linux-kernel - Re: [patch] x64, fpu: fix possible FPU leakage in error conditions

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Message-ID: <20080724202728.GL14380@linux-os.sc.intel.com>
Date:	Thu, 24 Jul 2008 13:27:28 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"stable@...nel.org" <stable@...nel.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch] x64, fpu: fix possible FPU leakage in error conditions

On Thu, Jul 24, 2008 at 11:59:03AM -0700, Linus Torvalds wrote:
> 
> On Thu, 24 Jul 2008, Suresh Siddha wrote:
> >
> > If we don't do init_fpu() + restore from the sane init state, process has
> > to be killed, in the paranoid failing scenario of math_state_restore()
> 
> Umm. I'm still not seeing why the right answer is not just to do "stts +
> math_used = 0".
> 
> And the specific case of math_state_restore(), that will also fix the
> problem - next time around.

Ok.

> As far as I can tell, your patch causes serious problems in case
> init_fpu() fails. Which it can do, afaik.

Only the first init_fpu() can fail (memory allocation failure). But here,
it is def not the first time.

Anyhow, your suggestion is simple and clean. Will post the patch shortly.

I have to do clear_fpu() though(stts + math_used = 0 may not be enough).

thanks,
suresh
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