linux-kernel - Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?

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Message-ID: <CALCETrWPW2mk==HK6KXNWMm8R1CnFb-CtQZzgdd=e8kaob80fQ@mail.gmail.com>
Date:	Mon, 21 Dec 2015 15:02:26 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>, Christoph Hellwig <hch@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Brian Gerst <brgerst@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?

On Mon, Dec 21, 2015 at 3:00 PM, Dave Hansen
<dave.hansen@...ux.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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