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

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Date:   Wed, 19 Sep 2018 07:55:51 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Rik van Riel <riel@...riel.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     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 18/09/2018 19:29, Rik van Riel wrote:
> On Tue, 2018-09-18 at 18:04 +0200, Sebastian Andrzej Siewior wrote:
>> On 2018-09-18 17:29:52 [+0200], Paolo Bonzini wrote:
>>>> I don't think it matters what the PKRU state is
>>>> for kernel threads, since kernel PTEs should not
>>>> be using protection keys anyway.
>>>
>>> What about copy_from/to_user?
>>
>> This doesn't work for a kernel thread, does it? I mean they share the
>> init's MM and never do copy_{from|to}_user.
> 
> Indeed, copy_from/to_user only works if current->mm
> points at an mm_struct with userspace memory.

A kthread can do use_mm/unuse_mm.

Paolo



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