Protection for PE image

In UEFI/PI firmware, the SMM image is a normal PE/COFF image loaded by the SmmCore. If a given section of the SMM image is page aligned, it may be protected according to the section attributes, such as read-only for the code and non-executable for data. See the top right of figure 1.

In EDK II, the PiSmmCore (https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Core/PiSmmCore/MemoryAttributesTable.c) checks the PE image alignment and builds an EDKII_PI_SMM_MEMORY_ATTRIBUTES_TABLE (https://github.com/tianocore/edk2/blob/master/MdeModulePkg/Include/Guid/PiSmmMemoryAttributesTable.h) to record such information. If the PI SMM image is not page aligned, this table will not be published. If the EDKII_PI_SMM_MEMORY_ATTRIBUTES_TABLE is published, that means the EfiRuntimeServicesCode contains only code and it is EFI_MEMORY_RO, and the EfiRuntimeServicesData contains only data and it is EFI_MEMORY_XP.

Later the PiSmmCpu driver (https://github.com/tianocore/edk2/blob/master/UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c)` SetMemMapAttributes()` API consumes the EDKII_PI_SMM_MEMORY_ATTRIBUTES_TABLE and sets the page table attribute.

There are several assumptions to support the PE image protection in SMM:

1. The PE code section and data sections are not merged. If those 2 sections are merged, a #PF exception might be generated because the CPU might try to write a RO data item in the data section or execute a non-executable (NX) instruction in code section.
2. The PE image can be protected if it is page aligned. There should not be any self-modified-code in the code region. If there is, a platform should not set this PE image to be page aligned.

A platform may disable the XD in the UEFI environment, but this does not impact the SMM environment. The SMM environment may choose to always enable the XD upon SMM entry, and restore the XD state at the SMM exit point.