Re: [PATCH 33/33] x86, pkeys: execute-only support

[Kees Cook]

On Fri, Feb 12, 2016 at 1:02 PM, 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.
>
> This patch uses protection keys to set up mappings to do just that.
> If a user calls:
>
>         mmap(..., PROT_EXEC);
> or
>         mprotect(ptr, sz, PROT_EXEC);
>
> (note PROT_EXEC-only without PROT_READ/WRITE), the kernel will
> notice this, and set a special protection key on the memory.  It
> also sets the appropriate bits in the Protection Keys User Rights
> (PKRU) register so that the memory becomes unreadable and
> unwritable.
>
> I haven't found any userspace that does this today.  With this
> facility in place, we expect userspace to move to use it
> eventually.  Userspace _could_ start doing this today.  Any
> PROT_EXEC calls get converted to PROT_READ inside the kernel, and
> would transparently be upgraded to "true" PROT_EXEC with this
> code.  IOW, userspace never has to do any PROT_EXEC runtime
> detection.

Random thought while skimming email:

Is there a way to detect this feature's availability without userspace
having to set up a segv handler and attempting to read a
PROT_EXEC-only region? (i.e. cpu flag for protection keys, or a way to
check the protection to see if PROT_READ got added automatically,
etc?)

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security