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Date:   Wed, 18 Sep 2019 19:50:23 +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 18:59, Alexander E. Patrakov пишет:
> 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.
> 

Well, I have to provide another data point. On Arch Linux and MSI Z87I 
desktop board:

$ lsmod | grep rng
<nothing>
$ modinfo rng_core
<yes, the module does exist>

So this particular board has no sources of randomness except interrupts 
(which are scarce), RDRAND (which is not trusted in Arch Linux by 
default) and jitter entropy (which is not collected by the kernel and 
needs haveged or equivalent).

-- 
Alexander E. Patrakov


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