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Subject: Re: Linux 5.3-rc8
From:   "Alexander E. Patrakov" <patrakov@gmail.com>
To:     Lennart Poettering <mzxreary@0pointer.de>, Willy Tarreau <w@1wt.eu>
Cc:     "Theodore Y. Ts'o" <tytso@mit.edu>,
        Matthew Garrett <mjg59@srcf.ucam.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Ahmed S. Darwish" <darwish.07@gmail.com>,
        Vito Caputo <vcaputo@pengaru.com>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jan Kara <jack@suse.cz>, Ray Strode <rstrode@redhat.com>,
        William Jon McCann <mccann@jhu.edu>,
        zhangjs <zachary@baishancloud.com>, linux-ext4@vger.kernel.org,
        lkml <linux-kernel@vger.kernel.org>
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 <20190917162137.GA27921@1wt.eu> <20190917171328.GA31798@gardel-login>
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Message-ID: <97a49761-701c-c5d9-0a35-65c01368017e@gmail.com>
Date:   Wed, 18 Sep 2019 19:50:23 +0500
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18.09.2019 18:59, Alexander E. Patrakov =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
> 18.09.2019 18:38, Lennart Poettering =D0=BF=D0=B8=D1=88=D0=B5=D1=82:
>> On Di, 17.09.19 19:29, Willy Tarreau (w@1wt.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 source=
s
>>>> the same as older netbooks with few and crappy sources, ...). They=20
>>>> 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, ver=
y
>>>> common usecase to install a system via SSH, and it's also very commo=
n
>>>> 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 temporar=
y
>>> SSH key. I do make a strong distinction between the installation phas=
e
>>> 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 th=
e
>>> 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=3D1000 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=3Dy 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.
>=20
> Very well said. However, 1000 is more than the hard-coded quality of=20
> some existing rngs, and so would send a misleading message that they ar=
e=20
> somehow worse. I would suggest case-by-case reevaluation of all existin=
g=20
> hwrng drivers by their maintainers, and then setting the default to=20
> something like 899, so that evaluated drivers have priority.
>=20

Well, I have to provide another data point. On Arch Linux and MSI Z87I=20
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=20
(which are scarce), RDRAND (which is not trusted in Arch Linux by=20
default) and jitter entropy (which is not collected by the kernel and=20
needs haveged or equivalent).

--=20
Alexander E. Patrakov


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