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From: Andy Lutomirski <luto@amacapital.net>
Date: Mon, 21 Dec 2015 15:02:26 -0800
Message-ID: <CALCETrWPW2mk==HK6KXNWMm8R1CnFb-CtQZzgdd=e8kaob80fQ@mail.gmail.com>
Subject: Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Borislav Petkov <bp@alien8.de>,
        Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Brian Gerst <brgerst@gmail.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>
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On Mon, Dec 21, 2015 at 3:00 PM, Dave Hansen
<dave.hansen@linux.intel.com> wrote:
> On 12/21/2015 02:52 PM, Andy Lutomirski wrote:
>> Perhaps this is silly, but what if the default were changed to deny
>> reads and writes for unallocated keys?  Is there a use case that
>> breaks?
>
> It's probably a reasonable debugging feature.
>
> But, anything that takes an XSAVE feature out of its "init state" has
> the potential to do a bit of harm because it increases the potential
> size of writes during XSAVE.  XSAVEOPT will _help_ here, but we probably
> don't want to go out of our way to take things out of the init state
> when we're unsure of the benefits.

Aren't you already doing that with your magic execute-only thing?

Also, if we ever do the deferred-xstate-restore thing that Rik was
playing with awhile back, then we'll want to switch to using rdpkru
and wrpkru in-kernel directly, and we'll explicitly mask PKRU out of
the XRSTOR and XSAVEOPT state, and this particular issue will become
irrelevant.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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