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

[H. Peter Anvin]

On November 7, 2025 1:22:30 AM PST, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>On Fri, 7 Nov 2025 at 10:04, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>
>> On Fri, Oct 31, 2025 at 11:12:53AM -0700, Dave Hansen wrote:
>>
>> > But there's a pretty broad set of things that are for "security" that
>> > aren't necessary while you're just running trusted ring0 code:
>> >
>> >  * SMAP/SMEP
>> >  * CR pinning itself
>> >  * MSR_IA32_SPEC_CTRL
>> >  * MSR_IA32_TSX_CTRL
>> >
>> > They just haven't mattered until now because they don't have any
>> > practical effect until you actually have code running on _PAGE_USER
>> > mappings trying to attack the kernel.
>>
>> 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.
>
>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>
>
>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.

Firmware update is a big one.