Re: [PATCH 26/26] x86, pkeys: Documentation

From	Dave Hansen <dave@sr71.net>
Newsgroups	linux.kernel
Subject	Re: [PATCH 26/26] x86, pkeys: Documentation
Date	2015-10-07 22:50 +0200
Message-ID	<qh3OW-7uS-17@gated-at.bofh.it> (permalink)
References	(3 earlier) <qb13r-4sC-1@gated-at.bofh.it> <qcbtN-7ST-37@gated-at.bofh.it> <qfqcV-4Ce-5@gated-at.bofh.it> <qh3vA-78b-21@gated-at.bofh.it> <qh3OV-7uS-3@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Back to linux.kernel | Previous | Next — Previous in thread | Find similar | Unroll thread

Thread

Re: [PATCH 26/26] x86, pkeys: Documentation Dave Hansen <dave@sr71.net> - 2015-10-07 22:30 +0200
  Re: [PATCH 26/26] x86, pkeys: Documentation Andy Lutomirski <luto@amacapital.net> - 2015-10-07 22:50 +0200
    Re: [PATCH 26/26] x86, pkeys: Documentation Dave Hansen <dave@sr71.net> - 2015-10-07 22:50 +0200

csiph-web