linux-kernel - Re: [patch 0/5] Support for sanitization flag in low-level page allocator

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Message-ID: <4A20E601.9070405@cs.helsinki.fi>
Date:	Sat, 30 May 2009 10:53:37 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	"Larry H." <research@...reption.com>
CC:	Alan Cox <alan@...rguk.ukuu.org.uk>, Ingo Molnar <mingo@...e.hu>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...l.org>, linux-mm@...ck.org,
	Ingo Molnar <mingo@...hat.com>, pageexec@...email.hu,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 0/5] Support for sanitization flag in low-level page	allocator

Hi Larry,

On 10:35 Sat 30 May, Pekka Enberg wrote:
>> The GFP_SENSITIVE flag looks like a big hammer that we don't really
>> need IMHO. It seems to me that most of the actual call-sites (crypto
>> code, wireless keys, etc.) should probably just use kzfree()
>> unconditionally to make sure we don't leak sensitive data. I did not
>> look too closely but I don't think any of the sensitive kfree() calls
>> are in fastpaths so the performance impact is negligible.

Larry H. wrote:
> That's hopeless, and kzfree is broken. Like I said in my earlier reply,
> please test that yourself to see the results. Whoever wrote that ignored
> how SLAB/SLUB work and if kzfree had been used somewhere in the kernel
> before, it should have been noticed long time ago.

An open-coded version of kzfree was being used in the kernel:

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=00fcf2cb6f6bb421851c3ba062c0a36760ea6e53

Can we now get to the part where you explain how it's broken because I 
obviously "ignored how SLAB/SLUB works"?

Thanks!

			Pekka
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