Re: [PATCH v11 4/9] x86/alternatives: Disable LASS when patching kernel code

[H. Peter Anvin]

On November 12, 2025 8:18:20 AM PST, Sohil Mehta <sohil.mehta@xxxxxxxxx> wrote:
>On 11/12/2025 7:28 AM, Ard Biesheuvel wrote:
>
>>>> d) calling SetVirtualAddressMap() does not result in all 1:1
>>>> references being converted to the new mapping.
>>>>
>>>>
>>>> To address d), the x86_64 implementation of efi_map_region() indeed
>>>> maps an 1:1 alias of each remapped runtime regions, so that stray
>>>> accesses don't fault. But the code addresses are all remapped, and so
>>>> the firmware routines are always invoked via their remapped aliases in
>>>> the kernel VA space. Not calling SetVirtualAddressMap() at all, or
>>>> calling it with a 1:1 mapping is not feasible, essentially because
>>>> Windows doesn't do that, and that is the only thing that is tested on
>>>> all x86 PCs by the respective OEMs.
>>>>
>>>> Given that remapping the code is dealt with by the firmware's PE/COFF
>>>> loader, whereas remapping [dynamically allocated] data requires effort
>>>> on the part of the programmer, I'd hazard a guess that 99.9% of those
>>>> bugs do not involve attempts to execute via the lower mapping, but
>>>> stray references to data objects that were not remapped properly.
>>>>
>>>> So we might consider
>>>> a) remapping those 1:1 aliases NX, so we don't have those patches of
>>>> RWX memory around
>>>> b) keeping LASS enabled during ordinary EFI runtime calls, as you suggest.
>>>
>>> Unless someone has a code pointer in their code.
>> 
>> That is a good point, especially because the EFI universe is
>> constructed out of GUIDs and so-called protocols, which are just
>> structs with function pointers.
>> 
>> However, EFI protocols are only supported at boot time, and the
>> runtime execution context is much more restricted. So I'd still expect
>> the code pointer case to be much less likely.
>
>But, that still leaves the stray data accesses. We would still need to
>disable the LASS data access enforcement by toggling RFLAGS.AC during
>the runtime calls.
>
>Can we rely on EFI to not mess up RFLAGS and keep the AC bit intact?

No.