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

[Dave Hansen <dave.hansen@intel.com>]

On 07/11/2016 07:45 AM, Andy Lutomirski wrote:
> On Mon, Jul 11, 2016 at 7:34 AM, Dave Hansen <dave@sr71.net> 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?

> 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.