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

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Date:   Fri, 19 Oct 2018 10:37:16 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andrew Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        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 Oct 19, 2018, at 10:01 AM, Dave Hansen <dave.hansen@...ux.intel.com> wrote:
> 
> On 10/19/2018 09:59 AM, Andy Lutomirski wrote:
>>> That looks like a good API in general.  The ffs_user_copy_worker that
>>> Sebastian mentioned seems to be used by AIO, in which case of course it
>>> has to happen in a kernel thread.
>>> 
>>> But while the API is good, deciding on the desired semantics is
>>> "interesting".  The submitting thread might be changing PKRU between the
>>> time the I/O operation is submitted and the time it is completed, for
>>> example.
>> I think there’s only one sensible answer: capture PKRU at the time of submission.
> 
> I think it's much more straightforward to just not enforce pkeys.
> Having this "phantom" value could cause a very odd, nearly undebuggable
> I/O failure.

But now we have the reverse. The IO can work if it’s truly async but, if the kernel decides to synchronously complete IO (with GUP or copy_to_user), it’ll fail, right. This isn’t exactly friendly either.