On Fri, May 28, 2021, at 9:11 AM, Dave Hansen wrote:

On 5/28/21 8:32 AM, Thomas Gleixner wrote:

quoted

quoted

This series:
 * Moves the PKRU manipulation to a more appropriate location,
   away from the page table code
 * Wraps get_xsave_addr() with more structured, less error-prone
   interfaces.
 * Conditionally hides a pkey debugfs file, eliminating the need
   for new runtime checks to work with the new interface.
 * Add a selftest to make it more likely to catch bugs like this
   in the future.  This improved selftest catches this issue on
   Intel CPUs.  Without the improvement, it only triggers on AMD.

I think all of this is fundamentaly wrong.

Contrary to FPU state, PKRU has to be updated at context switch
time. There is absolutely no point in having PKRU XSAVES managed.

It's broken in several ways. Anything which clears and loads the FPU
will load the wrong PKRU value. Go figure...

So the right thing is to disable PKRU in XCR0 and on sched out simply do

   task->thread.pkru = read_pkru();

and on sched in

   write_pkru(task->thread.pkru);

Simple, trivial and not going to be wreckaged by anything which fiddles
with xstates. We all know by now that xstates is a trainwreck and not
having stuff like that in there is making the fixes I'm doing way
simpler.

As for the general sentiment that PKRU is not suitable for management
with XSAVE, I'm with you.

I have a few concerns about moving away from XSAVE management, though.
I'm not nixing the whole idea, but there are some things we need to resolve.

First is that there _may_ be ABI concerns.  

I tend to think that, for -stable, we should fix the bug without an ABI change.