Am Montag, 23. Dezember 2019, 09:20:43 CET schrieb Andy Lutomirski:
Hi Andy,
>
> 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.
>
> 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.
If a non-blocking DRNG serving /dev/urandom or getrandom(2) needs reseeding,
at least 512 bits of entropy is left in the pool. Each DRNG seeding operation
requires at least 128 bits and at most 256 bits of entropy. This means that at
least 2 reseed operations worth of entropy is found in the entropy pool even
though massive amount of true random numbers are requested by unprivileged
users.
If a privileged caller requests true random numbers, the entropy pool is
allowed to be exhausted.
Access to the true random number generator is provided with getrandom(2) and
the GRND_TRUERANDOM flag. If the true random number generator (TRNG) is not
compiled or not present, -EOPNOTSUPP is returned.
Entire patch set:
Reviewed-by: Stephan M?ller <[email protected]>
Ciao
Stephan
On Thu, Dec 26, 2019 at 10:29:00AM +0100, Stephan M?ller wrote:
>
> 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.
I think it's reasonable to offer such an option as long as it's made
clear that it'll break userland and should only be enabled under very
weird circumstances. We don't want to end up in a situation where
userland developers feel that they need to code to handle such
situations - the only people who care about this distinction should be
in control of their userland stack and able to cope with the
consequences.
> 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.
I also feel like describing any of this as "true random data" is
misleading. Most of our entropy sources are devices that could, given
sufficient information, be modelled accurately. We're not sampling
quantum events here.
--
Matthew Garrett | [email protected]