AMD NBIO register lock bits - MMIO routing configuration: MULTI-TENANT ISOLATION: The sibling of the SMN issue

Impact

MULTI-TENANT ISOLATION: The sibling of the SMN issue: unprotected NBIO lock bits let a local administrator rewrite MMIO routing configuration and break SEV-SNP guest integrity. The host operator can point address windows at confidential guest memory that the RMP was supposed to fence off.

Who can reach it

Local, host administrator privilege.

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. Same OEM BIOS package as the SMN lock-bit issue; do not patch one and leave the other.

References