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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Thu, 18 Oct 2018 13:46:22 -0700
Message-ID: <CALCETrVjEOkNhdyAF10PaYi_rnFUWKRqPhLRCq3ZV=gxQFETxg@mail.gmail.com>
Subject: Re: [PATCH 05/11] x86/fpu: set PKRU state for kernel threads
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     Andrew Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim Krcmar <rkrcmar@redhat.com>,
        kvm list <kvm@vger.kernel.org>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Rik van Riel <riel@surriel.com>
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On Thu, Oct 18, 2018 at 11:25 AM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> On 2018-10-18 09:48:24 [-0700], Andy Lutomirski wrote:
> > > On Oct 18, 2018, at 9:26 AM, Sebastian Andrzej Siewior <bigeasy@linut=
ronix.de> wrote:
> > >> On 2018-10-12 11:02:18 [-0700], Andy Lutomirski wrote:
> > >> On Fri, Oct 12, 2018 at 10:54 AM Dave Hansen
> > >>> 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?
> > >
> >
> > I think we *do* want the patch. It=E2=80=99s a bugfix for use_mm users,=
 right?
>
> This is the loophole that has been pointed out. I am not convinced what
> the correct behaviour should be here (and we have five users of that
> interface). For instance f_fs[0].  It reads data from the USB EP and
> then writes it to userland task. Due to $circumstances it happens in a
> workqueue instead of the task's context.  So it borrows the mm with
> use_mm().  The current behaviour random because the PKRU value can not
> be predicted. It may or may not work.
>
> Setting it to allow-all/none would let the operation always fail or
> succeed which might be an improvement in terms of debugging. However it
> is hard to judge what the correct behaviour should be. Should fail or
> succeed.
> But this is not the only loophole: There is ptrace interface which is
> used by gdb (just checked) and also bypasses PKRU. So=E2=80=A6
>
> [0] drivers/usb/gadget/function/f_fs.c::ffs_user_copy_worker()
>
> Sebastian

I think we need an entirely new API:

user_mm_ctx_t ctx =3D user_mm_ctx_get();

...

use_user_mm_ctx(ctx);
unuse_user_mm_ctx(ctx);

...

user_mm_ctx_put(ctx);

and ctx will store a copy of mm and PKRU.