Re: [RFC PATCH v2 2/6] x86/arch_prctl: add ARCH_GET_NOPTI and ARCH_SET_NOPTI to enable/disable PTI

[Ingo Molnar]

* Andy Lutomirski <luto@xxxxxxxxxx> wrote:

> On Tue, Jan 9, 2018 at 6:54 AM, Willy Tarreau <w@xxxxxx> wrote:
> > On Tue, Jan 09, 2018 at 03:51:57PM +0100, Borislav Petkov wrote:
> >> On Tue, Jan 09, 2018 at 03:36:53PM +0100, Willy Tarreau wrote:
> >> > I see and am not particularly against this, but what use case do you
> >> > have in mind precisely ? I doubt it's just saving a few tens of bytes,
> >> > so probably you're more concerned about the potential risks this opens ?
> >> > But given we only allow this for CAP_SYS_RAWIO and these ones already
> >> > have access to /dev/mem and many other things, don't you think there
> >> > are much easier ways to dump kernel memory in this case than trying to
> >> > inject some meltdown code into the victim process ? Or maybe you have
> >> > other cases in mind that I'm not seeing.
> >>
> >> I'd like this to be config-controllable so that distros can make the
> >> decision whether/if they want to support the whole per-mm thing.
> >
> > OK.
> >
> >> Also, if CAP_SYS_RAWIO is going to protect, please make the
> >> ARCH_GET_NOPTI variant check it too.
> >
> > Interestingly I removed the check consecutive to the discussions. But
> > I think I'll simply remove the whole ARCH_GET_NOPTI as it has no real
> > value beyond initial development.
> >
> 
> I've thought about this a bit more.  Here are my thoughts:
> 
> 1. I don't like it being per-mm.  I think it should be a per-thread
> control so that a program can have a thread with PTI that runs
> less-trusted JavaScript and other network threads with PTI off.
> Obviously we lose NX protection mm-wide if any threads have PTI off.
> I think the way to implement this is:

Btw., the "NX protection", the NX bit set in the PTI kernel pagetables for the 
user range really just matters for non-SMEP hardware, right? On SMEP a CPU in 
kernel privilege mode cannot execute user pages, i.e. the fact that it's user 
pages is already NX, guaranteed by the CPU.

And note how there's a happy circumstance for users, regarding SMEP and PTI NX:

- All Intel desktop/server CPUs currently sold and those built in the last ~3 
  years have SMEP enabled already, so are not affected.

- AMD CPUs don't have PTI enabled, so they already don't have NX for their user 
  pages - no change in behavior.

I.e.: non-issue and not a real constraint on the flexibility of this ABI, AFAICS - 
it's "only" an implementational matter.

Thanks,

	Ingo