linux-kernel - Re: [RFC PATCH 04/10 v2 ] x86/fpu: eager switch PKRU state

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Message-Id: <27496B23-92A0-462C-A198-6A4413634488@amacapital.net>
Date:   Wed, 19 Sep 2018 12:49:35 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Rik van Riel <riel@...riel.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: [RFC PATCH 04/10 v2 ] x86/fpu: eager switch PKRU state



> On Sep 19, 2018, at 10:00 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
>> On 19/09/2018 18:57, Sebastian Andrzej Siewior wrote:
>>> On 2018-09-19 07:55:51 [+0200], Paolo Bonzini wrote:
>>> A kthread can do use_mm/unuse_mm.
>> 
>> indeed. The FPU struct for the kernel thread isn't valid / does not
>> contain the expected PKRU value. So loading the pkru value from the
>> struct FPU does not work as expected. We could set it to 0 for a kernel
>> thread so we don't end up with a random value.
>> If we want to get this usecase working then we would have to move pkru
>> value from FPU to mm_struct and consider it in use_mm(). Do we want
>> this?
> 
> As a start, I think keeping it in the FPU struct but loading it
> unconditionally will work.  kthreads will not obey PKU but it will be
> better already.
> 
> I honestly don't know if PKRU should be per-mm, I don't know mm very
> well despite my brilliant observation above. :)
> 
> 

It must be per thread.  I don’t think it’s possible to have sane semantics per mm.

I also think that use_mm should set PKRU to the same value that signal handlers use.  If we use 0, it’s a recipe for accidental PK bypass.