From the email discussion:
> > Usage recommendations
> > The kernel random-number generator relies on entropy gathered
> > from device drivers and other sources of environmental noise.
> > It is designed to produce a small amount of high-quality seed
> > material to seed a cryptographically secure pseudorandom number
> > generator (CSPRNG). It is designed for security, not speed,
> > and is poorly suited to generating large amounts of crypto‐
> > graphic random data. Users should be economical in the amount
> > of seed material that they consume via getrandom(2), /dev/uran‐
> > dom, and /dev/random.
> >
> > ┌─────────────────────────────────────────────────────┐
> > │FIXME │
> > ├─────────────────────────────────────────────────────┤
> > │Is it really necessary to avoid consuming large │
> > │amounts from /dev/urandom? Various sources linked to │
> > │by https://bugzilla.kernel.org/show_bug.cgi?id=71211 │
> > │suggest it is not. │
> > │ │
> > │And: has the answer to the previous question changed │
> > │across kernel versions? │
> > └─────────────────────────────────────────────────────┘
> > Consuming unnecessarily large quantities of data via these
> > interfaces will have a negative impact on other consumers of
> > randomness.
[Ted T'so:]
> So "poorly suited" is definitely true. Also true is that urandom is
> not engineered for use for non-cryptographic uses. It's always going
> to be faster to use random(3) for those purposes.
>
> As far as whether or not it has a negative impact, it depends on how
> much you trust the underlying cryptographic algorithms. If the CSPRNG
> is seeded correctly with at least 256 bits of entropy that can't be
> guessed by the attacker, and if the underlying cryptographic
> primitives are secure, then it won't matter. But *if* there is an
> unknown vulnerability in the underlying primitive, and *if* large
> amounts of data generated by the CSPRNG would help exploit that
> vulnerability, and *if* that bulk amount of CSPRNG output is made
> available to an attacker with the capability to break the underlying
> cryptographic vulnerability, then there would be a problem.
>
> Obviously, no one knows of such a vulnerability, and I'm fairly
> confident that there won't be such a vulnerability across the
> different ways we've used to generate the urandom source --- but some
> people are professional paranoids, and would argue that we shouldn't
> make bulk output of the CSPRNG available for no good reason, just in
> case.
[Nikos Mavrogiannopoulos:]
The above is certainly accurate, however, I think that such a
discussion or text, when reflected to a man-page is going to
cause problems. The audience of a man-page are not crypto people,
and seeing such text would create confusion rather than clarify
how these devices/apis should be used. The *if* part is not put
into a perspective, suggesting that such an *if* is possible.
However, if one clarifies, i.e., in that case, your TLS or SSH
connection is most likely broken as well, and not because of any
attack on /dev/urandom, then one can see that we are heading
towards a theoretical discussion.
My suggestion, on that particular text would be to remove it,
but make it explicit somewhere in the text that all the
assurances for the devices depend on the crypto primitives,
rather than describing risks that may arise on particular
usage patterns *if* primitives are broken.
Reviewed-by: Nikos Mavrogiannopoulos <nmav@redhat.com>
Reported-by: Nikos Mavrogiannopoulos <nmav@redhat.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Michael Kerrisk