From: David Laight <David.Laight@ACULAB.COM>
To: "'Theodore Y. Ts'o'" <tytso@mit.edu>, George Spelvin <lkml@SDF.ORG>
Cc: Dan Williams <dan.j.williams@intel.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Qian Cai <cai@lca.pw>, Kees Cook <keescook@chromium.org>,
Michal Hocko <mhocko@suse.com>,
Andrew Morton <akpm@linux-foundation.org>,
Linux MM <linux-mm@kvack.org>
Subject: RE: [RFC PATCH v1 00/52] Audit kernel random number use
Date: Mon, 30 Mar 2020 09:27:17 +0000 [thread overview]
Message-ID: <7923d2289ec044579a3eb00ca339a018@AcuMS.aculab.com> (raw)
In-Reply-To: <20200329214214.GB768293@mit.edu>
From: Theodore Y. Ts'o
> Sent: 29 March 2020 22:42
> On Sun, Mar 29, 2020 at 05:41:22PM +0000, George Spelvin wrote:
> > > Using xor was particularly stupid.
> > > The whole generator was then linear and trivially reversable.
> > > Just using addition would have made it much stronger.
> >
> > I considered changing it to addition (actually, add pairs and XOR the
> > sums), but that would break its self-test. And once I'd done that,
> > there are much better possibilities.
> >
> > Actually, addition doesn't make it *much* stronger. To start
> > with, addition and xor are the same thing at the lsbit, so
> > observing 113 lsbits gives you a linear decoding problem.
>
> David,
>
> If anyone is trying to rely on prandom_u32() as being "strong" in any
> sense of the word in terms of being reversable by attacker --- they
> shouldn't be using prandom_u32(). That's going to be true no matter
> *what* algorithm we use.
Indeed, but xor merging of 4 LFSR gives an appearance of an
improvements (over a single LFSR) but gives none and just
increases the complexity.
David
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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2020-03-30 9:27 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-03 9:51 [RFC PATCH v1 46/50] mm/shuffle.c: use get_random_max() George Spelvin
2020-03-28 18:23 ` Dan Williams
2020-03-28 18:28 ` [RFC PATCH v1 00/52] Audit kernel random number use George Spelvin
2020-03-29 12:21 ` David Laight
2020-03-29 17:41 ` George Spelvin
2020-03-29 21:42 ` Theodore Y. Ts'o
2020-03-30 9:27 ` David Laight [this message]
2020-04-01 5:17 ` lib/random32.c security George Spelvin
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