linux-kernel - Re: [x86.git#mm] stack protector fixes, vmsplice exploit

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Date:	Thu, 14 Feb 2008 13:00:03 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	pageexec@...email.hu, Sam Ravnborg <sam@...nborg.org>,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [x86.git#mm] stack protector fixes, vmsplice exploit

On Thu, 14 Feb 2008 21:25:35 +0100
Ingo Molnar <mingo@...e.hu> wrote:

> 
> * pageexec@...email.hu <pageexec@...email.hu> wrote:
> 
> > really, the best defense is to reduce the useful lifetime of any 
> > leaked canary, and you can't get better than syscall granularity 
> > without disproportional effort and impact elsewhere (and i'm sure
> > some would find even this disproportional ;).
> 
> hm, i think per syscall canaries are really expensive.


it's not that bad. Assuming you use a PNR that you re-seed periodically,
it's 
* go to the next random number with PNR
* write to PDA and task struct

give or take 10 cycles total if you squeeze it hard, 20 if you don't.
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