linux-kernel - Re: [PATCH 00/11 v3] x86: load FPU registers on return to userland

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Message-Id: <1DDD1BB3-A306-4366-A941-4F7321672C6B@amacapital.net>
Date:   Thu, 4 Oct 2018 09:50:51 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Rik van Riel <riel@...riel.com>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 00/11 v3] x86: load FPU registers on return to userland



> On Oct 4, 2018, at 9:45 AM, Rik van Riel <riel@...riel.com> wrote:
> 
> On Thu, 2018-10-04 at 16:05 +0200, Sebastian Andrzej Siewior wrote:
> 
> 
>> In v3 I dropped that decouple idea. I also learned that the wrpkru
>> instruction is not privileged and so caching it in kernel does not
>> work.
> 
> Wait, so any thread can bypass its memory protection
> keys, even if there is a seccomp filter preventing
> it from calling the PKRU syscalls?
> 
> Is that intended?
> 
> Is that simply a hardware limitation, or something
> where we can set a flag somewhere to force tasks to
> go through the kernel?
> 
> 

Hardware limitation.