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

From: Kees Cook
Date: Mon Dec 14 2015 - 18:39:39 EST

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?

Yeah, dunno. I was curious to see this working on a real example
first, and then we could figure out how the linker should behave
generally. Sounds like we need some kind of ELF flag to say "please
use unreadable-exec memory mappings for this program, too.


Kees Cook
Chrome OS & Brillo Security
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