[nrf noup] entropy: Add fake entropy nRF PRNG driver

This adds temporary entropy driver simulation for
nRF54h20 device since final entropy source is not
available yet.

TODO: Remove this commit when proper solution will
be available.

Jira: NCSDK-25947

Signed-off-by: Kamil Gawor <[email protected]>

boards/arm/nrf54h20dk_nrf54h20/nrf54h20dk_nrf54h20_cpuapp.dts

@@ -22,6 +22,7 @@
 		zephyr,code-partition = &cpuapp_slot0_partition;
 		zephyr,flash = &mram1x;
 		zephyr,sram = &cpuapp_data;
+		zephyr,entropy = &prng;
 	};
 
 	aliases {
@@ -86,6 +87,11 @@
 			label = "Green LED 3";
 		};
 	};
+
+	prng: prng {
+		compatible = "nordic,entropy-prng";
+		status = "okay";
+	};
 };
 
 &cpuapp_ram0x_region {

boards/arm/nrf54h20dk_nrf54h20/nrf54h20dk_nrf54h20_cpurad.dts

@@ -23,6 +23,12 @@
 		zephyr,code-partition = &cpurad_slot0_partition;
 		zephyr,flash = &mram1x;
 		zephyr,sram = &cpurad_ram0;
+		zephyr,entropy = &prng;
+	};
+
+	prng: prng {
+		compatible = "nordic,entropy-prng";
+		status = "okay";
 	};
 };
 

drivers/entropy/CMakeLists.txt

@@ -34,6 +34,13 @@ zephyr_library_sources_ifdef(CONFIG_ENTROPY_BT_HCI             entropy_bt_hci.c)
 zephyr_library_sources_ifdef(CONFIG_ENTROPY_GECKO_SE           entropy_gecko_se.c)
 zephyr_library_sources_ifdef(CONFIG_ENTROPY_PSA_CRYPTO_RNG     entropy_psa_crypto.c)
 
+if (CONFIG_FAKE_ENTROPY_NRF_PRNG)
+  zephyr_library_sources(fake_entropy_nrf_prng.c)
+
+  message(WARNING "\nA nRF PRNG is used, which does not produce real random bits."
+  "This is not secure and should therefore never be used in a product.")
+endif()
+
 if (CONFIG_BUILD_WITH_TFM)
   target_include_directories(${ZEPHYR_CURRENT_LIBRARY} PRIVATE
     $<TARGET_PROPERTY:tfm,TFM_BINARY_DIR>/api_ns/interface/include

drivers/entropy/Kconfig

@@ -35,6 +35,7 @@ source "drivers/entropy/Kconfig.gecko"
 source "drivers/entropy/Kconfig.neorv32"
 source "drivers/entropy/Kconfig.bt_hci"
 source "drivers/entropy/Kconfig.psa_crypto"
+source "drivers/entropy/Kconfig.nrf_prng"
 
 config ENTROPY_HAS_DRIVER
 	bool

drivers/entropy/Kconfig.nrf_prng

@@ -0,0 +1,19 @@
+# nRF fake entropy prng generator driver configuration
+
+# Copyright (c) 2024 Nordic Semiconductor ASA
+# SPDX-License-Identifier: Apache-2.0
+
+if ENTROPY_GENERATOR
+
+config FAKE_ENTROPY_NRF_PRNG
+	bool "A fake nRF entropy driver"
+	default y
+	depends on DT_HAS_NORDIC_ENTROPY_PRNG_ENABLED
+	depends on SOC_SERIES_NRF54HX
+	select ENTROPY_HAS_DRIVER
+	help
+	  This is a super simple PRNG driver that can be used on nRF platforms that
+	  do not have an entropy source.
+	  This is NOT SAFE to use for cryptographic operations!
+
+endif

drivers/entropy/fake_entropy_nrf_prng.c

@@ -0,0 +1,101 @@
+/*
+ * Copyright (c) 2024 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/init.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <assert.h>
+#include <zephyr/drivers/entropy.h>
+
+#define DT_DRV_COMPAT nordic_entropy_prng
+
+/* This file implements a pseudo-RNG
+ * https://vigna.di.unimi.it/xorshift/xoshiro128plus.c
+ */
+
+static uint32_t s[4];
+
+static inline uint32_t rotl(const uint32_t x, int k)
+{
+	return (x << k) | (x >> (32 - k));
+}
+
+static uint32_t rng_next(void)
+{
+	const uint32_t result = rotl(s[0] + s[3], 7) + s[0];
+
+	const uint32_t t = s[1] << 9;
+
+	s[2] ^= s[0];
+	s[3] ^= s[1];
+	s[1] ^= s[2];
+	s[0] ^= s[3];
+
+	s[2] ^= t;
+
+	s[3] = rotl(s[3], 11);
+
+	return result;
+}
+
+static int entropy_prng_get_entropy(const struct device *dev, uint8_t *buffer, uint16_t length)
+{
+	ARG_UNUSED(dev);
+
+	while (length) {
+		/*
+		 * Note that only 1 thread (Zephyr thread or HW models), runs at
+		 * a time, therefore there is no need to use random_r()
+		 */
+		uint32_t value = rng_next();
+
+		size_t to_copy = MIN(length, sizeof(long));
+
+		memcpy(buffer, &value, to_copy);
+		buffer += to_copy;
+		length -= to_copy;
+	}
+
+	return 0;
+}
+
+static int entropy_prng_get_entropy_isr(const struct device *dev, uint8_t *buf, uint16_t len,
+					uint32_t flags)
+{
+	ARG_UNUSED(flags);
+
+	int err;
+
+	/*
+	 * entropy_prng_get_entropy() is also safe for ISRs
+	 * and always produces data.
+	 */
+	err = entropy_prng_get_entropy(dev, buf, len);
+	if (err < 0) {
+		return err;
+	} else {
+		return len;
+	}
+}
+
+static int entropy_prng_init(const struct device *dev)
+{
+	ARG_UNUSED(dev);
+
+	/* Picked some arbitrary initial seed. */
+	s[0] = 0xAF568BC0;
+	s[1] = 0xAC34307E;
+	s[2] = 0x9B7F6DD1;
+	s[3] = 0xD84319FC;
+	return 0;
+}
+
+static const struct entropy_driver_api entropy_prng_api_funcs = {
+	.get_entropy = entropy_prng_get_entropy, .get_entropy_isr = entropy_prng_get_entropy_isr};
+
+DEVICE_DT_INST_DEFINE(0, entropy_prng_init, NULL, NULL, NULL, PRE_KERNEL_1,
+		      CONFIG_ENTROPY_INIT_PRIORITY, &entropy_prng_api_funcs);

dts/bindings/rng/nordic,nrf-prng.yaml

@@ -0,0 +1,8 @@
+# Copyright (c) 2024 Nordic Semiconductor ASA
+# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+
+description: This is a super simple PRNG
+
+compatible: "nordic,entropy-prng"
+
+include: base.yaml