SMM Callout Privilege Escalation, which security research Danny Odler discovered, enables an attacker with physical or administrative access to the victim system to manipulate the AMD Generic Encapsulated Software Architecture (AGESA) microcode inside the motherboard's firmware. This allows for the execution of malicious code that's not detectable by the operating system.
Luckily, this vulnerability can be mitigated with a simple microcode update, which seemingly doesn't bear a performance impact on the system. AMD has already distributed updated versions of its AGESA microcodes to its motherboard partners and will deliver the remaining versions by the end of this month.
As usual, AMD recommends users to update their systems to the latest firmware once it's available.