AMD PSP boot ROM - integrity of decrypted firmware image: MULTI-TENANT ISOLATION: The PSP boot ROM authenticates

Impact

MULTI-TENANT ISOLATION: The PSP boot ROM authenticates and decrypts firmware but does not sufficiently verify the integrity of the *decrypted* image before using it. An attacker who can influence the encrypted blob can therefore get the boot ROM to execute content it never really validated - code execution in the earliest, most privileged stage of the platform, in mask ROM territory where no patch can reach the flawed check itself.

Who can reach it

Local, requires the ability to modify the firmware image in SPI ROM - root plus flash write, a compromised BMC, or supply-chain access.

What to do

Fixed in AMD reference firmware (AGESA / PSP / SEV firmware) and delivered only as an OEM SBIOS/BIOS package - Dell, HPE, Supermicro, Lenovo and the ODMs each rebuild and requalify AMD's AGESA drop before shipping. **Expect one to six months of OEM lag**, and on end-of-support platforms expect nothing. Applying it is a drain plus full power cycle, not a driver reload. Verify by reading back the PSP/SMU firmware version afterwards rather than trusting the BIOS version string. Since the flawed logic is in boot ROM, the mitigation is in the firmware AMD ships around it rather than a fix to the ROM. Practical compensating controls: enforce SPI write protection, require signed BIOS update packages, and restrict which management paths can drive host flash.

References