RE: [PATCHv6 01/16] x86/cpu: Enumerate the LASS feature bits

[H. Peter Anvin]

On June 23, 2025 4:13:34 PM PDT, "Luck, Tony" <tony.luck@xxxxxxxxx> wrote:
>> >> Logically there are two completely different things:
>> >>
>> >>       1. Touching userspace
>> >>       2. Touching the lower half of the address space
>> >>
>> >> If it's only userspace in the lower half of the address space, then
>> >> there's no controversy. But the problem obviously occurs when you want
>> >> to touch kernel mappings in the lower half of the address space.
>> >
>> > Why does the kernel create the mappings to poke kernel text
>> > for ALTERNATIVE patching in the lower half of the address space?
>> >
>> > Instead of special "we really to want to access the lower addresses"
>> > code, wouldn't it be easier to map the "poke" virtual addresses in normal
>> > kernel upper-half space?
>>
>> The upper half of the address space is shared kernel space, right? Every
>> PGD has identical contents in the upper half. So if we create a mapping
>> there,everybody get access to it. Every mm can access it. Every
>> *process* can access it. It still has kernel permissions of course, but
>> it's still a place that everybody can get at.
>>
>> The lower half is *ONLY* accessible to the local mm. In this case, only
>> the text poking mm. It's a natural, safe, place to create a mapping that
>> you want to be private and not be exploited.
>>
>> So, doing it in the upper half is risky.
>>
>> If we *wanted*, we could have a non-shared PGD entry in the top half of
>> the address space. But we'd need to reserve its address space and all
>> that jazz. I'm not sure it's any better than just disabling LASS
>> enforcement for a moment.
>
>Maybe it’s a thing to put on the list for "when x86 drops support for 32-bit".
>
>Reserving a PGD entry in the kernel half of the address space for
>local CPU use would be practical then. Perhaps there might be other
>uses too.
>
>-Tony
>

Are we actually doing patching on more than one CPU at a time?