Re: [PATCH v11 5/9] x86/efi: Disable LASS while mapping the EFI runtime services

Peter Zijlstra
2025-11-07T10:10:22Z

On Fri, Nov 07, 2025 at 10:22:30AM +0100, Ard Biesheuvel wrote:

> There is also PRM, which is much worse, as it permits devices in the
> ACPI namespace to call firmware routines that are mapped privileged in
> the OS address space in the same way. I objected to this at the time,
> and asked for a facility where we could at least mark such code as
> unprivileged (and run it as such) but this was ignored, as Intel and
> MS had already sealed the deal and put this into production. This is
> much worse than typical EFI routines, as the PRM code is ODM/OEM code
> rather than something that comes from the upstream EFI implementation.
> It is basically a dumping ground for code that used to run in SMM
> because it was too ugly to run anywhere else. </rant>

'https://uefi.org/sites/default/files/resources/Platform Runtime Mechanism - with legal notice.pdf'

Has on page 16, section 3.1:

8. PRM handlers must not contain any privileged instructions.

So we should be able to actually run this crap in ring3, right?