Re: [PATCH v25 10/12] LRNG - add TRNG support

From: Alexander E. Patrakov
Date: Wed Nov 20 2019 - 14:57:40 EST


21.11.2019 00:51, Stephan MÃller ÐÐÑÐÑ:
Am Mittwoch, 20. November 2019, 14:29:18 CET schrieb Greg Kroah-Hartman:

Hi Greg,

On Wed, Nov 20, 2019 at 09:58:35AM +0100, Stephan MÃller wrote:
Am Dienstag, 19. November 2019, 13:41:50 CET schrieb Greg Kroah-Hartman:

Hi Greg,

On Tue, Nov 19, 2019 at 02:07:40AM -0800, Andy Lutomirski wrote:
As this would introduce a new device file now, is there a special
process that I need to follow or do I need to copy? Which
major/minor
number should I use?

Looking into static const struct memdev devlist[] I see

[8] = { "random", 0666, &random_fops, 0 },
[9] = { "urandom", 0666, &urandom_fops, 0 },

Shall a true_random be added here with [10]?

I am not at all an expert on chardevs, but this sounds generally
reasonable. gregkh is probably the real authority here.

[10] is the aio char device node, so you better not try to overlap it or
bad things will happen :(

Thanks for your insights.

Which device minor number could we use?

Get your own dynamic one by using a misc device if you _REALLY_ want to
add yet-another-char-node-for-random-data.

But I would have thought that we all realize that this is not the way to
do things. Let's not have "random", "urandom", and "true_random" be
something we want to totally confuse userspace with, that way is insane.

Please just make the existing userspace api "just work", don't add to
the mess.

Thank you, I think we should follow that advise.

With that and considering Alexander's rightful remark we have a challenge. So,
changing the syscall may not be the right way unless we find a way to restrict
the permissions somehow (capability? LSM? None of that seems to be a good
fit).

What about providing a /sys file? I.e. adding a file that:

a) has permissions 440 per default and maybe the ownership of root:root

b) allow user space to perform a chown/chgrp

c) only supports reading of data from user space

But then, how could we provide a common interface for the existing random.c
and the LRNG?

Or should we use a proc file for that? If yes, I guess it should not be a
sysctl, but a "regular" proc file that should allow a chown(2) operation. On
the other hand, is proc the right place to provide a user space interface for
exporting data to user?

Thanks a lot.

Ciao
Stephan



I'd say that a sys or proc file is worse than a device node, because the wanted semantics are exactly those of a device node. Besides, a chown of a sysfs file is something not friendly to containers. We may need different uids in different containers to be able to access true random data.

--
Alexander E. Patrakov

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