linux-kernel - Re: [PATCH 1/3] Make /dev/urandom scalable

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Message-ID: <20150924165204.GA2835@unpythonic.net>
Date:	Thu, 24 Sep 2015 11:52:04 -0500
From:	Jeff Epler <jepler@...ythonic.net>
To:	Austin S Hemmelgarn <ahferroin7@...il.com>
Cc:	Theodore Ts'o <tytso@....edu>, Andi Kleen <andi@...stfloor.org>,
	linux-kernel@...r.kernel.org, kirill.shutemov@...ux.intel.com,
	herbert@...dor.apana.org.au, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/3] Make /dev/urandom scalable

On Thu, Sep 24, 2015 at 12:00:44PM -0400, Austin S Hemmelgarn wrote:
> I've had cases where I've done thousands of dieharder runs, and it
> failed almost 10% of the time, while stuff like mt19937 fails in
> otherwise identical tests only about 1-2% of the time

That is a startling result.  Please say what architecture, kernel
version, dieharder version and commandline arguments you are using to
get 10% WEAK or FAILED assessments from dieharder on /dev/urandom.

Since the structure of linux urandom involves taking a cryptographic
hash the basic expectation is that it would fail statistical randomness
tests at similar rates to e.g., dieharder's AES_OFB (-g 205) even in the
absence of any entropy in the kernel pools.

So if 10% failures at correct statistical tests can be replicated it is
important and needs attention.

I did take a few moments to look into this today and got starling
failures (p-value 0.00000000) with e.g., 
    dieharder -g 501 -d 10
(and a few other tests) using dieharder 3.31.1 on both debian
linux-4.1-rt-amd64 and debian kfreebsd-10-amd64, but this seems to be an
upstream bug known at least to debian and redhat, possibly fixed in
current Fedora but apparently not in Debian.
    https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=745742
    https://bugzilla.redhat.com/show_bug.cgi?format=multiple&id=803292
if you have an affected version, these failures are seen only with -g
501, not with -g 200 < /dev/urandom.  They are probably also not seen
with 32-bit dieharder.

 diehard_parking_lot|   0|     12000|     100|0.00000000|  FAILED  
    diehard_2dsphere|   2|      8000|     100|0.00000000|  FAILED  
    diehard_3dsphere|   3|      4000|     100|0.00000000|  FAILED  
     diehard_squeeze|   0|    100000|     100|0.00000000|  FAILED  
        diehard_sums|   0|       100|     100|0.00000000|  FAILED  

Jeff
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