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From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 7 Oct 2015 13:39:25 -0700
Message-ID: <CALCETrXsQrVstLe4WAAWy-scMmS4Yxe95Lx05j3dmu41L76dMg@mail.gmail.com>
Subject: Re: [PATCH 26/26] x86, pkeys: Documentation
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To: Dave Hansen <dave@sr71.net>
Cc: Ingo Molnar <mingo@kernel.org>, X86 ML <x86@kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, Linus Torvalds <torvalds@linux-foundation.org>, Andrew Morton <akpm@linux-foundation.org>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>, Kees Cook <keescook@google.com>, Brian Gerst <brgerst@gmail.com>

On Wed, Oct 7, 2015 at 1:24 PM, Dave Hansen <dave@sr71.net> wrote:
> On 10/03/2015 01:17 AM, Ingo Molnar wrote:
>> Right now the native x86 PTE format allows two protection related bits for
>> user-space pages:
>>
>>   _PAGE_BIT_RW:                   if 0 the page is read-only,  if 1 then it's read-write
>>   _PAGE_BIT_NX:                   if 0 the page is executable, if 1 then it's not executable
>>
>> As discussed previously, pkeys allows 'true execute only (--x)' mappings.
>>
>> Another possibility would be 'true write-only (-w-)' mappings.
>
> How would those work?
>
> Protection Keys has a Write-Disable and an Access-Disable bit.  But,
> Access-Disable denies _all_ data access to the region.  There's no way
> to allow only writes.

Weird.  I wonder why Intel did that.

I also wonder whether EPT can do write-only.

--Andy

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