal management interfaces (no raw MSR access, no vendor overclocking or power-tuning drivers in tenant images) - that mitigation is under your control and does not wait on a BIOS drop. Add out-of-band power and thermal telemetry so an SMU-driven event is distinguishable from a genuine cooling fault.

References