From 243b20ab41748e898abcf298b0cb836f04391afd Mon Sep 17 00:00:00 2001
From: Stephan Mueller <smueller@chronox.de>
Date: Sun, 15 May 2022 18:43:30 +0200
Subject: [PATCH 25/25] LRNG - add hwrand framework interface

The LRNG can be registered with the hardware random number generator
framework. This offers a random number generator with the name "lrng"
that is accessible via the framework. For example it allows pulling
data from the LRNG via the /dev/hwrng file.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
---
 drivers/char/lrng/Kconfig                 | 20 +++----
 drivers/char/lrng/Makefile                |  1 +
 drivers/char/lrng/lrng_interface_hwrand.c | 68 +++++++++++++++++++++++
 3 files changed, 79 insertions(+), 10 deletions(-)
 create mode 100644 drivers/char/lrng/lrng_interface_hwrand.c

--- a/drivers/char/lrng/Kconfig
+++ b/drivers/char/lrng/Kconfig
@@ -101,16 +101,16 @@ config LRNG_KCAPI_IF
 	  is intended to be higher than the existing RNG
 	  implementations.
 
-# config LRNG_HWRAND_IF
-# 	tristate "Interface with Hardware Random Number Generator Framework"
-# 	depends on HW_RANDOM
-# 	select LRNG_DRNG_ATOMIC
-# 	help
-# 	  The LRNG can be registered with the hardware random number
-# 	  generator framework. This offers a random number generator
-# 	  with the name "lrng" that is accessible via the framework.
-# 	  For example it allows pulling data from the LRNG via the
-# 	  /dev/hwrng file.
+config LRNG_HWRAND_IF
+	tristate "Interface with Hardware Random Number Generator Framework"
+	depends on HW_RANDOM
+	select LRNG_DRNG_ATOMIC
+	help
+	  The LRNG can be registered with the hardware random number
+	  generator framework. This offers a random number generator
+	  with the name "lrng" that is accessible via the framework.
+	  For example it allows pulling data from the LRNG via the
+	  /dev/hwrng file.
 
 config LRNG_DEV_IF
 	bool "Character device file interface"
--- a/drivers/char/lrng/Makefile
+++ b/drivers/char/lrng/Makefile
@@ -36,3 +36,4 @@ obj-$(CONFIG_LRNG_RANDOM_IF)		+= lrng_in
 					   lrng_interface_aux.o
 obj-$(CONFIG_LRNG_KCAPI_IF)		+= lrng_interface_kcapi.o
 obj-$(CONFIG_LRNG_DEV_IF)		+= lrng_interface_dev.o
+obj-$(CONFIG_LRNG_HWRAND_IF)		+= lrng_interface_hwrand.o
--- /dev/null
+++ b/drivers/char/lrng/lrng_interface_hwrand.c
@@ -0,0 +1,68 @@
+// SPDX-License-Identifier: GPL-2.0 OR BSD-2-Clause
+/*
+ * LRNG interface with the HW-Random framework
+ *
+ * Copyright (C) 2022, Stephan Mueller <smueller@chronox.de>
+ */
+
+#include <linux/lrng.h>
+#include <linux/hw_random.h>
+#include <linux/module.h>
+
+static int lrng_hwrand_if_random(struct hwrng *rng, void *buf, size_t max,
+				 bool wait)
+{
+	/*
+	 * lrng_get_random_bytes_full not called as we cannot block.
+	 *
+	 * Note: We should either adjust .quality below depending on
+	 * rng_is_initialized() or block here, but neither is not supported by
+	 * the hw_rand framework.
+	 */
+	lrng_get_random_bytes(buf, max);
+	return (int)max;
+}
+
+static struct hwrng lrng_hwrand = {
+	.name		= "lrng",
+	.init		= NULL,
+	.cleanup	= NULL,
+	.read		= lrng_hwrand_if_random,
+
+	/*
+	 * We set .quality only in case the LRNG does not provide the common
+	 * interfaces or does not use the legacy RNG as entropy source. This
+	 * shall avoid that the LRNG automatically spawns the hw_rand
+	 * framework's hwrng kernel thread to feed data into
+	 * add_hwgenerator_randomness. When the LRNG implements the common
+	 * interfaces, this function feeds the data directly into the LRNG.
+	 * If the LRNG uses the legacy RNG as entropy source,
+	 * add_hwgenerator_randomness is implemented by the legacy RNG, but
+	 * still eventually feeds the data into the LRNG. We should avoid such
+	 * circular loops.
+	 *
+	 * We can specify full entropy here, because the LRNG is designed
+	 * to provide full entropy.
+	 */
+#if !defined(CONFIG_LRNG_RANDOM_IF) && \
+    !defined(CONFIG_LRNG_KERNEL_RNG)
+	.quality	= 1024,
+#endif
+};
+
+static int __init lrng_hwrand_if_mod_init(void)
+{
+	return hwrng_register(&lrng_hwrand);
+}
+
+static void __exit lrng_hwrand_if_mod_exit(void)
+{
+	hwrng_unregister(&lrng_hwrand);
+}
+
+module_init(lrng_hwrand_if_mod_init);
+module_exit(lrng_hwrand_if_mod_exit);
+
+MODULE_LICENSE("Dual BSD/GPL");
+MODULE_AUTHOR("Stephan Mueller <smueller@chronox.de>");
+MODULE_DESCRIPTION("Entropy Source and DRNG Manager HW-Random Interface");