AMD Secure Processor firmware - MMIO routing lock (Zen 5): MULTI-TENANT ISOLATION: A missing lock check in ASP firmware

Impact

MULTI-TENANT ISOLATION: A missing lock check in ASP firmware on some Zen 5 parts lets a locally authenticated administrator alter MMIO routing after the point where it should have been frozen. Redirecting MMIO means pointing a device's window somewhere it should not go - which is how a host administrator reaches into a confidential guest's memory despite SEV-SNP.

Who can reach it

Local, administrative privilege on the host. Affects Zen 5 (Turin / EPYC 9005) generation parts.

What to do

Fixed in AMD reference firmware (AGESA / SEV firmware) and delivered to you only as an OEM SBIOS/BIOS package - Dell, HPE, Supermicro, Lenovo, Gigabyte and the ODMs each rebuild and requalify AMD's AGESA drop before it ships. **Expect months, not weeks**: AMD publishes the bulletin, the OEM ships BIOS somewhere between one and six months later, and for platforms past their support window it may never arrive at all. Applying it is a full node power cycle with the host drained - not a driver reload, not a live patch. Track it as a firmware campaign per server SKU, not per kernel version, and verify afterwards by reading back the SMU/PSP firmware version rather than trusting the BIOS revision string. Because this sits inside the SEV-SNP trust boundary, the update also moves the platform's reported TCB version: after patching you must refresh VCEK certificates from AMD's KDS and update whatever attestation policy your tenants (or your own confidential-VM control plane) pin against, or every guest launch will start failing validation.

References