Re: [RFC PATCH v1 00/52] Audit kernel random number use

[Theodore Y. Ts'o]

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