coreboot-kgpe-d16/Documentation/security/smm.md

x86 System Management Mode

Introduction

The code running in System Management Mode (SMM) provides runtime services to applications running in ring0. It has a higher privilege level than ring0 and resides in the SMRAM region which cannot be accessed from ring0.

SMM can be entered by issuing System Management Interrupts (SMIs).

Secure data exchange

In order to not leak SMM internals or accidentally overwrite parts of SMM, ring0 provided data (pointers, offsets, sizes, ...) must be checked before using them in SMM.

There exist two methods to verify data:

/* Returns true if the region overlaps with the SMM */
bool smm_region_overlaps_handler(struct region *r);

/* Returns true if the memory pointed to overlaps with SMM reserved memory. */
static inline bool smm_points_to_smram(const void *ptr, const size_t len);