Linux 6.9 Makes A Change To Satisfy Microsoft For EFI x86 Shim Loader Signing

The EFI updates were merged today for the ongoing Linux 6.9 merge window. This cycle the EFI kernel code is seeing enhancements for confidential computing as well as for satisfy Microsoft's requirements for getting them to sign the x86 shim loader again for UEFI Secure Boot handling.

The EFI changes for Linux 6.9 allow using the Confidential Computing (CC) protocol should the TCG2 protocol not be supported, such as the case for Intel Trusted Domain Extensions (TDX) confidential virtual machines. The Microsoft change is around ensuring mappings are not both writable and executable when running in the EFI boot services. Ensuring not writable and executable is good security practice in general but important for getting Microsoft to re-sign the x86 shim loader so that Linux distributions will play nicely on Secure Boot enabled systems.

The merge request by Ard Biesheuvel notes:

- Measure initrd and command line using the CC protocol if the ordinary TCG2 protocol is not implemented, typically on TDX confidential VMs

- Avoid creating mappings that are both writable and executable while running in the EFI boot services. This is a prerequisite for getting the x86 shim loader signed by MicroSoft again, which allows the distros to install on x86 PCs that ship with EFI secure boot enabled.

- API update for struct platform_driver::remove()

This new EFI code is good to go for Linux 6.9 that will debut as stable around the middle of 2024.