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

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Date:	Wed, 7 Oct 2015 13:24:00 -0700
From:	Dave Hansen <dave@...1.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.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 10/03/2015 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.

Or am I missing something?
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