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Date:   Fri, 28 May 2021 10:13:32 -0700
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Dave Hansen" <dave.hansen@intel.com>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        "Dave Hansen" <dave.hansen@linux.intel.com>, linux-mm@kvack.org
Cc:     "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
        "Ingo Molnar" <mingo@redhat.com>, "Borislav Petkov" <bp@alien8.de>,
        "the arch/x86 maintainers" <x86@kernel.org>, shuah@kernel.org,
        "Babu Moger" <babu.moger@amd.com>, dave.kleikamp@oracle.com,
        linuxram@us.ibm.com, bauerman@linux.ibm.com
Subject: Re: [PATCH 0/5] x86/pkeys: PKRU manipulation bug fixes and cleanups
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Precedence: bulk


On Fri, May 28, 2021, at 9:11 AM, Dave Hansen wrote:
> On 5/28/21 8:32 AM, Thomas Gleixner wrote:
> >>
> >> 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.