On Mon, Sep 9, 2019 at 2:42 AM Pavel Machek [off-list ref] wrote:

On Thu 2019-08-29 18:11:35, Andy Lutomirski wrote:

quoted

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

quoted

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