On Wed, Oct 7, 2015 at 1:24 PM, Dave Hansen [off-list ref] wrote:

On 10/03/2015 01:17 AM, Ingo Molnar wrote:

quoted

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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