linux-kernel - Re: [PATCH 26/26] x86, pkeys: Documentation

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Date:	Wed, 7 Oct 2015 13:47:10 -0700
From:	Dave Hansen <dave@...1.net>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Kees Cook <keescook@...gle.com>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH 26/26] x86, pkeys: Documentation

On 10/07/2015 01:39 PM, Andy Lutomirski wrote:
> On Wed, Oct 7, 2015 at 1:24 PM, Dave Hansen <dave@...1.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

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