path: root/sys/dev/rnd.c

/*	$OpenBSD: rnd.c,v 1.199 2018/04/28 15:44:59 jasper Exp $	*/

/*
 * Copyright (c) 2011 Theo de Raadt.
 * Copyright (c) 2008 Damien Miller.
 * Copyright (c) 1996, 1997, 2000-2002 Michael Shalayeff.
 * Copyright (c) 2013 Markus Friedl.
 * Copyright Theodore Ts'o, 1994, 1995, 1996, 1997, 1998, 1999.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, and the entire permission notice in its entirety,
 *    including the disclaimer of warranties.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote
 *    products derived from this software without specific prior
 *    written permission.
 *
 * ALTERNATIVELY, this product may be distributed under the terms of
 * the GNU Public License, in which case the provisions of the GPL are
 * required INSTEAD OF the above restrictions.  (This clause is
 * necessary due to a potential bad interaction between the GPL and
 * the restrictions contained in a BSD-style copyright.)
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * Computers are very predictable devices.  Hence it is extremely hard
 * to produce truly random numbers on a computer --- as opposed to
 * pseudo-random numbers, which can be easily generated by using an
 * algorithm.  Unfortunately, it is very easy for attackers to guess
 * the sequence of pseudo-random number generators, and for some
 * applications this is not acceptable.  Instead, we must try to
 * gather "environmental noise" from the computer's environment, which
 * must be hard for outside attackers to observe and use to
 * generate random numbers.  In a Unix environment, this is best done
 * from inside the kernel.
 *
 * Sources of randomness from the environment include inter-keyboard
 * timings, inter-interrupt timings from some interrupts, and other
 * events which are both (a) non-deterministic and (b) hard for an
 * outside observer to measure.  Randomness from these sources is
 * added to the "rnd states" queue; this is used as much of the
 * source material which is mixed on occasion using a CRC-like function
 * into the "entropy pool".  This is not cryptographically strong, but
 * it is adequate assuming the randomness is not chosen maliciously,
 * and it is very fast because the interrupt-time event is only to add
 * a small random token to the "rnd states" queue.
 *
 * When random bytes are desired, they are obtained by pulling from
 * the entropy pool and running a SHA512 hash. The SHA512 hash avoids
 * exposing the internal state of the entropy pool.  Even if it is
 * possible to analyze SHA512 in some clever way, as long as the amount
 * of data returned from the generator is less than the inherent
 * entropy in the pool, the output data is totally unpredictable.  For
 * this reason, the routine decreases its internal estimate of how many
 * bits of "true randomness" are contained in the entropy pool as it
 * outputs random numbers.
 *
 * If this estimate goes to zero, the SHA512 hash will continue to generate
 * output since there is no true risk because the SHA512 output is not
 * exported outside this subsystem.  It is next used as input to seed a
 * ChaCha20 stream cipher, which is re-seeded from time to time.  This
 * design provides very high amounts of output data from a potentially
 * small entropy base, at high enough speeds to encourage use of random
 * numbers in nearly any situation.  Before OpenBSD 5.5, the RC4 stream
 * cipher (also known as ARC4) was used instead of ChaCha20.
 *
 * The output of this single ChaCha20 engine is then shared amongst many
 * consumers in the kernel and userland via a few interfaces:
 * arc4random_buf(), arc4random(), arc4random_uniform(), randomread()
 * for the set of /dev/random nodes and the system call getentropy(),
 * which provides seeds for process-context pseudorandom generators.
 *
 * Acknowledgements:
 * =================
 *
 * Ideas for constructing this random number generator were derived
 * from Pretty Good Privacy's random number generator, and from private
 * discussions with Phil Karn.  Colin Plumb provided a faster random
 * number generator, which speeds up the mixing function of the entropy
 * pool, taken from PGPfone.  Dale Worley has also contributed many
 * useful ideas and suggestions to improve this driver.
 *
 * Any flaws in the design are solely my responsibility, and should
 * not be attributed to the Phil, Colin, or any of the authors of PGP.
 *
 * Further background information on this topic may be obtained from
 * RFC 1750, "Randomness Recommendations for Security", by Donald
 * Eastlake, Steve Crocker, and Jeff Schiller.
 *
 * Using a RC4 stream cipher as 2nd stage after the MD5 (now SHA512) output
 * is the result of work by David Mazieres.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/disk.h>
#include <sys/event.h>
#include <sys/limits.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <sys/malloc.h>
#include <sys/fcntl.h>
#include <sys/timeout.h>
#include <sys/mutex.h>
#include <sys/task.h>
#include <sys/msgbuf.h>
#include <sys/mount.h>
#include <sys/syscallargs.h>

#include <crypto/sha2.h>

#define KEYSTREAM_ONLY
#include <crypto/chacha_private.h>

#include <dev/rndvar.h>

#include <uvm/uvm_param.h>
#include <uvm/uvm_extern.h>

/*
 * For the purposes of better mixing, we use the CRC-32 polynomial as
 * well to make a twisted Generalized Feedback Shift Register
 *
 * (See M. Matsumoto & Y. Kurita, 1992.  Twisted GFSR generators.  ACM
 * Transactions on Modeling and Computer Simulation 2(3):179-194.
 * Also see M. Matsumoto & Y. Kurita, 1994.  Twisted GFSR generators
 * II.  ACM Transactions on Modeling and Computer Simulation 4:254-266)
 *
 * Thanks to Colin Plumb for suggesting this.
 *
 * We have not analyzed the resultant polynomial to prove it primitive;
 * in fact it almost certainly isn't.  Nonetheless, the irreducible factors
 * of a random large-degree polynomial over GF(2) are more than large enough
 * that periodicity is not a concern.
 *
 * The input hash is much less sensitive than the output hash.  All
 * we want from it is to be a good non-cryptographic hash -
 * i.e. to not produce collisions when fed "random" data of the sort
 * we expect to see.  As long as the pool state differs for different
 * inputs, we have preserved the input entropy and done a good job.
 * The fact that an intelligent attacker can construct inputs that
 * will produce controlled alterations to the pool's state is not
 * important because we don't consider such inputs to contribute any
 * randomness.  The only property we need with respect to them is that
 * the attacker can't increase his/her knowledge of the pool's state.
 * Since all additions are reversible (knowing the final state and the
 * input, you can reconstruct the initial state), if an attacker has
 * any uncertainty about the initial state, he/she can only shuffle
 * that uncertainty about, but never cause any collisions (which would
 * decrease the uncertainty).
 *
 * The chosen system lets the state of the pool be (essentially) the input
 * modulo the generator polynomial.  Now, for random primitive polynomials,
 * this is a universal class of hash functions, meaning that the chance
 * of a collision is limited by the attacker's knowledge of the generator
 * polynomial, so if it is chosen at random, an attacker can never force
 * a collision.  Here, we use a fixed polynomial, but we *can* assume that
 * ###--> it is unknown to the processes generating the input entropy. <-###
 * Because of this important property, this is a good, collision-resistant
 * hash; hash collisions will occur no more often than chance.
 */

/*
 * Stirring polynomials over GF(2) for various pool sizes. Used in
 * add_entropy_words() below.
 *
 * The polynomial terms are chosen to be evenly spaced (minimum RMS
 * distance from evenly spaced; except for the last tap, which is 1 to
 * get the twisting happening as fast as possible.
 *
 * The resultant polynomial is:
 *   2^POOLWORDS + 2^POOL_TAP1 + 2^POOL_TAP2 + 2^POOL_TAP3 + 2^POOL_TAP4 + 1
 */
#define POOLWORDS	2048
#define POOLBYTES	(POOLWORDS*4)
#define POOLMASK	(POOLWORDS - 1)
#define	POOL_TAP1	1638
#define	POOL_TAP2	1231
#define	POOL_TAP3	819
#define	POOL_TAP4	411

/*
 * Raw entropy collection from device drivers; at interrupt context or not.
 * enqueue_randomness() provide data which is put into the entropy queue.
 */

#define QEVLEN	128		 /* must be a power of 2 */
#define QEVSLOW (QEVLEN * 3 / 4) /* yet another 0.75 for 60-minutes hour /-; */

#define KEYSZ	32
#define IVSZ	8
#define BLOCKSZ	64
#define RSBUFSZ	(16*BLOCKSZ)
#define EBUFSIZE KEYSZ + IVSZ

struct rand_event {
	u_int re_time;
	u_int re_val;
} rnd_event_space[QEVLEN];

u_int rnd_event_cons;
u_int rnd_event_prod;

struct mutex rnd_enqlck = MUTEX_INITIALIZER(IPL_HIGH);
struct mutex rnd_deqlck = MUTEX_INITIALIZER(IPL_HIGH);

struct timeout rnd_timeout;

static u_int32_t entropy_pool[POOLWORDS];
u_int32_t entropy_pool0[POOLWORDS] __attribute__((section(".openbsd.randomdata")));
u_int	entropy_add_ptr;
u_char	entropy_input_rotate;

void	dequeue_randomness(void *);
void	add_entropy_words(const u_int32_t *, u_int);
void	extract_entropy(u_int8_t *)
    __attribute__((__bounded__(__minbytes__,1,EBUFSIZE)));

int	filt_randomread(struct knote *, long);
void	filt_randomdetach(struct knote *);
int	filt_randomwrite(struct knote *, long);

static void _rs_seed(u_char *, size_t);
static void _rs_clearseed(const void *p, size_t s);

struct filterops randomread_filtops =
	{ 1, NULL, filt_randomdetach, filt_randomread };
struct filterops randomwrite_filtops =
	{ 1, NULL, filt_randomdetach, filt_randomwrite };

static inline struct rand_event *
rnd_get(void)
{
	u_int idx;

	/* nothing to do if queue is empty */
	if (rnd_event_prod == rnd_event_cons)
		return NULL;

	if (rnd_event_prod - rnd_event_cons > QEVLEN)
		rnd_event_cons = rnd_event_prod - QEVLEN;
	idx = rnd_event_cons++;
	return &rnd_event_space[idx & (QEVLEN - 1)];
}

static inline struct rand_event *
rnd_put(void)
{
	u_int idx = rnd_event_prod++;

	/* allow wrapping. caller will mix it in. */
	return &rnd_event_space[idx & (QEVLEN - 1)];
}

static inline u_int
rnd_qlen(void)
{
	return rnd_event_prod - rnd_event_cons;
}

/*
 * This function adds entropy to the entropy pool by using timing delays.
 *
 * The number "val" is also added to the pool - it should somehow describe
 * the type of event which just happened.  Currently the values of 0-255
 * are for keyboard scan codes, 256 and upwards - for interrupts.
 */
void
enqueue_randomness(u_int val)
{
	struct rand_event *rep;
	struct timespec	ts;
	u_int qlen;

	if (timeout_initialized(&rnd_timeout))
		nanotime(&ts);

	mtx_enter(&rnd_enqlck);
	rep = rnd_put();
	rep->re_time += ts.tv_nsec ^ (ts.tv_sec << 20);
	rep->re_val += val;
	qlen = rnd_qlen();
	mtx_leave(&rnd_enqlck);

	if (qlen > QEVSLOW/2 && timeout_initialized(&rnd_timeout) &&
	    !timeout_pending(&rnd_timeout))
		timeout_add(&rnd_timeout, 1);
}

/*
 * This function adds a byte into the entropy pool.  It does not
 * update the entropy estimate.  The caller must do this if appropriate.
 *
 * The pool is stirred with a polynomial of degree POOLWORDS over GF(2);
 * see POOL_TAP[1-4] above
 *
 * Rotate the input word by a changing number of bits, to help assure
 * that all bits in the entropy get toggled.  Otherwise, if the pool
 * is consistently fed small numbers (such as keyboard scan codes)
 * then the upper bits of the entropy pool will frequently remain
 * untouched.
 */
void
add_entropy_words(const u_int32_t *buf, u_int n)
{
	/* derived from IEEE 802.3 CRC-32 */
	static const u_int32_t twist_table[8] = {
		0x00000000, 0x3b6e20c8, 0x76dc4190, 0x4db26158,
		0xedb88320, 0xd6d6a3e8, 0x9b64c2b0, 0xa00ae278
	};

	for (; n--; buf++) {
		u_int32_t w = (*buf << entropy_input_rotate) |
		    (*buf >> ((32 - entropy_input_rotate) & 31));
		u_int i = entropy_add_ptr =
		    (entropy_add_ptr - 1) & POOLMASK;
		/*
		 * Normally, we add 7 bits of rotation to the pool.
		 * At the beginning of the pool, add an extra 7 bits
		 * rotation, so that successive passes spread the
		 * input bits across the pool evenly.
		 */
		entropy_input_rotate =
		    (entropy_input_rotate + (i ? 7 : 14)) & 31;

		/* XOR pool contents corresponding to polynomial terms */
		w ^= entropy_pool[(i + POOL_TAP1) & POOLMASK] ^
		     entropy_pool[(i + POOL_TAP2) & POOLMASK] ^
		     entropy_pool[(i + POOL_TAP3) & POOLMASK] ^
		     entropy_pool[(i + POOL_TAP4) & POOLMASK] ^
		     entropy_pool[(i + 1) & POOLMASK] ^
		     entropy_pool[i]; /* + 2^POOLWORDS */

		entropy_pool[i] = (w >> 3) ^ twist_table[w & 7];
	}
}

/*
 * Pulls entropy out of the queue and merges it into the pool
 * with the CRC.
 */
/* ARGSUSED */
void
dequeue_randomness(void *v)
{
	struct rand_event *rep;
	u_int32_t buf[2];

	if (timeout_initialized(&rnd_timeout))
		timeout_del(&rnd_timeout);

	mtx_enter(&rnd_deqlck);
	while ((rep = rnd_get())) {
		buf[0] = rep->re_time;
		buf[1] = rep->re_val;
		mtx_leave(&rnd_deqlck);
		add_entropy_words(buf, 2);
		mtx_enter(&rnd_deqlck);
	}
	mtx_leave(&rnd_deqlck);
}

/*
 * Grabs a chunk from the entropy_pool[] and slams it through SHA512 when
 * requested.
 */
void
extract_entropy(u_int8_t *buf)
{
	static u_int32_t extract_pool[POOLWORDS];
	u_char digest[SHA512_DIGEST_LENGTH];
	SHA2_CTX shactx;

#if SHA512_DIGEST_LENGTH < EBUFSIZE
#error "need more bigger hash output"
#endif

	/*
	 * INTENTIONALLY not protected by any lock.  Races during
	 * memcpy() result in acceptable input data; races during
	 * SHA512Update() would create nasty data dependencies.  We
	 * do not rely on this as a benefit, but if it happens, cool.
	 */
	memcpy(extract_pool, entropy_pool, sizeof(extract_pool));

	/* Hash the pool to get the output */
	SHA512Init(&shactx);
	SHA512Update(&shactx, (u_int8_t *)extract_pool, sizeof(extract_pool));
	SHA512Final(digest, &shactx);

	/* Copy data to destination buffer */
	memcpy(buf, digest, EBUFSIZE);

	/* Modify pool so next hash will produce different results */
	enqueue_randomness(EBUFSIZE);
	dequeue_randomness(NULL);

	/* Wipe data from memory */
	explicit_bzero(extract_pool, sizeof(extract_pool));
	explicit_bzero(digest, sizeof(digest));
}

/* random keystream by ChaCha */

void arc4_reinit(void *v);		/* timeout to start reinit */
void arc4_init(void *);			/* actually do the reinit */

struct mutex rndlock = MUTEX_INITIALIZER(IPL_HIGH);
struct timeout arc4_timeout;
struct task arc4_task = TASK_INITIALIZER(arc4_init, NULL);

static chacha_ctx rs;		/* chacha context for random keystream */
/* keystream blocks (also chacha seed from boot) */
static u_char rs_buf[RSBUFSZ];
u_char rs_buf0[RSBUFSZ] __attribute__((section(".openbsd.randomdata")));
static size_t rs_have;		/* valid bytes at end of rs_buf */
static size_t rs_count;		/* bytes till reseed */

void
suspend_randomness(void)
{
	struct timespec ts;

	getnanotime(&ts);
	enqueue_randomness(ts.tv_sec);
	enqueue_randomness(ts.tv_nsec);

	dequeue_randomness(NULL);
	rs_count = 0;
	arc4random_buf(entropy_pool, sizeof(entropy_pool));
}

void
resume_randomness(char *buf, size_t buflen)
{
	struct timespec ts;

	if (buf && buflen)
		_rs_seed(buf, buflen);
	getnanotime(&ts);
	enqueue_randomness(ts.tv_sec);
	enqueue_randomness(ts.tv_nsec);

	dequeue_randomness(NULL);
	rs_count = 0;
}

static inline void _rs_rekey(u_char *dat, size_t datlen);

static inline void
_rs_init(u_char *buf, size_t n)
{
	KASSERT(n >= KEYSZ + IVSZ);
	chacha_keysetup(&rs, buf, KEYSZ * 8);
	chacha_ivsetup(&rs, buf + KEYSZ, NULL);
}

static void
_rs_seed(u_char *buf, size_t n)
{
	_rs_rekey(buf, n);

	/* invalidate rs_buf */
	rs_have = 0;
	memset(rs_buf, 0, RSBUFSZ);

	rs_count = 1600000;
}

static void
_rs_stir(int do_lock)
{
	struct timespec ts;
	u_int8_t buf[EBUFSIZE], *p;
	int i;

	/*
	 * Use SHA512 PRNG data and a system timespec; early in the boot
	 * process this is the best we can do -- some architectures do
	 * not collect entropy very well during this time, but may have
	 * clock information which is better than nothing.
	 */
	extract_entropy(buf);

	nanotime(&ts);
	for (p = (u_int8_t *)&ts, i = 0; i < sizeof(ts); i++)
		buf[i] ^= p[i];

	if (do_lock)
		mtx_enter(&rndlock);
	_rs_seed(buf, sizeof(buf));
	if (do_lock)
		mtx_leave(&rndlock);

	explicit_bzero(buf, sizeof(buf));
}

static inline void
_rs_stir_if_needed(size_t len)
{
	static int rs_initialized;

	if (!rs_initialized) {
		memcpy(entropy_pool, entropy_pool0, sizeof entropy_pool);
		memcpy(rs_buf, rs_buf0, sizeof rs_buf);
		/* seeds cannot be cleaned yet, random_start() will do so */
		_rs_init(rs_buf, KEYSZ + IVSZ);
		rs_count = 1024 * 1024 * 1024;	/* until main() runs */
		rs_initialized = 1;
	} else if (rs_count <= len)
		_rs_stir(0);
	else
		rs_count -= len;
}

static void
_rs_clearseed(const void *p, size_t s)
{
	struct kmem_dyn_mode kd_avoidalias;
	vaddr_t va = trunc_page((vaddr_t)p);
	vsize_t off = (vaddr_t)p - va;
	vsize_t len;
	vaddr_t rwva;
	paddr_t pa;

	while (s > 0) {
		pmap_extract(pmap_kernel(), va, &pa);

		memset(&kd_avoidalias, 0, sizeof kd_avoidalias);
		kd_avoidalias.kd_prefer = pa;
		kd_avoidalias.kd_waitok = 1;
		rwva = (vaddr_t)km_alloc(PAGE_SIZE, &kv_any, &kp_none,
		    &kd_avoidalias);
		if (!rwva)
			panic("_rs_clearseed");

		pmap_kenter_pa(rwva, pa, PROT_READ | PROT_WRITE);
		pmap_update(pmap_kernel());

		len = MIN(s, PAGE_SIZE - off);
		explicit_bzero((void *)(rwva + off), len);

		pmap_kremove(rwva, PAGE_SIZE);
		km_free((void *)rwva, PAGE_SIZE, &kv_any, &kp_none);

		va += PAGE_SIZE;
		s -= len;
		off = 0;
	}
}

static inline void
_rs_rekey(u_char *dat, size_t datlen)
{
#ifndef KEYSTREAM_ONLY
	memset(rs_buf, 0, RSBUFSZ);
#endif
	/* fill rs_buf with the keystream */
	chacha_encrypt_bytes(&rs, rs_buf, rs_buf, RSBUFSZ);
	/* mix in optional user provided data */
	if (dat) {
		size_t i, m;

		m = MIN(datlen, KEYSZ + IVSZ);
		for (i = 0; i < m; i++)
			rs_buf[i] ^= dat[i];
	}
	/* immediately reinit for backtracking resistance */
	_rs_init(rs_buf, KEYSZ + IVSZ);
	memset(rs_buf, 0, KEYSZ + IVSZ);
	rs_have = RSBUFSZ - KEYSZ - IVSZ;
}

static inline void
_rs_random_buf(void *_buf, size_t n)
{
	u_char *buf = (u_char *)_buf;
	size_t m;

	_rs_stir_if_needed(n);
	while (n > 0) {
		if (rs_have > 0) {
			m = MIN(n, rs_have);
			memcpy(buf, rs_buf + RSBUFSZ - rs_have, m);
			memset(rs_buf + RSBUFSZ - rs_have, 0, m);
			buf += m;
			n -= m;
			rs_have -= m;
		}
		if (rs_have == 0)
			_rs_rekey(NULL, 0);
	}
}

static inline void
_rs_random_u32(u_int32_t *val)
{
	_rs_stir_if_needed(sizeof(*val));
	if (rs_have < sizeof(*val))
		_rs_rekey(NULL, 0);
	memcpy(val, rs_buf + RSBUFSZ - rs_have, sizeof(*val));
	memset(rs_buf + RSBUFSZ - rs_have, 0, sizeof(*val));
	rs_have -= sizeof(*val);
}

/* Return one word of randomness from a ChaCha20 generator */
u_int32_t
arc4random(void)
{
	u_int32_t ret;

	mtx_enter(&rndlock);
	_rs_random_u32(&ret);
	mtx_leave(&rndlock);
	return ret;
}

/*
 * Fill a buffer of arbitrary length with ChaCha20-derived randomness.
 */
void
arc4random_buf(void *buf, size_t n)
{
	mtx_enter(&rndlock);
	_rs_random_buf(buf, n);
	mtx_leave(&rndlock);
}

/*
 * Allocate a new ChaCha20 context for the caller to use.
 */
struct arc4random_ctx *
arc4random_ctx_new()
{
	char keybuf[KEYSZ + IVSZ];

	chacha_ctx *ctx = malloc(sizeof(chacha_ctx), M_TEMP, M_WAITOK);
	arc4random_buf(keybuf, KEYSZ + IVSZ);
	chacha_keysetup(ctx, keybuf, KEYSZ * 8);
	chacha_ivsetup(ctx, keybuf + KEYSZ, NULL);
	explicit_bzero(keybuf, sizeof(keybuf));
	return (struct arc4random_ctx *)ctx;
}

/*
 * Free a ChaCha20 context created by arc4random_ctx_new()
 */
void
arc4random_ctx_free(struct arc4random_ctx *ctx)
{
	explicit_bzero(ctx, sizeof(chacha_ctx));
	free(ctx, M_TEMP, sizeof(chacha_ctx));
}

/*
 * Use a given ChaCha20 context to fill a buffer
 */
void
arc4random_ctx_buf(struct arc4random_ctx *ctx, void *buf, size_t n)
{
	chacha_encrypt_bytes((chacha_ctx *)ctx, buf, buf, n);
}

/*
 * Calculate a uniformly distributed random number less than upper_bound
 * avoiding "modulo bias".
 *
 * Uniformity is achieved by generating new random numbers until the one
 * returned is outside the range [0, 2**32 % upper_bound).  This
 * guarantees the selected random number will be inside
 * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
 * after reduction modulo upper_bound.
 */
u_int32_t
arc4random_uniform(u_int32_t upper_bound)
{
	u_int32_t r, min;

	if (upper_bound < 2)
		return 0;

	/* 2**32 % x == (2**32 - x) % x */
	min = -upper_bound % upper_bound;

	/*
	 * This could theoretically loop forever but each retry has
	 * p > 0.5 (worst case, usually far better) of selecting a
	 * number inside the range we need, so it should rarely need
	 * to re-roll.
	 */
	for (;;) {
		r = arc4random();
		if (r >= min)
			break;
	}

	return r % upper_bound;
}

/* ARGSUSED */
void
arc4_init(void *null)
{
	_rs_stir(1);
}

/*
 * Called by timeout to mark arc4 for stirring,
 */
void
arc4_reinit(void *v)
{
	task_add(systq, &arc4_task);
	/* 10 minutes, per dm@'s suggestion */
	timeout_add_sec(&arc4_timeout, 10 * 60);
}

/*
 * Start periodic services inside the random subsystem, which pull
 * entropy forward, hash it, and re-seed the random stream as needed.
 */
void
random_start(void)
{
	extern char etext[];

#if !defined(NO_PROPOLICE)
	extern long __guard_local;

	if (__guard_local == 0)
		printf("warning: no entropy supplied by boot loader\n");
#endif

	_rs_clearseed(entropy_pool0, sizeof entropy_pool0);
	_rs_clearseed(rs_buf0, sizeof rs_buf0);

	/* Message buffer may contain data from previous boot */
	if (msgbufp->msg_magic == MSG_MAGIC)
		add_entropy_words((u_int32_t *)msgbufp->msg_bufc,
		    msgbufp->msg_bufs / sizeof(u_int32_t));
	add_entropy_words((u_int32_t *)etext - 32*1024,
	    8192/sizeof(u_int32_t));

	dequeue_randomness(NULL);
	arc4_init(NULL);
	timeout_set(&arc4_timeout, arc4_reinit, NULL);
	arc4_reinit(NULL);
	timeout_set(&rnd_timeout, dequeue_randomness, NULL);
}

int
randomopen(dev_t dev, int flag, int mode, struct proc *p)
{
	return 0;
}

int
randomclose(dev_t dev, int flag, int mode, struct proc *p)
{
	return 0;
}

/*
 * Maximum number of bytes to serve directly from the main ChaCha
 * pool. Larger requests are served from a discrete ChaCha instance keyed
 * from the main pool.
 */
#define ARC4_MAIN_MAX_BYTES	2048

int
randomread(dev_t dev, struct uio *uio, int ioflag)
{
	u_char		lbuf[KEYSZ+IVSZ];
	chacha_ctx	lctx;
	size_t		total = uio->uio_resid;
	u_char		*buf;
	int		myctx = 0, ret = 0;

	if (uio->uio_resid == 0)
		return 0;

	buf = malloc(POOLBYTES, M_TEMP, M_WAITOK);
	if (total > ARC4_MAIN_MAX_BYTES) {
		arc4random_buf(lbuf, sizeof(lbuf));
		chacha_keysetup(&lctx, lbuf, KEYSZ * 8);
		chacha_ivsetup(&lctx, lbuf + KEYSZ, NULL);
		explicit_bzero(lbuf, sizeof(lbuf));
		myctx = 1;
	}

	while (ret == 0 && uio->uio_resid > 0) {
		size_t	n = ulmin(POOLBYTES, uio->uio_resid);

		if (myctx) {
#ifndef KEYSTREAM_ONLY
			memset(buf, 0, n);
#endif
			chacha_encrypt_bytes(&lctx, buf, buf, n);
		} else
			arc4random_buf(buf, n);
		ret = uiomove(buf, n, uio);
		if (ret == 0 && uio->uio_resid > 0)
			yield();
	}
	if (myctx)
		explicit_bzero(&lctx, sizeof(lctx));
	explicit_bzero(buf, POOLBYTES);
	free(buf, M_TEMP, POOLBYTES);
	return ret;
}

int
randomwrite(dev_t dev, struct uio *uio, int flags)
{
	int		ret = 0, newdata = 0;
	u_int32_t	*buf;

	if (uio->uio_resid == 0)
		return 0;

	buf = malloc(POOLBYTES, M_TEMP, M_WAITOK);

	while (ret == 0 && uio->uio_resid > 0) {
		size_t	n = ulmin(POOLBYTES, uio->uio_resid);

		ret = uiomove(buf, n, uio);
		if (ret != 0)
			break;
		while (n % sizeof(u_int32_t))
			((u_int8_t *)buf)[n++] = 0;
		add_entropy_words(buf, n / 4);
		if (uio->uio_resid > 0)
			yield();
		newdata = 1;
	}

	if (newdata)
		arc4_init(NULL);

	explicit_bzero(buf, POOLBYTES);
	free(buf, M_TEMP, POOLBYTES);
	return ret;
}

int
randomkqfilter(dev_t dev, struct knote *kn)
{
	switch (kn->kn_filter) {
	case EVFILT_READ:
		kn->kn_fop = &randomread_filtops;
		break;
	case EVFILT_WRITE:
		kn->kn_fop = &randomwrite_filtops;
		break;
	default:
		return (EINVAL);
	}

	return (0);
}

void
filt_randomdetach(struct knote *kn)
{
}

int
filt_randomread(struct knote *kn, long hint)
{
	kn->kn_data = ARC4_MAIN_MAX_BYTES;
	return (1);
}

int
filt_randomwrite(struct knote *kn, long hint)
{
	kn->kn_data = POOLBYTES;
	return (1);
}

int
randomioctl(dev_t dev, u_long cmd, caddr_t data, int flag, struct proc *p)
{
	switch (cmd) {
	case FIOASYNC:
		/* No async flag in softc so this is a no-op. */
		break;
	case FIONBIO:
		/* Handled in the upper FS layer. */
		break;
	default:
		return ENOTTY;
	}
	return 0;
}

int
sys_getentropy(struct proc *p, void *v, register_t *retval)
{
	struct sys_getentropy_args /* {
		syscallarg(void *) buf;
		syscallarg(size_t) nbyte;
	} */ *uap = v;
	char buf[256];
	int error;

	if (SCARG(uap, nbyte) > sizeof(buf))
		return (EIO);
	arc4random_buf(buf, SCARG(uap, nbyte));
	if ((error = copyout(buf, SCARG(uap, buf), SCARG(uap, nbyte))) != 0)
		return (error);
	explicit_bzero(buf, sizeof(buf));
	retval[0] = 0;
	return (0);
}