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Subject: Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?
To: Andy Lutomirski <luto@amacapital.net>
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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>,
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        Linus Torvalds <torvalds@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <567885AE.9000202@linux.intel.com>
Date: Mon, 21 Dec 2015 15:05:18 -0800
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On 12/21/2015 03:02 PM, Andy Lutomirski wrote:
> 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?

Yep, but that's with a concrete benefit in mind.

> 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.

Yep, agreed.