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

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Date:   Fri, 12 Oct 2018 11:02:18 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Andrew Lutomirski <luto@...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 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().