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

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Message-ID: <20121215191549.GA11036@domone.kolej.mff.cuni.cz>
Date:	Sat, 15 Dec 2012 20:15:49 +0100
From:	Ondřej Bílka <neleai@...nam.cz>
To:	Stephan Mueller <smueller@...onox.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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

Why not use nonblocking pool and seed nonblocking pool only with half of
collected entropy to get /dev/random in almost all practical scenarios
nonblocking?

On Thu, Dec 13, 2012 at 08:44:36AM +0100, Stephan Mueller wrote:
> 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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