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

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Date:	Sat, 30 May 2009 19:39:06 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	"Larry H." <research@...reption.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	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


* Pekka Enberg <penberg@...helsinki.fi> wrote:

> 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"?

Yeah, kzfree() sounds like the right approach for all places that 
know it for sure that they dont want information to persist.

	Ingo
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