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

From:  Pekka Enberg <penberg-AT-cs.helsinki.fi>
To:  "Larry H." <research-AT-subreption.com>
Subject:  Re: [patch 0/5] Support for sanitization flag in low-level page allocator
Date:  Sat, 30 May 2009 10:53:37 +0300
Message-ID:  <4A20E601.9070405@cs.helsinki.fi>
Cc:  Alan Cox <alan-AT-lxorguk.ukuu.org.uk>, Ingo Molnar <mingo-AT-elte.hu>, Rik van Riel <riel-AT-redhat.com>, linux-kernel-AT-vger.kernel.org, Linus Torvalds <torvalds-AT-osdl.org>, linux-mm-AT-kvack.org, Ingo Molnar <mingo-AT-redhat.com>, pageexec-AT-freemail.hu, Linus Torvalds <torvalds-AT-linux-foundation.org>
Archive-link:  Article

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-...

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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