Re: [PATCH v2 02/03]: hwrng: create filler thread

From: "H. Peter Anvin" <hpa@zytor.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Torsten Duwe <duwe@lst.de>, "Theodore Ts'o" <tytso@mit.edu>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Matt Mackall <mpm@selenic.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Arnd Bergmann <arnd@arndb.de>,
	Rusty Russell <rusty@rustcorp.com.au>,
	Satoru Takeuchi <satoru.takeuchi@gmail.com>,
	ingo.tuchscherer@de.ibm.com,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Hans-Georg Markgraf <MGRF@de.ibm.com>,
	Gerald Schaefer <gerald.schaefer@de.ibm.com>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Joe Perches <joe@perches.com>, Greg Price <price@mit.edu>
Subject: Re: [PATCH v2 02/03]: hwrng: create filler thread
Date: Wed, 26 Mar 2014 18:55:17 -0700	[thread overview]
Message-ID: <56e20125-95d3-465f-b582-fba5ba74d2f3@email.android.com> (raw)
In-Reply-To: <CALCETrUa_y6nGAtHmKkMwfTttXEabXCWPsXv==RrJN0_T3imZA@mail.gmail.com>

There are a number of things wrong with this post, but I'll respond in detail when I get to a keyboard.

On March 26, 2014 6:11:53 PM PDT, Andy Lutomirski <luto@amacapital.net> wrote:
>[cc: Greg Price, might be working on this stuff]
>
>On Wed, Mar 26, 2014 at 6:03 PM, H. Peter Anvin <hpa@zytor.com> wrote:
>> I'm wondering more about the default.  We default to 50% for
>arch_get_random_seed, and this is supposed to be the default for in
>effect unverified hwrngs...
>
>TBH I'm highly skeptical of this kind of entropy estimation.
>/dev/random is IMO just silly, since you need to have very
>conservative entropy estimates for the concept to really work, and
>that ends up being hideously slow.  Also, in the /dev/random sense,
>most hardware RNGs have no entropy at all, since they're likely to be
>FIPS-approved DRBGs that don't have a real non-deterministic source.
>
>For the kernel's RNG to be secure, I think it should have the property
>that it still works if you rescale all the entropy estimates by any
>constant that's decently close to 1.
>
>If entropy estimates are systematically too low, then a naive
>implementation results in an excessively long window during early
>bootup in which /dev/urandom is completely insecure.
>
>If entropy estimates are systematically too high, then a naive
>implementation fails to do a catastrophic reseed, and the RNG can be
>brute-forced.
>
>So I think that the core code should do something along the lines of
>using progressively larger reseeds.  Since I think that /dev/random is
>silly, this means that we only really care about the extent to which
>"entropy" measures entropy conditioned on whatever an attacker can
>actually compute.  Since this could vary widely between devices (e.g.
>if your TPM is malicious), I think that the best we can do is to
>collect ~256 bits from everything available, shove it all in to the
>core together, and repeat.  For all I know, the core code already does
>this.
>
>The upshot is that the actual rescaling factor should barely matter.
>50% is probably fine.  So is 100% and 25%.  10% is probably asking for
>trouble during early boot if all you have is a TPM.
>
>--Andy

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.