Register Lock

When the system powers on, most of the silicon registers are unlocked. The firmware code needs to configure the system and lock the critical resources by setting the lock bit in a silicon register. Examples include but are not limited to flash chip lock, SMM lock, SMI lock, MMIO BAR configuration lock, Model Specific Register (MSR) configuration lock, etc.

Previous Vulnerabilities:

Flash

In 1998, older platforms did not properly lock access to the flash parts, allowing anyone to overwrite BIOS code. Sixty million computers were believed to be infected by the CIH) virus.

In Power Of Community 2007, a new attack appeared which took advantage of the Intel top swap feature, if the latter capability was unlocked.

Today, there are several ways to lock the flash part, and the firmware should lock all the possible ways, in proper time, and in all boot paths. These paths include a normal boot, S3, S4, capsule update, recovery, etc.

SMRAM

It is likely the first documented SMM attack, which occurred because the SMM memory range was not locked.

Platforms must lock SMRAM in silicon and setup SMRR for all processors to protect SMRAM. This lock must happen in all boot paths (normal boot, S3, S4, capsule update, recovery, etc.).

MMIO BAR

In BlackHat 2008, Invisible lab demonstrated how to use unlocked remap registers for SMM or Management Engine (ME) firmware to inject code.

Today, all critical MMIO bars are required to be locked without overlap. The configuration is checked by the ACM during a TXT DRTM launch.