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

Peter Zijlstra
2025-11-07T09:40:31Z

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

> > But that's just the thing EFI is *NOT* trusted! We're basically
> > disabling all security features (not listed above are CET and CFI) to
> > run this random garbage we have no control over.
> >
> > How about we just flat out refuse EFI runtime services? What are they
> > actually needed for? Why are we bending over backwards and subverting
> > our security for this stuff?
>
> On x86, it is mostly the EFI variable services that user space has
> come to rely on, not only for setting the boot path (which typically
> happens only once at installation time, when the path to GRUB is set
> as the first boot option). Unfortunately, the systemd folks have taken
> a liking to this feature too, and have started storing things in
> there.

*groan*, so booting with noefi (I just went and found that option) will
cause a modern Linux system to fail booting?

> 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>

What the actual fuck!! And we support this garbage? Without
pr_err(FW_BUG ) notification?

How can one find such devices? I need to check my machine.

> It would be nice if we could
>
> a) Get rid of SetVirtualAddressMap(), which is another insane hack
> that should never have been supported on 64-bit systems. On arm64, we
> no longer call it unless there is a specific need for it (some Ampere
> Altra systems with buggy firmware will crash otherwise). On x86,
> though, it might be tricky because there so much buggy firmware.
> Perhaps we should phase it out by checking for the UEFI version, so
> that future systems will avoid it. This would mean, however, that EFI
> code remains in the low user address space, which may not be what you
> want (unless we do c) perhaps?)
>
> b) Run EFI runtime calls in a sandbox VM - there was a PoC implemented
> for arm64 a couple of years ago, but it was very intrusive and the ARM
> intern in question went on to do more satisyfing work.
>
> c) Unmap the kernel KPTI style while the runtime calls are in
> progress? This should be rather straight-forward, although it might
> not help a lot as the code in question still runs privileged.

At the very least I think we should start printing scary messages about
disabling security to run untrusted code. This is all quite insane :/