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Subject: Re: [PATCH 26/26] x86, pkeys: Documentation
To: Andy Lutomirski <luto@amacapital.net>
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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>
From: Dave Hansen <dave@sr71.net>
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Date: Wed, 7 Oct 2015 13:47:10 -0700
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On 10/07/2015 01:39 PM, Andy Lutomirski wrote:
> 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.

The SDM makes it look that way.  There appear to be completely separate
r/w/x bits.  r=0/w=0/x=0 means !present.

The bit 0 definition says, for instance:

	Read access; indicates whether reads are allowed from the
	4-KByte page referenced by this entry