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

[Andy Lutomirski]

On Wed, Jan 6, 2016 at 4:01 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.
> could lose the bits in PKRU that enforce execute-only
> permissions.  To avoid this, we suggest avoiding ever calling
> mmap() or mprotect() when the PKRU value is expected to be
> stable.

s/stable/unstable/

This may be a bit unfortunate for people who call mmap from signal
handlers.  Admittedly, the failure mode isn't that bad.

Out of curiosity, do you have timing information for WRPKRU and
RDPKRU?  If they're fast and if anyone ever implements my deferred
xstate restore idea, then the performance issue goes away and we can
stop caring about whether PKRU is in the init state.

--Andy