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Date:   Mon, 7 Nov 2022 13:44:34 +0100
From:   Dominik Brodowski <linux@dominikbrodowski.net>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     herbert@gondor.apana.org.au, linux-crypto@vger.kernel.org,
        Ard Biesheuvel <ardb@kernel.org>
Subject: Re: [PATCH v2] hw_random: treat default_quality as a maximum and
 default to 1024
Message-ID: <20221107124434.GA9166@isilmar-4.linta.de>
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 <20221107122455.6169-1-Jason@zx2c4.com>
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On Mon, Nov 07, 2022 at 01:24:55PM +0100, Jason A. Donenfeld wrote:
> Most hw_random devices return entropy which is assumed to be of full
> quality, but driver authors don't bother setting the quality knob. Some
> hw_random devices return less than full quality entropy, and then driver
> authors set the quality knob. Therefore, the entropy crediting should be
> opt-out rather than opt-in per-driver, to reflect the actual reality on
> the ground.
> 
> For example, the two Raspberry Pi RNG drivers produce full entropy
> randomness, and both EDK2 and U-Boot's drivers for these treat them as
> such. The result is that EFI then uses these numbers and passes the to
> Linux, and Linux credits them as boot, thereby initializing the RNG.
> Yet, in Linux, the quality knob was never set to anything, and so on the
> chance that Linux is booted without EFI, nothing is ever credited.
> That's annoying.
> 
> The same pattern appears to repeat itself throughout various drivers. In
> fact, very very few drivers have bothered setting quality=1024.
> 
> Looking at the git history of existing drivers and corresponding mailing
> list discussion, this conclusion tracks. There's been a decent amount of
> discussion about drivers that set quality < 1024 -- somebody read and
> interepreted a datasheet, or made some back of the envelope calculation
> somehow. But there's been very little, if any, discussion about most
> drivers where the quality is just set to 1024 or unset (or set to 1000
> when the authors misunderstood the API and assumed it was base-10 rather
> than base-2); in both cases the intent was fairly clear of, "this is a
> hardware random device; it's fine."
> 
> So let's invert this logic. A hw_random struct's quality knob now
> controls the maximum quality a driver can produce, or 0 to specify 1024.
> Then, the module-wide switch called "default_quality" is changed to
> represent the maximum quality of any driver. By default it's 1024, and
> the quality of any particular driver is then given by:
> 
>     min(default_quality, rng->quality ?: 1024);
> 
> This way, the user can still turn this off for weird reasons (and we can
> replace whatever driver-specific disabling hacks existed in the past),
> yet we get proper crediting for relevant RNGs.
> 
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Herbert Xu <herbert@gondor.apana.org.au>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Thanks for the additional explanation!

	Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>

Thanks,
	Dominik