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

[Dave Hansen]

On 02/22/2018 02:21 PM, Nadav Amit wrote:
> 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 don't think we can help out-of-tree modules getting this wrong.
They're OK if they use PAGE_KERNEL*, btw.