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Subject: Re: [RFC][PATCH 03/10] x86/mm: introduce "default" kernel PTE mask
To:     Dave Hansen <dave.hansen@linux.intel.com>,
        Nadav Amit <namit@vmware.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "keescook@google.com" <keescook@google.com>,
        "hughd@google.com" <hughd@google.com>,
        "jgross@suse.com" <jgross@suse.com>,
        "x86@kernel.org" <x86@kernel.org>
References: <20180222203651.B776810C@viggo.jf.intel.com>
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 <57A9625E-3F25-402B-8B54-129A68449C48@vmware.com>
 <61c68587-3ad5-edf7-539c-8af99c6220ea@linux.intel.com>
From:   Tom Lendacky <thomas.lendacky@amd.com>
Message-ID: <5b801b73-bedd-e7f5-765a-6e00759a5786@amd.com>
Date:   Thu, 22 Feb 2018 17:11:36 -0600
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On 2/22/2018 4:26 PM, Dave Hansen wrote:
> On 02/22/2018 02:21 PM, Nadav Amit wrote:
>> Dave Hansen <dave.hansen@linux.intel.com> wrote:
>>> From: Dave Hansen <dave.hansen@linux.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

>