Date:   Wed, 18 Sep 2019 18:59:31 +0500
From:   "Alexander E. Patrakov" <patrakov@...il.com>
To:     Lennart Poettering <mzxreary@...inter.de>, Willy Tarreau <w@....eu>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Ahmed S. Darwish" <darwish.07@...il.com>,
        Vito Caputo <vcaputo@...garu.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.cz>, Ray Strode <rstrode@...hat.com>,
        William Jon McCann <mccann@....edu>,
        zhangjs <zachary@...shancloud.com>, linux-ext4@...r.kernel.org,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: Linux 5.3-rc8

18.09.2019 18:38, Lennart Poettering пишет:
> On Di, 17.09.19 19:29, Willy Tarreau (w@....eu) wrote:
> 
>>> What do you expect these systems to do though?
>>>
>>> I mean, think about general purpose distros: they put together live
>>> images that are supposed to work on a myriad of similar (as in: same
>>> arch) but otherwise very different systems (i.e. VMs that might lack
>>> any form of RNG source the same as beefy servers with muliple sources
>>> the same as older netbooks with few and crappy sources, ...). They can't
>>> know what the specific hw will provide or won't. It's not their
>>> incompetence that they build the image like that. It's a common, very
>>> common usecase to install a system via SSH, and it's also very common
>>> to have very generic images for a large number varied systems to run
>>> on.
>>
>> I'm totally file with installing the system via SSH, using a temporary
>> SSH key. I do make a strong distinction between the installation phase
>> and the final deployment. The SSH key used *for installation* doesn't
>> need to the be same as the final one. And very often at the end of the
>> installation we'll have produced enough entropy to produce a correct
>> key.
> 
> That's not how systems are built today though. And I am not sure they
> should be. I mean, the majority of systems at this point probably have
> some form of hardware (or virtualized) RNG available (even raspi has
> one these days!), so generating these keys once at boot is totally
> OK. Probably a number of others need just a few seconds to get the
> entropy needed, where things are totally OK too. The only problem is
> systems that lack any reasonable source of entropy and where
> initialization of the pool will take overly long.
> 
> I figure we can reduce the number of systems where entropy is scarce
> quite a bit if we'd start crediting entropy by default from various hw
> rngs we currently don't credit entropy for. For example, the TPM and
> older intel/amd chipsets. You currently have to specify
> rng_core.default_quality=1000 on the kernel cmdline to make them
> credit entropy. I am pretty sure this should be the default now, in a
> world where CONFIG_RANDOM_TRUST_CPU=y is set anyway. i.e. why say
> RDRAND is fine but those chipsets are not? That makes no sense to me.
> 
> I am very sure that crediting entropy to chipset hwrngs is a much
> better way to solve the issue on those systems than to just hand out
> rubbish randomness.

Very well said. However, 1000 is more than the hard-coded quality of 
some existing rngs, and so would send a misleading message that they are 
somehow worse. I would suggest case-by-case reevaluation of all existing 
hwrng drivers by their maintainers, and then setting the default to 
something like 899, so that evaluated drivers have priority.

-- 
Alexander E. Patrakov


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