On Dec 26, 2019, at 5:29 PM, Stephan Müller [off-list ref] wrote:

Am Montag, 23. Dezember 2019, 09:20:43 CET schrieb Andy Lutomirski:

Hi Andy,

quoted

There are some open questions and future work here:

Should the kernel provide an interface to get software-generated
"true random" numbers?  I can think of only one legitimate reason to
use such an interface: compliance with government standards.  If the
kernel provides such an interface going forward, I think it should
be a brand new character device, and it should have a default mode
0440 or similar.  Software-generated "true random numbers" are a
very limited resource, and resource exhaustion is a big deal.  Ask
anyone who has twiddled their thumbs while waiting for gnupg to
generate a key.  If we think the kernel might do such a thing, then
patches 5-8 could be tabled for now.

What about offering a compile-time option to enable or disable such code?
Note, with the existing random.c code base, there is no need to have a 
separate blocking_pool. The ChaCha20 DRNG could be used for that very same 
purpose, provided that in case these true random numbers are generated when 
the Chacha20 DRNG received an equal amount of "unused" entropy.

This scares me. The DRNG should be simple and easy to understand. If we’re tapping extra numbers in some weird way, then I would be more comfortable with some clear assurance that this doesn’t break the security. If we’re tapping numbers in the same way as normal urandom, then I don’t really see the point.

quoted

Alternatively, perhaps the kernel should instead provide a
privileged interface to read out raw samples from the various
entropy sources, and users who care could have a user daemon that
does something intelligent with them.  This would push the mess of
trying to comply with whatever standards are involved to userspace.
Userspace could then export "true randomness" via CUSE if it is so
inclined, or could have a socket with a well-known name, or whatever
else seems appropriate.

With the patch set v26 of my LRNG I offer another possible alternative 
avoiding any additional character device file and preventing the starvation of 
legitimate use cases: the LRNG has an entropy pool that leaves different 
levels of entropy in the pool depending on the use cases of this data.

If an unprivileged caller requests true random data, at least 1024 bits of 
entropy is left in the pool. I.e. all entropy above that point is available 
for this request type. Note, even namespaces fall into this category 
considering that unprivileged users can create a user name space in which they 
can become root.

This doesn’t solve the problem. If two different users run stupid programs like gnupg, they will starve each other.

As I see it, there are two major problems with /dev/random right now: it’s prone to DoS (i.e. starvation, malicious or otherwise), and, because no privilege is required, it’s prone to misuse. Gnupg is misuse, full stop. If we add a new unprivileged interface, gnupg and similar programs will use it, and we lose all over again.