Re: [PATCH v2 1/2] hwrng: iproc-rng200 - Set the quality value

From: Lukasz Stelmach <l.stelmach@samsung.com>
To: Stefan Wahren <stefan.wahren@i2se.com>
Cc: Matt Mackall <mpm@selenic.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Arnd Bergmann <arnd@arndb.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Ray Jui <rjui@broadcom.com>,
	Scott Branden <sbranden@broadcom.com>,
	bcm-kernel-feedback-list@broadcom.com,
	Kukjin Kim <kgene@kernel.org>,
	Krzysztof Kozlowski <krzk@kernel.org>,
	Florian Fainelli <f.fainelli@gmail.com>,
	Markus Elfring <elfring@users.sourceforge.net>,
	Matthias Brugger <mbrugger@suse.com>,
	linux-crypto@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-samsung-soc@vger.kernel.org,
	Stephan Mueller <smueller@chronox.de>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Subject: Re: [PATCH v2 1/2] hwrng: iproc-rng200 - Set the quality value
Date: Thu, 21 May 2020 21:14:02 +0200	[thread overview]
Message-ID: <dleftjftbtyubp.fsf%l.stelmach@samsung.com> (raw)
In-Reply-To: <573b2eff-3c74-90e9-50fa-644264d0a0e5@i2se.com> (Stefan Wahren's message of "Thu, 21 May 2020 13:00:33 +0200")

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It was <2020-05-21 czw 13:00>, when Stefan Wahren wrote:
> Hi Lukasz,
>
> Am 19.05.20 um 23:25 schrieb Łukasz Stelmach:
>> The value was estimaded with ea_iid[1] using on 10485760 bytes read from
>> the RNG via /dev/hwrng. The min-entropy value calculated using the most
>> common value estimate (NIST SP 800-90P[2], section 6.3.1) was 7.964464.
>
> could you please mention in the commit the used hardware
> implementation(s) of iproc-rng200 to get this quality?
>
> AFAIK there is still no public register description at least for the
> bcm2711. So is it safe to assume that the suggested quality applies to
> all possible configurations?

I've learnt that there is a post-processing unit in RNG200 that tests
the output of the noise generator and fills FIFO only with data that
passes FIPS tests. Unlike simmilar unit in Exynos, it cannot be disabled
or bypassed. Therefore, after Stephan Mueller's thorough explanations I
am considering dropping this patch in v3.

However, I stil am still not 100% convinced that it is impossible to
assign the quality a reasonable non-zero value in such case.

> Thanks
> Stefan
>
>>
>> [1] https://protect2.fireeye.com/url?k=da4735b2-87d99b28-da46befd-0cc47a336fae-e1c21080bc6ab1e4&q=1&u=https%3A%2F%2Fgithub.com%2Fusnistgov%2FSP800-90B_EntropyAssessment
>> [2] https://csrc.nist.gov/publications/detail/sp/800-90b/final
>>
>> Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
>> ---
>>  drivers/char/hw_random/iproc-rng200.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/drivers/char/hw_random/iproc-rng200.c b/drivers/char/hw_random/iproc-rng200.c
>> index 32d9fe61a225..95669ece050f 100644
>> --- a/drivers/char/hw_random/iproc-rng200.c
>> +++ b/drivers/char/hw_random/iproc-rng200.c
>> @@ -199,6 +199,7 @@ static int iproc_rng200_probe(struct platform_device *pdev)
>>  	priv->rng.read = iproc_rng200_read,
>>  	priv->rng.init = iproc_rng200_init,
>>  	priv->rng.cleanup = iproc_rng200_cleanup,
>> +	priv->rng.quality = 1000,
>>  
>>  	/* Register driver */
>>  	ret = devm_hwrng_register(dev, &priv->rng);
>
>

-- 
Łukasz Stelmach
Samsung R&D Institute Poland
Samsung Electronics

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