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

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Date:   Thu, 22 Feb 2018 17:11:36 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>,
        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 2/22/2018 4:26 PM, Dave Hansen wrote:
> 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.

You will probably need to change __default_kernel_pte_mask to be an
EXPORT_SYMBOL instead of an EXPORT_SYMBOL_GPL for (some) out-of-tree
modules.

Thanks,
Tom

>