Re: [PATCH 0/7] Rework random blocking

Andy Lutomirski
2019-09-09T22:58:01Z

On Mon, Sep 9, 2019 at 2:42 AM Pavel Machek <pavel@ucw.cz> wrote:
>
> On Thu 2019-08-29 18:11:35, Andy Lutomirski wrote:
> > This makes two major semantic changes to Linux's random APIs:
> >
> > It adds getentropy(..., GRND_INSECURE). This causes getentropy to
> > always return *something*. There is no guarantee whatsoever that
> > the result will be cryptographically random or even unique, but the
> > kernel will give the best quality random output it can. The name is
> > a big hint: the resulting output is INSECURE.
> >
> > The purpose of this is to allow programs that genuinely want
> > best-effort entropy to get it without resorting to /dev/urandom.
> > Plenty of programs do this because they need to do *something*
> > during boot and they can't afford to wait. Calling it "INSECURE" is
> > probably the best we can do to discourage using this API for things
> > that need security.
> >
> > This series also removes the blocking pool and makes /dev/random
> > work just like getentropy(..., 0) and makes GRND_RANDOM a no-op. I
> > believe that Linux's blocking pool has outlived its usefulness.
> > Linux's CRNG generates output that is good enough to use even for
> > key generation. The blocking pool is not stronger in any material
> > way, and keeping it around requires a lot of infrastructure of
> > dubious value.
>
> Could you give some more justification? If crng is good enough for
> you, you can use /dev/urandom...

Take a look at the diffstat. The random code is extremely security
sensitive, and it's made considerably more complicated by the need to
support the blocking semantics for /dev/random. My primary argument
is that there is no real reason for the kernel to continue to support
it.

>
>
> are
>
> > This series should not break any existing programs. /dev/urandom is
> > unchanged. /dev/random will still block just after booting, but it
> > will block less than it used to. getentropy() with existing flags
> > will return output that is, for practical purposes, just as strong
> > as before.
>
> So what is the exact semantic of /dev/random after your change?

Reads return immediately if the CRNG is initialized, i.e reads return
immediately if and only if getentropy(..., 0) would succeed.
Otherwise reads block.

--Andy