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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>,
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        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
Thread-Topic: [RFC PATCH v1 00/52] Audit kernel random number use
Thread-Index: AQHWBS7wHKwkXcotX0GE2ObBbkw++6hffddggACdo1OAAMNccA==
Date:   Mon, 30 Mar 2020 09:27:17 +0000
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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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