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

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Message-ID: <567885AE.9000202@linux.intel.com>
Date:	Mon, 21 Dec 2015 15:05:18 -0800
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Andy Lutomirski <luto@...capital.net>
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 12/21/2015 03:02 PM, Andy Lutomirski wrote:
> 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?

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.
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