Re: [PATCH 6/9] x86, pkeys: add pkey set/get syscalls

[Andy Lutomirski]

On Jul 11, 2016 8:48 AM, "Dave Hansen" <dave.hansen@xxxxxxxxx> wrote:
>
> On 07/11/2016 07:45 AM, Andy Lutomirski wrote:
> > On Mon, Jul 11, 2016 at 7:34 AM, Dave Hansen <dave@xxxxxxxx> wrote:
> >> Should we instead just recommend to userspace that they lock down access
> >> to keys by default in all threads as a best practice?
> >
> > Is that really better than doing it in-kernel?  My concern is that
> > we'll find library code that creates a thread, and that code could run
> > before the pkey-aware part of the program even starts running.
>
> Yeah, so let's assume we have some pkey-unaware thread.  The upside of a
> scheme where the kernel preemptively (and transparently to the thread)
> locks down PKRU is that the thread can't go corrupting any non-zero-pkey
> structures that came from other threads.
>
> But, the downside is that the thread can not access any non-zero-pkey
> structures without taking some kind of action with PKRU.  That obviously
> won't happen since the thread is pkeys-unaware to begin with.  Would
> that break these libraries unless everything using pkeys knows to only
> share pkey=0 data with those threads?
>

Yes, but at least for the cases I can think of, that's probably a good
thing.  OTOH, I can see cases where you want everyone to be able to
read but only specific code paths to be able to write.

I think it's more or less impossible to get sensible behavior passing
pkey != 0 data to legacy functions.  If you call:

void frob(struct foo *p);

If frob in turn passes p to a thread, what PKRU is it supposed to use?

> > So how is user code supposed lock down all of its threads?
> >
> > seccomp has TSYNC for this, but I don't think that PKRU allows
> > something like that.
>
> I'm not sure this is possible for PKRU.  Think of a simple PKRU
> manipulation in userspace:
>
>         pkru = rdpkru();
>         pkru |= PKEY_DENY_ACCESS<<key*2;
>         wrpkru(pkru);
>
> If we push a PKRU value into a thread between the rdpkru() and wrpkru(),
> we'll lose the content of that "push".  I'm not sure there's any way to
> guarantee this with a user-controlled register.

We could try to insist that user code uses some vsyscall helper that
tracks which bits are as-yet-unassigned.  That's quite messy, though.

We could also arbitrarily partition the key space into
initially-wide-open, initially-read-only, and initially-no-access and
let pkey_alloc say which kind it wants.

--Andy