From 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/Kconfig
index 59f25286befe..ff2102c0159c 100644
--- a/drivers/char/hw_random/Kconfig
+++ b/drivers/char/hw_random/Kconfig
@@ -35,6 +35,26 @@ config HW_RANDOM_TIMERIOMEM
 
 	  If unsure, say Y.
 
+config HW_RANDOM_JITTERENTROPY
+        tristate "Jitter Entropy HW Random Number Generator support"
+        select CRYPTO_JITTERENTROPY
+        ---help---
+          This driver provides kernel-side support for extracting entropy
+          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, build
+          this driver instead, it has an advantage of being available very
+          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/Makefile
index 7c9ef4a7667f..9c6d1d3626f6 100644
--- a/drivers/char/hw_random/Makefile
+++ b/drivers/char/hw_random/Makefile
@@ -6,6 +6,7 @@
 obj-$(CONFIG_HW_RANDOM) += rng-core.o
 rng-core-y := core.o
 obj-$(CONFIG_HW_RANDOM_TIMERIOMEM) += timeriomem-rng.o
+obj-$(CONFIG_HW_RANDOM_JITTERENTROPY) += jitterentropy-hwrng.o
 obj-$(CONFIG_HW_RANDOM_INTEL) += intel-rng.o
 obj-$(CONFIG_HW_RANDOM_AMD) += amd-rng.o
 obj-$(CONFIG_HW_RANDOM_ATMEL) += 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 generator
+ */
+
+#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 else */
+	if (wait)
+		schedule();
+
+	err = crypto_rng_get_bytes(drng, data, max);
+	if (err)
+		return err;
+	return max;
+}
+
+static struct hwrng jitterentropy_rng = {
+	.name		= KBUILD_MODNAME,
+	.read		= jitterentropy_rng_read,
+	.quality	= 4, /* minimum that guarantees progress in hwrng_fill thread */
+};
+
+static int __init mod_init(void)
+{
+	int ret;
+
+	pr_info("Registering the driver\n");
+	drng = crypto_alloc_rng("jitterentropy_rng", 0, 0);
+	if (!drng) {
+		pr_err("crypto_alloc_rng() failed\n");
+		return -ENODEV;
+	}
+
+	ret = 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");
-- 
2.23.0