AMD CPUs -- Guest Memory Vulnerabilities

AMD reports:

   Researchers from IOActive have reported that it may be possible for
   an attacker with ring 0 access to modify the configuration of System
   Management Mode (SMM) even when SMM Lock is enabled.  Improper
   validation in a model specific register (MSR) could allow a malicious
   program with ring0 access to modify SMM configuration while SMI lock
   is enabled, potentially leading to arbitrary code execution.