Re: [PATCH RFC v3] random: getrandom(2): optionally block when CRNG is uninitialized

From: Ahmed S. Darwish
Date: Sun Sep 15 2019 - 06:02:16 EST

On Sun, Sep 15, 2019 at 11:30:57AM +0200, Willy Tarreau wrote:
> On Sun, Sep 15, 2019 at 10:59:07AM +0200, Lennart Poettering wrote:
> > We live in a world where people run HTTPS, SSH, and all that stuff in
> > the initrd already. It's where SSH host keys are generated, and plenty
> > session keys.
> It is exactly the type of crap that create this situation : making
> people developing such scripts believe that any random source was OK
> to generate these, and as such forcing urandom to produce crypto-solid
> randoms!

Willy, let's tone it down please... the thread is already getting a
bit toxic.

> No, distro developers must know that it's not acceptable to
> generate lifetime crypto keys from the early boot when no entropy is
> available. At least with this change they will get an error returned
> from getrandom() and will be able to ask the user to feed entropy, or
> be able to say "it was impossible to generate the SSH key right now,
> the daemon will only be started once it's possible", or "the SSH key
> we produced will not be saved because it's not safe and is only usable
> for this recovery session".
> > If Linux lets all that stuff run with awful entropy then
> > you pretend things where secure while they actually aren't. It's much
> > better to fail loudly in that case, I am sure.
> This is precisely what this change permits : fail instead of block
> by default, and let applications decide based on the use case.

Unfortunately, not exactly.

Linus didn't want getrandom to return an error code / "to fail" in
that case, but to silently return CRNG-uninitialized /dev/urandom
data, to avoid user-space even working around the error code through

I understand the rationale behind that, of course, and this is what
I've done so far in the V3 RFC.

Nonetheless, this _will_, for example, make systemd-random-seed(8)
save week seeds under /var/lib/systemd/random-seed, since the kernel
didn't inform it about such weakness at all..

The situation is so bad now, that it's more of "some user-space are
more equal than others".. Let's just at least admit this while
discussing the RFC patch in question.


> > Quite frankly, I don't think this is something to fix in the
> > kernel.
> As long as it offers a single API to return randoms, and that it is
> not possible not to block for low-quality randoms, it needs to be
> at least addressed there. Then userspace can adapt. For now userspace
> does not have this option just due to the kernel's way of exposing
> randoms.
> Willy