linux-kernel - Re: [PATCH 1/5] random: fix crng

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Date:   Fri, 13 Apr 2018 13:00:37 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Stephan Mueller <smueller@...onox.de>
Cc:     linux-crypto@...r.kernel.org,
        Linux Kernel Developers List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] random: fix crng_ready() test

On Fri, Apr 13, 2018 at 03:05:01PM +0200, Stephan Mueller wrote:
> 
> What I would like to point out that more and more folks change to 
> getrandom(2). As this call will now unblock much later in the boot cycle, 
> these systems see a significant departure from the current system behavior.
> 
> E.g. an sshd using getrandom(2) would be ready shortly after the boot finishes 
> as of now. Now it can be a matter minutes before it responds. Thus, is such 
> change in the kernel behavior something for stable?

It will have some change on the kernel behavior, but not as much as
you might think.  That's because in older kernels, we were *already*
blocking until crng_init > 2 --- if the getrandom(2) call happened
while crng_init was in state 0.

Even before this patch series, we didn't wake up a process blocked on
crng_init_wait until crng_init state 2 is reached:

static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
{
	...
	if (crng == &primary_crng && crng_init < 2) {
		invalidate_batched_entropy();
		crng_init = 2;
		process_random_ready_list();
		wake_up_interruptible(&crng_init_wait);
		pr_notice("random: crng init done\n");
	}
}

This is the reason why there are reports like this: "Boot delayed for
about 90 seconds until 'random: crng init done'"[1]

[1] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1685794

So we have the problem already.  There will be more cases of this
after this patch series is applied, true.  But what we have already is
an inconsistent state where if you call getrandom(2) while the kernel
is in crng_init state 0, you will block until crng_init state 2, but
if you are in crng_init state 1, you will assume the CRNG is fully
initialized.

Given the documentation of how getrandom(2) works what its documented
guarantees are, I think it does justify making its behavior both more
consistent with itself, and more consistent what the security
guarantees we have promised people.

I was a little worried that on VM's this could end up causing things
to block for a long time, but an experiment on a GCE VM shows that
isn't a problem:

[    0.000000] Linux version 4.16.0-rc3-ext4-00009-gf6b302ebca85 (tytso@...c) (gcc version 7.3.0 (Debian 7.3.0-15)) #16 SMP Thu Apr 12 16:57:17 EDT 2018
[    1.282220] random: fast init done
[    3.987092] random: crng init done
[    4.376787] EXT4-fs (sda1): re-mounted. Opts: (null)

There are some desktops where the "crng_init done" report doesn't
happen until 45-90 seconds into the boot.  I don't think I've seen
reports where it takes _minutes_ however.  Can you give me some
examples of such cases?

					- Ted

P.S.  Of course, in a VM environment, if the host supports virtio-rng,
the boot delay problem is completely not an issue.  You just have to
enable virtio-rng in the guest kernel, which I believe is already the
case for most distro kernels.

BTW, for KVM, it's fairly simple to set it the host-side support for
virtio-rng.  Just add to the kvm command-line options:

    -object rng-random,filename=/dev/urandom,id=rng0 \
	-device virtio-rng-pci,rng=rng0