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Subject: Re: [PATCH RFC v4 1/1] random: WARN on large getrandom() waits and
 introduce getrandom2()
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        "Theodore Y. Ts'o" <tytso@mit.edu>
Cc:     "Ahmed S. Darwish" <darwish.07@gmail.com>,
        Lennart Poettering <mzxreary@0pointer.de>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        lkml <linux-kernel@vger.kernel.org>, linux-ext4@vger.kernel.org,
        linux-man@vger.kernel.org
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From:   "Alexander E. Patrakov" <patrakov@gmail.com>
Message-ID: <6adb02d4-c486-a945-7f51-d007d6de45b2@gmail.com>
Date:   Fri, 20 Sep 2019 01:45:40 +0500
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20.09.2019 01:04, Linus Torvalds =D0=BF=D0=B8=D1=88=D0=B5=D1=82:

> instead. Yeah, it still doesn't help on machines that don't even have
> a cycle counter, but it at least means that you don't have to have a
> CPU rdrand (or equivalent) but you do have a cycle counter, now the
> extraction of randomness from the pool doesn't just do the
> (predictable) mutation for the backtracking, but actually means that
> you have some very hard to predict timing effects.
>=20
> Again, in this case a cycle counter really does add a small amount of
> entropy (everybody agrees that modern CPU's are simply too complex to
> be predictable at a cycle level), but that's not really the point. The
> point is that now doing the extraction really fundamentally changes
> the state in unpredictable ways, so that you don't have that "if I
> recognize a value, I know what the next value will be" kind of attack.

This already resembles in-kernel haveged (except that it doesn't credit=20
entropy), and Willy Tarreau said "collect the small entropy where it is, =

period" today. So, too many people touched upon the topic in one day,=20
and therefore I'll bite.

We already have user-space software (haveged and modern versions of=20
rngd) that extract supposed entropy from clock jitter and feed it back=20
to the kernel via /dev/random (crediting it). Indeed, at present, on=20
some hardware this is the only way for distributions and users to=20
collect enough entropy during boot and avoid stalls - all other=20
suggestions are simply non-constructive. Also, Google's Fuchsia OS does=20
use and credit jitter entropy.

For the record: I do not have a justifiable opinion whether haveged/rngd =

output (known as jitter entropy) actually contains any entropy. I=20
understand that there are two possible viewpoints here. The rest of the=20
email is written under the assumption that haveged does provide real=20
entropy and not fake one.

The problem that I have with the current situation is that distributions =

and users, when they set up their systems to run haveged or rngd, often=20
do it incorrectly (even, as mentioned, under the assumption that haveged =

is something valid and useful). The most common mistake is relying on=20
systemd-provided default dependencies, thus not starting such software=20
as early as possible. Even worse, no initramfs generator allows one to=20
easily include haveged/rngd in the initramfs and run it there. And for=20
me, the first urandom warning comes from the initramfs, so anything=20
started from the main system is, arguably, already too late.

Therefore, I think, an in-kernel hwrng that exposes jitter entropy is=20
something useful (for those who agree that jitter entropy is not fake),=20
because it avoids the pitfall-ridden userspace setup. Just as an=20
exercise, I have implemented a very simple driver (attached as a patch)=20
that does just that. I am only half-serious here, the driver is only=20
lightly tested in KVM without any devices except an unconnected virtio=20
network card, not on any real hardware. Someone else can also find it=20
useful as a test/fake hwrng driver.

I am aware that there was an earlier decision that jitter entropy should =

not be credited, i.e. effectively a pre-existing NAK from Theodore Ts'o. =

But, well, distributions are already overriding this decision in=20
userspace, and do it badly, so in my viewpoint, the driver would be a=20
net win if some mechanism is added that makes it a no-op by default even =

if the driver is built-in. E.g. an explicit "enable" parameter, but I am =

open to other suggestions, too.

--=20
Alexander E. Patrakov

--------------84DCDD92DB3AA9B299024B99
Content-Type: text/x-patch; charset=UTF-8;
 name="0001-hw_random-Add-jitterentropy_hwrng.patch"
Content-Transfer-Encoding: quoted-printable
Content-Disposition: attachment;
 filename="0001-hw_random-Add-jitterentropy_hwrng.patch"

=46rom 2836990aff5bc1dab6a4e927304247dae469c774 Mon Sep 17 00:00:00 2001
From: "Alexander E. Patrakov" <patrakov@gmail.com>
Date: Thu, 19 Sep 2019 01:18:39 +0500
Subject: [PATCH] hw_random: Add jitterentropy_hwrng

This re-exports the existing "jitterentropy_rng" cryptoapi RNG as a
hwrng. The use case is to replace haveged, which distributions
often misconfigure by running it too late, while it is really needed
even in the initramfs on some systems.

Signed-off-by: Alexander E. Patrakov <patrakov@gmail.com>
---
 drivers/char/hw_random/Kconfig               | 20 ++++++
 drivers/char/hw_random/Makefile              |  1 +
 drivers/char/hw_random/jitterentropy-hwrng.c | 70 ++++++++++++++++++++
 3 files changed, 91 insertions(+)
 create mode 100644 drivers/char/hw_random/jitterentropy-hwrng.c

diff --git a/drivers/char/hw_random/Kconfig b/drivers/char/hw_random/Kcon=
fig
index 59f25286befe..ff2102c0159c 100644
--- a/drivers/char/hw_random/Kconfig
+++ b/drivers/char/hw_random/Kconfig
@@ -35,6 +35,26 @@ config HW_RANDOM_TIMERIOMEM
=20
 	  If unsure, say Y.
=20
+config HW_RANDOM_JITTERENTROPY
+        tristate "Jitter Entropy HW Random Number Generator support"
+        select CRYPTO_JITTERENTROPY
+        ---help---
+          This driver provides kernel-side support for extracting entrop=
y
+          from CPU and memory clock jitter.
+
+          jitterentropy-hwrng serves the same purpose as haveged, but is=
 in
+          the kernel. So, if you otherwise would have to run haveged, bu=
ild
+          this driver instead, it has an advantage of being available ve=
ry
+          early in the boot process.
+
+          Note that it is still not known whether clock jitter provides =
any
+          actual entropy.
+
+          To compile this driver as a module, choose M here: the
+          module will be called jitterentropy-hwrng.
+
+          If unsure, say N.
+
 config HW_RANDOM_INTEL
 	tristate "Intel HW Random Number Generator support"
 	depends on (X86 || IA64) && PCI
diff --git a/drivers/char/hw_random/Makefile b/drivers/char/hw_random/Mak=
efile
index 7c9ef4a7667f..9c6d1d3626f6 100644
--- a/drivers/char/hw_random/Makefile
+++ b/drivers/char/hw_random/Makefile
@@ -6,6 +6,7 @@
 obj-$(CONFIG_HW_RANDOM) +=3D rng-core.o
 rng-core-y :=3D core.o
 obj-$(CONFIG_HW_RANDOM_TIMERIOMEM) +=3D timeriomem-rng.o
+obj-$(CONFIG_HW_RANDOM_JITTERENTROPY) +=3D jitterentropy-hwrng.o
 obj-$(CONFIG_HW_RANDOM_INTEL) +=3D intel-rng.o
 obj-$(CONFIG_HW_RANDOM_AMD) +=3D amd-rng.o
 obj-$(CONFIG_HW_RANDOM_ATMEL) +=3D atmel-rng.o
diff --git a/drivers/char/hw_random/jitterentropy-hwrng.c b/drivers/char/=
hw_random/jitterentropy-hwrng.c
new file mode 100644
index 000000000000..b7aeefe4f47d
--- /dev/null
+++ b/drivers/char/hw_random/jitterentropy-hwrng.c
@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2019 Alexander E. Patrakov <patrakov@gmail.com>
+ *
+ * Driver that exposes CPU clock jitter as a hardware random number gene=
rator
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/delay.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/hw_random.h>
+#include <crypto/rng.h>
+
+static struct crypto_rng *drng;
+
+static int jitterentropy_rng_read(struct hwrng *rng, void *data, size_t =
max, bool wait)
+{
+	int err;
+
+	/* Prevent the hwrng_fill thread from impeding progress of everything e=
lse */
+	if (wait)
+		schedule();
+
+	err =3D crypto_rng_get_bytes(drng, data, max);
+	if (err)
+		return err;
+	return max;
+}
+
+static struct hwrng jitterentropy_rng =3D {
+	.name		=3D KBUILD_MODNAME,
+	.read		=3D jitterentropy_rng_read,
+	.quality	=3D 4, /* minimum that guarantees progress in hwrng_fill threa=
d */
+};
+
+static int __init mod_init(void)
+{
+	int ret;
+
+	pr_info("Registering the driver\n");
+	drng =3D crypto_alloc_rng("jitterentropy_rng", 0, 0);
+	if (!drng) {
+		pr_err("crypto_alloc_rng() failed\n");
+		return -ENODEV;
+	}
+
+	ret =3D hwrng_register(&jitterentropy_rng);
+	if (ret) {
+		crypto_free_rng(drng);
+		return ret;
+	}
+
+	return 0;
+}
+
+static void __exit mod_exit(void)
+{
+	hwrng_unregister(&jitterentropy_rng);
+	crypto_free_rng(drng);
+}
+
+module_init(mod_init);
+module_exit(mod_exit);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Alexander E. Patrakov <patrakov@gmail.com>");
+MODULE_DESCRIPTION("Exposes clock jitter as a hwrng");
+MODULE_SOFTDEP("pre: jitterentropy_rng");
--=20
2.23.0


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