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Date:   Sun, 29 Mar 2020 17:42:14 -0400
From:   "Theodore Y. Ts'o" <tytso@mit.edu>
To:     George Spelvin <lkml@SDF.ORG>
Cc:     David Laight <David.Laight@ACULAB.COM>,
        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
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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.

Better distribution?  Sure.  Making prandom_u32() faster?  Absolutely;
that's its primary Raison d'Etre.

George,

Did you send the full set of patches to a single mailing list?  Or can
you make it available on a git tree somewhere?  I've y seen this
message plus the ext4 related change, and I can't find the full patch
series anywhere.  If you can send the next version such that it's
fully cc'ed to linux-kernel, that would be really helpful.

Thanks!!

						- Ted