Re: [PATCH 31/32] x86, pkeys: execute-only support

[Andy Lutomirski]

On Mon, Dec 14, 2015 at 3:37 PM, Dave Hansen <dave@xxxxxxxx> wrote:
> On 12/14/2015 12:05 PM, Kees Cook wrote:
>> On Mon, Dec 14, 2015 at 11:06 AM, Dave Hansen <dave@xxxxxxxx> wrote:
>>> > From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>>> > Protection keys provide new page-based protection in hardware.
>>> > But, they have an interesting attribute: they only affect data
>>> > accesses and never affect instruction fetches.  That means that
>>> > if we set up some memory which is set as "access-disabled" via
>>> > protection keys, we can still execute from it.
> ...
>>> > I haven't found any userspace that does this today.
>> To realistically take advantage of this, it sounds like the linker
>> would need to know to keep bss and data page-aligned away from text,
>> and then set text to PROT_EXEC only?
>>
>> Do you have any example linker scripts for this?
>
> Nope.  My linker-fu is weak.
>
> Can we even depend on the linker by itself?  Even if the sections were
> marked --x, we can't actually use them with those permissions unless we
> have protection keys.
>
> Do we need some special tag on the section to tell the linker to map it
> as --x under some conditions and r-x for others?
>

Why?  Wouldn't --x just end up acting like r-x if PKRU is absent?

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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