Re: [RFC][PATCH 03/10] x86/mm: introduce "default" kernel PTE mask

From: Nadav Amit
Date: Thu Feb 22 2018 - 17:21:46 EST

Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> wrote:

> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> The __PAGE_KERNEL_* page permissions are "raw". They contain bits
> that may or may not be supported on the current processor. They
> need to be filtered by a mask (currently __supported_pte_mask) to
> turn them into a value that we can actually set in a PTE.
> These __PAGE_KERNEL_* values all contain _PAGE_GLOBAL. But, with
> PTI, we want to be able to support _PAGE_GLOBAL (have the bit set
> in __supported_pte_mask) but not have it appear in any of these
> masks by default.

There might be a potential issue with this approach. __supported_pte_mask is
exported, so out-of-tree modules might use it. They therefore can
unknowingly use this value to set PTEs with _PAGE_GLOBAL set.

I do not know if it is a real issue, but leaving __supported_pte_mask as it
is now (with _PAGE_GLOBAL masked), and using a different variable for with
_PAGE_GLOBAL unmasked (i.e., the real â__supported_pte_maskâ) can solve it.