Limitation
Setting up RO and NX attribute for SMRAM is a good enhancement to prevent a code overriding attack. However it has some limitations:
It cannot resist a Return-Oriented-Programming (ROP) attack. [ROP]. We might need ASLR to mitigate the ROP attack. [ASLR] With the code region randomized, an attacker cannot accurately predict the location of instructions in order to leverage gadgets.
Not all important data structure are set to Read-Only. This is the current SMM driver limitation. The SMM driver can be updated to allocate the important structures to be read-only instead of a read-write global variable.
To set not-present bit for non-fixed DRAM region in SmmReadyToLock is a good enhancement to enforce the protection policy. However, it cannot cover below cases:
Memory Hot Plug. Take a server platform as the example, A RAS server may hot plug more DRAM during OS runtime, and rely on SMM to initialize those DRAM. This SMM Memory Initialization module may need access the DRAM for the memory test.
Memory Mapped IO (MMIO). Ideally, not all MMIO regions are configured to be accessible to SMM. Some MMIO BARs are important such as VTd or SPI controller. VTd BAR is important because OS need setup VTd to configuration the DMA protection. SPI controller BAR is important because BIOS SMM handler need access it to program the flash device. It should be a platform policy to configure which one should be accessible. The SMI handler must consider the case that the MMIO BAR might be modified by the malicious software and check if the MMIO BAR is in the valid region.