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

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Message-ID: <57A9625E-3F25-402B-8B54-129A68449C48@vmware.com>
Date:   Thu, 22 Feb 2018 22:21:34 +0000
From:   Nadav Amit <namit@...are.com>
To:     Dave Hansen <dave.hansen@...ux.intel.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

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 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 “__supported_pte_mask”) can solve it.