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

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Date:   Thu, 22 Feb 2018 14:26:43 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Nadav Amit <namit@...are.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "keescook@...gle.com" <keescook@...gle.com>,
        "hughd@...gle.com" <hughd@...gle.com>,
        "jgross@...e.com" <jgross@...e.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC][PATCH 03/10] x86/mm: introduce "default" kernel PTE mask

On 02/22/2018 02:21 PM, Nadav Amit wrote:
> Dave Hansen <dave.hansen@...ux.intel.com> wrote:
>> From: Dave Hansen <dave.hansen@...ux.intel.com>
>> 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.