Re: [PATCH] x86/mm/cpa: set PAGE_KERNEL in __set_pages_p()

[Dave Hansen]

On 5/10/22 06:35, Tom Lendacky wrote:
> I'm wondering if adding a specific helper that takes a boolean to
> indicate whether to set the global flag would be best. I'll let some of
> the MM maintainers comment about that.

First of all, I'm not positive that _PAGE_BIT_PROTNONE is ever used for
kernel mappings.  This would all get a lot easier if we decided that
_PAGE_BIT_PROTNONE is only for userspace mappings and we don't have to
worry about it when _PAGE_USER is clear.

Second, the number of places that do these
__set_pages_p()/__set_pages_np() pairs is pretty limited.  Some of them
are *quite* unambiguous over whether they are dealing with the direct map:

> int set_direct_map_invalid_noflush(struct page *page)
> {
>         return __set_pages_np(page, 1);
> }
> 
> int set_direct_map_default_noflush(struct page *page)
> {
>         return __set_pages_p(page, 1);
> }

which would make it patently obvious whether __set_pages_p() should
restore the global bit.  That would have been a problem in the "old" PTI
days where _some_ of the direct map was exposed to Meltdown.  I don't
think we have any of those mappings left, though.  They're all aliases
like text and cpu_entry_area.

It would be nice if someone could look into unraveling
_PAGE_BIT_PROTNONE.  We could even probably move it to another bit for
kernel mappings if we actually need it (I'm not convinced we do).