linux-kernel - Re: [PATCH] avoid entropy starvation due to stack protection

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Message-ID: <50C98764.6050104@chronox.de>
Date:	Thu, 13 Dec 2012 08:44:36 +0100
From:	Stephan Mueller <smueller@...onox.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Ted Ts'o <tytso@....edu>, lkml <linux-kernel@...r.kernel.org>,
	Jeff Liu <jeff.liu@...cle.com>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] avoid entropy starvation due to stack protection

On 13.12.2012 01:43:21, +0100, Andrew Morton
<akpm@...ux-foundation.org> wrote:

Hi Andrew,
> On Tue, 11 Dec 2012 13:33:04 +0100
> Stephan Mueller <smueller@...onox.de> wrote:
>
>> Some time ago, I noticed the fact that for every newly
>> executed process, the function create_elf_tables requests 16 bytes of
>> randomness from get_random_bytes. This is easily visible when calling
>>
>> while [ 1 ]
>> do
>> 	cat /proc/sys/kernel/random/entropy_avail
>> 	sleep 1
>> done
> Please see
> http://ozlabs.org/~akpm/mmotm/broken-out/binfmt_elfc-use-get_random_int-to-fix-entropy-depleting.patch
>
> That patch is about one week from a mainline merge, btw.

Initially I was also thinking about get_random_int. But stack protection
depends on non-predictable numbers to ensure it cannot be defeated. As
get_random_int depends on MD5 which is assumed to be broken now, I
discarded the idea of using get_random_int.

Moreover, please consider that get_cycles is an architecture-specific
function that on some architectures only returns 0 (For all
architectures where this is implemented, you have no guarantee that it
increments as a high-resolution timer). So, the quality of
get_random_int is questionable IMHO for the use as a stack protector.

Also note, that other in-kernel users of get_random_bytes may be
converted to using the proposed kernel pool to avoid more entropy drainage.

Please note that the suggested approach of fully seeding a deterministic
RNG never followed by a re-seeding is used elsewhere (e.g. the OpenSSL
RNG). Therefore, I think the suggested approach is viable.

Ciao
Stephan

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