arch/s390/crypto/arch_random.c

// SPDX-License-Identifier: GPL-2.0
/*
 * s390 arch random implementation.
 *
 * Copyright IBM Corp. 2017, 2020
 * Author(s): Harald Freudenberger
 *
 * The s390_arch_random_generate() function may be called from random.c
 * in interrupt context. So this implementation does the best to be very
 * fast. There is a buffer of random data which is asynchronously checked
 * and filled by a workqueue thread.
 * If there are enough bytes in the buffer the s390_arch_random_generate()
 * just delivers these bytes. Otherwise false is returned until the
 * worker thread refills the buffer.
 * The worker fills the rng buffer by pulling fresh entropy from the
 * high quality (but slow) true hardware random generator. This entropy
 * is then spread over the buffer with an pseudo random generator PRNG.
 * As the arch_get_random_seed_long() fetches 8 bytes and the calling
 * function add_interrupt_randomness() counts this as 1 bit entropy the
 * distribution needs to make sure there is in fact 1 bit entropy contained
 * in 8 bytes of the buffer. The current values pull 32 byte entropy
 * and scatter this into a 2048 byte buffer. So 8 byte in the buffer
 * will contain 1 bit of entropy.
 * The worker thread is rescheduled based on the charge level of the
 * buffer but at least with 500 ms delay to avoid too much CPU consumption.
 * So the max. amount of rng data delivered via arch_get_random_seed is
 * limited to 4k bytes per second.
 */

#include <linux/kernel.h>
#include <linux/atomic.h>
#include <linux/random.h>
#include <linux/slab.h>
#include <linux/static_key.h>
#include <linux/workqueue.h>
#include <linux/moduleparam.h>
#include <asm/cpacf.h>

DEFINE_STATIC_KEY_FALSE(s390_arch_random_available);

atomic64_t s390_arch_random_counter = ATOMIC64_INIT(0);
EXPORT_SYMBOL(s390_arch_random_counter);

#define ARCH_REFILL_TICKS (HZ/2)
#define ARCH_PRNG_SEED_SIZE 32
#define ARCH_RNG_BUF_SIZE 2048

static DEFINE_SPINLOCK(arch_rng_lock);
static u8 *arch_rng_buf;
static unsigned int arch_rng_buf_idx;

static void arch_rng_refill_buffer(struct work_struct *);
static DECLARE_DELAYED_WORK(arch_rng_work, arch_rng_refill_buffer);

bool s390_arch_random_generate(u8 *buf, unsigned int nbytes)
{
	/* max hunk is ARCH_RNG_BUF_SIZE */
	if (nbytes > ARCH_RNG_BUF_SIZE)
		return false;

	/* lock rng buffer */
	if (!spin_trylock(&arch_rng_lock))
		return false;

	/* try to resolve the requested amount of bytes from the buffer */
	arch_rng_buf_idx -= nbytes;
	if (arch_rng_buf_idx < ARCH_RNG_BUF_SIZE) {
		memcpy(buf, arch_rng_buf + arch_rng_buf_idx, nbytes);
		atomic64_add(nbytes, &s390_arch_random_counter);
		spin_unlock(&arch_rng_lock);
		return true;
	}

	/* not enough bytes in rng buffer, refill is done asynchronously */
	spin_unlock(&arch_rng_lock);

	return false;
}
EXPORT_SYMBOL(s390_arch_random_generate);

static void arch_rng_refill_buffer(struct work_struct *unused)
{
	unsigned int delay = ARCH_REFILL_TICKS;

	spin_lock(&arch_rng_lock);
	if (arch_rng_buf_idx > ARCH_RNG_BUF_SIZE) {
		/* buffer is exhausted and needs refill */
		u8 seed[ARCH_PRNG_SEED_SIZE];
		u8 prng_wa[240];
		/* fetch ARCH_PRNG_SEED_SIZE bytes of entropy */
		cpacf_trng(NULL, 0, seed, sizeof(seed));
		/* blow this entropy up to ARCH_RNG_BUF_SIZE with PRNG */
		memset(prng_wa, 0, sizeof(prng_wa));
		cpacf_prno(CPACF_PRNO_SHA512_DRNG_SEED,
			   &prng_wa, NULL, 0, seed, sizeof(seed));
		cpacf_prno(CPACF_PRNO_SHA512_DRNG_GEN,
			   &prng_wa, arch_rng_buf, ARCH_RNG_BUF_SIZE, NULL, 0);
		arch_rng_buf_idx = ARCH_RNG_BUF_SIZE;
	}
	delay += (ARCH_REFILL_TICKS * arch_rng_buf_idx) / ARCH_RNG_BUF_SIZE;
	spin_unlock(&arch_rng_lock);

	/* kick next check */
	queue_delayed_work(system_long_wq, &arch_rng_work, delay);
}

/*
 * Here follows the implementation of s390_arch_get_random_long().
 *
 * The random longs to be pulled by arch_get_random_long() are
 * prepared in an 4K buffer which is filled from the NIST 800-90
 * compliant s390 drbg. By default the random long buffer is refilled
 * 256 times before the drbg itself needs a reseed. The reseed of the
 * drbg is done with 32 bytes fetched from the high quality (but slow)
 * trng which is assumed to deliver 100% entropy. So the 32 * 8 = 256
 * bits of entropy are spread over 256 * 4KB = 1MB serving 131072
 * arch_get_random_long() invocations before reseeded.
 *
 * How often the 4K random long buffer is refilled with the drbg
 * before the drbg is reseeded can be adjusted. There is a module
 * parameter 's390_arch_rnd_long_drbg_reseed' accessible via
 *   /sys/module/arch_random/parameters/rndlong_drbg_reseed
 * or as kernel command line parameter
 *   arch_random.rndlong_drbg_reseed=<value>
 * This parameter tells how often the drbg fills the 4K buffer before
 * it is re-seeded by fresh entropy from the trng.
 * A value of 16 results in reseeding the drbg at every 16 * 4 KB = 64
 * KB with 32 bytes of fresh entropy pulled from the trng. So a value
 * of 16 would result in 256 bits entropy per 64 KB.
 * A value of 256 results in 1MB of drbg output before a reseed of the
 * drbg is done. So this would spread the 256 bits of entropy among 1MB.
 * Setting this parameter to 0 forces the reseed to take place every
 * time the 4K buffer is depleted, so the entropy rises to 256 bits
 * entropy per 4K or 0.5 bit entropy per arch_get_random_long().  With
 * setting this parameter to negative values all this effort is
 * disabled, arch_get_random long() returns false and thus indicating
 * that the arch_get_random_long() feature is disabled at all.
 */

static unsigned long rndlong_buf[512];
static DEFINE_SPINLOCK(rndlong_lock);
static int rndlong_buf_index;

static int rndlong_drbg_reseed = 256;
module_param_named(rndlong_drbg_reseed, rndlong_drbg_reseed, int, 0600);
MODULE_PARM_DESC(rndlong_drbg_reseed, "s390 arch_get_random_long() drbg reseed");

static inline void refill_rndlong_buf(void)
{
	static u8 prng_ws[240];
	static int drbg_counter;

	if (--drbg_counter < 0) {
		/* need to re-seed the drbg */
		u8 seed[32];

		/* fetch seed from trng */
		cpacf_trng(NULL, 0, seed, sizeof(seed));
		/* seed drbg */
		memset(prng_ws, 0, sizeof(prng_ws));
		cpacf_prno(CPACF_PRNO_SHA512_DRNG_SEED,
			   &prng_ws, NULL, 0, seed, sizeof(seed));
		/* re-init counter for drbg */
		drbg_counter = rndlong_drbg_reseed;
	}

	/* fill the arch_get_random_long buffer from drbg */
	cpacf_prno(CPACF_PRNO_SHA512_DRNG_GEN, &prng_ws,
		   (u8 *) rndlong_buf, sizeof(rndlong_buf),
		   NULL, 0);
}

bool s390_arch_get_random_long(unsigned long *v)
{
	bool rc = false;
	unsigned long flags;

	/* arch_get_random_long() disabled ? */
	if (rndlong_drbg_reseed < 0)
		return false;

	/* try to lock the random long lock */
	if (!spin_trylock_irqsave(&rndlong_lock, flags))
		return false;

	if (--rndlong_buf_index >= 0) {
		/* deliver next long value from the buffer */
		*v = rndlong_buf[rndlong_buf_index];
		rc = true;
		goto out;
	}

	/* buffer is depleted and needs refill */
	if (in_interrupt()) {
		/* delay refill in interrupt context to next caller */
		rndlong_buf_index = 0;
		goto out;
	}

	/* refill random long buffer */
	refill_rndlong_buf();
	rndlong_buf_index = ARRAY_SIZE(rndlong_buf);

	/* and provide one random long */
	*v = rndlong_buf[--rndlong_buf_index];
	rc = true;

out:
	spin_unlock_irqrestore(&rndlong_lock, flags);
	return rc;
}
EXPORT_SYMBOL(s390_arch_get_random_long);

static int __init s390_arch_random_init(void)
{
	/* all the needed PRNO subfunctions available ? */
	if (cpacf_query_func(CPACF_PRNO, CPACF_PRNO_TRNG) &&
	    cpacf_query_func(CPACF_PRNO, CPACF_PRNO_SHA512_DRNG_GEN)) {

		/* alloc arch random working buffer */
		arch_rng_buf = kmalloc(ARCH_RNG_BUF_SIZE, GFP_KERNEL);
		if (!arch_rng_buf)
			return -ENOMEM;

		/* kick worker queue job to fill the random buffer */
		queue_delayed_work(system_long_wq,
				   &arch_rng_work, ARCH_REFILL_TICKS);

		/* enable arch random to the outside world */
		static_branch_enable(&s390_arch_random_available);
	}

	return 0;
}
arch_initcall(s390_arch_random_init);