linux-kernel - Re: [PATCH 05/11] x86/fpu: set PKRU state for kernel threads

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Date:   Thu, 18 Oct 2018 18:26:44 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH 05/11] x86/fpu: set PKRU state for kernel threads

On 2018-10-12 11:02:18 [-0700], Andy Lutomirski wrote:
> On Fri, Oct 12, 2018 at 10:54 AM Dave Hansen
> <dave.hansen@...ux.intel.com> wrote:
> >
> > On 10/04/2018 07:05 AM, Sebastian Andrzej Siewior wrote:
> > > The PKRU value is not set for kernel threads because they do not have
> > > the ->initialized value set. As a result the kernel thread has a random
> > > PKRU value set which it inherits from the previous task.
> > > It has been suggested by Paolo Bonzini to set it for kernel threads, too
> > > because it might be a fix.
> > > I *think* this is not required because the kernel threads don't copy
> > > data to/from userland and don't have access to any userspace mm in
> > > general.
> > > However there is this use_mm(). If we gain a mm by use_mm() we don't
> > > have a matching PKRU value because those are per thread. It has been
> > > suggested to use 0 as the PKRU value but this would bypass PKRU.
> > >
> > > Set the initial (default) PKRU value for kernel threads.
> >
> > We might want to do this for cleanliness reasons...  Maybe.
> >
> > But this *should* have no practical effects.  Kernel threads have no
> > real 'mm' and no user pages.  They should not have do access to user
> > mappings.  Protection keys *only* apply to user mappings.  Thus,
> > logically, they should never be affected by PKRU values.
> >
> > So I'm kinda missing the point of the patch.
> 
> use_mm().

So. I would drop that patch from queue. Anyone feels different about it?

Sebastian