@@ -524,6 +524,20 @@ config HW_RANDOM_XIPHERA
To compile this driver as a module, choose M here: the
module will be called xiphera-trng.
+config HW_RANDOM_ARM_SMCCC_TRNG
+ tristate "Arm SMCCC TRNG firmware interface support"
+ depends on HAVE_ARM_SMCCC_DISCOVERY
+ default HW_RANDOM
+ help
+ Say 'Y' to enable the True Random Number Generator driver using
+ the Arm SMCCC TRNG firmware interface. This reads entropy from
+ higher exception levels (firmware, hypervisor). Uses SMCCC for
+ communicating with the firmware:
+ https://developer.arm.com/documentation/den0098/latest/
+
+ To compile this driver as a module, choose M here: the
+ module will be called arm_smccc_trng.
+
endif # HW_RANDOM
config UML_RANDOM
@@ -45,3 +45,4 @@ obj-$(CONFIG_HW_RANDOM_OPTEE) += optee-rng.o
obj-$(CONFIG_HW_RANDOM_NPCM) += npcm-rng.o
obj-$(CONFIG_HW_RANDOM_CCTRNG) += cctrng.o
obj-$(CONFIG_HW_RANDOM_XIPHERA) += xiphera-trng.o
+obj-$(CONFIG_HW_RANDOM_ARM_SMCCC_TRNG) += arm_smccc_trng.o
new file mode 100644
@@ -0,0 +1,134 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Randomness driver for the ARM SMCCC TRNG Firmware Interface
+ * https://developer.arm.com/documentation/den0098/latest/
+ *
+ * Copyright (C) 2020 Arm Ltd.
+ *
+ * The ARM TRNG firmware interface specifies a protocol to read entropy
+ * from a higher exception level, to abstract from any machine specific
+ * implemenations and allow easier use in hypervisors.
+ *
+ * The firmware interface is realised using the SMCCC specification.
+ */
+
+#include <linux/bits.h>
+#include <linux/device.h>
+#include <linux/hw_random.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/arm-smccc.h>
+
+#ifdef CONFIG_ARM64
+#define ARM_SMCCC_TRNG_RND ARM_SMCCC_TRNG_RND64
+#define MAX_BITS_PER_CALL (3 * 64UL)
+#else
+#define ARM_SMCCC_TRNG_RND ARM_SMCCC_TRNG_RND32
+#define MAX_BITS_PER_CALL (3 * 32UL)
+#endif
+
+/* We don't want to allow the firmware to stall us forever. */
+#define SMCCC_TRNG_MAX_TRIES 20
+
+#define SMCCC_RET_TRNG_INVALID_PARAMETER -2
+#define SMCCC_RET_TRNG_NO_ENTROPY -3
+
+static int smccc_trng_init(struct hwrng *rng)
+{
+ return 0;
+}
+
+static int copy_from_registers(char *buf, struct arm_smccc_res *res,
+ size_t bytes)
+{
+ unsigned int chunk, copied;
+
+ if (bytes == 0)
+ return 0;
+
+ chunk = min(bytes, sizeof(long));
+ memcpy(buf, &res->a3, chunk);
+ copied = chunk;
+ if (copied >= bytes)
+ return copied;
+
+ chunk = min((bytes - copied), sizeof(long));
+ memcpy(&buf[copied], &res->a2, chunk);
+ copied += chunk;
+ if (copied >= bytes)
+ return copied;
+
+ chunk = min((bytes - copied), sizeof(long));
+ memcpy(&buf[copied], &res->a1, chunk);
+
+ return copied + chunk;
+}
+
+static int smccc_trng_read(struct hwrng *rng, void *data, size_t max, bool wait)
+{
+ struct arm_smccc_res res;
+ u8 *buf = data;
+ unsigned int copied = 0;
+ int tries = 0;
+
+ while (copied < max) {
+ size_t bits = min_t(size_t, (max - copied) * BITS_PER_BYTE,
+ MAX_BITS_PER_CALL);
+
+ arm_smccc_1_1_invoke(ARM_SMCCC_TRNG_RND, bits, &res);
+ if ((int)res.a0 < 0)
+ return (int)res.a0;
+
+ switch ((int)res.a0) {
+ case SMCCC_RET_SUCCESS:
+ copied += copy_from_registers(buf + copied, &res,
+ bits / BITS_PER_BYTE);
+ tries = 0;
+ break;
+ case SMCCC_RET_TRNG_NO_ENTROPY:
+ if (!wait)
+ return copied;
+ tries++;
+ if (tries >= SMCCC_TRNG_MAX_TRIES)
+ return copied;
+ cond_resched();
+ break;
+ }
+ }
+
+ return copied;
+}
+
+static int smccc_trng_probe(struct platform_device *pdev)
+{
+ struct hwrng *trng;
+ int ret;
+
+ trng = devm_kzalloc(&pdev->dev, sizeof(*trng), GFP_KERNEL);
+ if (!trng)
+ return -ENOMEM;
+
+ trng->name = "smccc_trng";
+ trng->init = smccc_trng_init;
+ trng->read = smccc_trng_read;
+
+ platform_set_drvdata(pdev, trng);
+ ret = devm_hwrng_register(&pdev->dev, trng);
+ if (!ret)
+ dev_info(&pdev->dev,
+ "ARM SMCCC TRNG firmware random number generator\n");
+
+ return ret;
+}
+
+static struct platform_driver smccc_trng_driver = {
+ .driver = {
+ .name = "smccc_trng",
+ },
+ .probe = smccc_trng_probe,
+};
+module_platform_driver(smccc_trng_driver);
+
+MODULE_ALIAS("platform:smccc_trng");
+MODULE_AUTHOR("Andre Przywara");
+MODULE_LICENSE("GPL");
The "Arm True Random Number Generator Firmware Interface"[1] provides an SMCCC based interface to a true hardware random number generator. So far we are using that in arch_get_random_seed(), but it might be useful to expose the entropy through the /dev/hwrng device as well. This allows to assess the quality of the implementation, by using "rngtest" from the rng-tools package, for example. Add a simple platform driver implementing the hw_random interface. The corresponding platform device is created by the SMCCC core code, we just match it here by name and provide a module alias. Since the firmware takes care about serialisation, this can happily coexist with the arch_get_random_seed() bits. Signed-off-by: Andre Przywara <andre.przywara@arm.com> [1] https://developer.arm.com/documentation/den0098/latest/ --- drivers/char/hw_random/Kconfig | 14 +++ drivers/char/hw_random/Makefile | 1 + drivers/char/hw_random/arm_smccc_trng.c | 134 ++++++++++++++++++++++++ 3 files changed, 149 insertions(+) create mode 100644 drivers/char/hw_random/arm_smccc_trng.c