miscutils/seedrng.c - codeaurora/busybox - Gitiles

 // SPDX-License-Identifier: GPL-2.0 OR MIT
 /*
  * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
  *
  * SeedRNG is a simple program made for seeding the Linux kernel random number
  * generator from seed files. It is is useful in light of the fact that the
  * Linux kernel RNG cannot be initialized from shell scripts, and new seeds
  * cannot be safely generated from boot time shell scripts either. It should
  * be run once at init time and once at shutdown time. It can be run at other
  * times on a timer as well. Whenever it is run, it writes existing seed files
  * into the RNG pool, and then creates a new seed file. If the RNG is
  * initialized at the time of creating a new seed file, then that new seed file
  * is marked as "creditable", which means it can be used to initialize the RNG.
  * Otherwise, it is marked as "non-creditable", in which case it is still used
  * to seed the RNG's pool, but will not initialize the RNG. In order to ensure
  * that entropy only ever stays the same or increases from one seed file to the
  * next, old seed values are hashed together with new seed values when writing
  * new seed files.
  *
  * This is based on code from <https://git.zx2c4.com/seedrng/about/>.
  */
 //config:config SEEDRNG
 //config:	bool "seedrng (1.3 kb)"
 //config:	default y
 //config:	help
 //config:	Seed the kernel RNG from seed files, meant to be called
 //config:	once during startup, once during shutdown, and optionally
 //config:	at some periodic interval in between.

 //applet:IF_SEEDRNG(APPLET(seedrng, BB_DIR_USR_SBIN, BB_SUID_DROP))

 //kbuild:lib-$(CONFIG_SEEDRNG) += seedrng.o

 //usage:#define seedrng_trivial_usage
 //usage:	"[-d DIR] [-n]"
 //usage:#define seedrng_full_usage "\n\n"
 //usage:	"Seed the kernel RNG from seed files"
 //usage:	"\n"
 //usage:	"\n	-d DIR	Use seed files in DIR (default: /var/lib/seedrng)"
 //usage:	"\n	-n	Do not credit randomness, even if creditable"

 #include "libbb.h"

 #include <linux/random.h>
 #include <sys/random.h>
 #include <sys/file.h>

 #ifndef GRND_INSECURE
 #define GRND_INSECURE 0x0004 /* Apparently some headers don't ship with this yet. */
 #endif

 #define DEFAULT_SEED_DIR         "/var/lib/seedrng"
 #define CREDITABLE_SEED_NAME     "seed.credit"
 #define NON_CREDITABLE_SEED_NAME "seed.no-credit"

 enum {
 	MIN_SEED_LEN = SHA256_OUTSIZE,
 	/* kernels < 5.18 could return short reads from getrandom()
 	 * if signal is pending and length is > 256.
 	 * Let's limit our reads to 256 bytes.
 	 */
 	MAX_SEED_LEN = 256,
 };

 static size_t determine_optimal_seed_len(void)
 {
 	char poolsize_str[12];
 	unsigned poolsize;
 	int n;

 	n = open_read_close("/proc/sys/kernel/random/poolsize", poolsize_str, sizeof(poolsize_str) - 1);
 	if (n < 0) {
 		bb_perror_msg("can't determine pool size, assuming %u bits", MIN_SEED_LEN * 8);
 		return MIN_SEED_LEN;
 	}
 	poolsize_str[n] = '\0';
 	poolsize = (bb_strtou(poolsize_str, NULL, 10) + 7) / 8;
 	return MAX(MIN(poolsize, MAX_SEED_LEN), MIN_SEED_LEN);
 }

 static bool read_new_seed(uint8_t *seed, size_t len)
 {
 	bool is_creditable;
 	ssize_t ret;

 	ret = getrandom(seed, len, GRND_NONBLOCK);
 	if (ret == (ssize_t)len) {
 		return true;
 	}
 	if (ret < 0 && errno == ENOSYS) {
 		int fd = xopen("/dev/random", O_RDONLY);
 		struct pollfd random_fd;
 		random_fd.fd = fd;
 		random_fd.events = POLLIN;
 		is_creditable = poll(&random_fd, 1, 0) == 1;
 //This is racy. is_creditable can be set to true here, but other process
 //can consume "good" random data from /dev/urandom before we do it below.
 		close(fd);
 	} else {
 		if (getrandom(seed, len, GRND_INSECURE) == (ssize_t)len)
 			return false;
 		is_creditable = false;
 	}

 	/* Either getrandom() is not implemented, or
 	 * getrandom(GRND_INSECURE) did not give us LEN bytes.
 	 * Fallback to reading /dev/urandom.
 	 */
 	errno = 0;
 	if (open_read_close("/dev/urandom", seed, len) != (ssize_t)len)
 		bb_perror_msg_and_die("can't read '%s'", "/dev/urandom");
 	return is_creditable;
 }

 static void seed_from_file_if_exists(const char *filename, int dfd, bool credit, sha256_ctx_t *hash)
 {
 	struct {
 		int entropy_count;
 		int buf_size;
 		uint8_t buf[MAX_SEED_LEN];
 	} req;
 	ssize_t seed_len;

 	seed_len = open_read_close(filename, req.buf, sizeof(req.buf));
 	if (seed_len < 0) {
 		if (errno != ENOENT)
 			bb_perror_msg_and_die("can't read '%s'", filename);
 		return;
 	}
 	xunlink(filename);
 	if (seed_len != 0) {
 		int fd;

 		/* We are going to use this data to seed the RNG:
 		 * we believe it to genuinely containing entropy.
 		 * If this just-unlinked file survives
 		 * (if machine crashes before deletion is recorded on disk)
 		 * and we reuse it after reboot, this assumption
 		 * would be violated, and RNG may end up generating
 		 * the same data. fsync the directory
 		 * to make sure file is gone:
 		 */
 		if (fsync(dfd) != 0)
 			bb_simple_perror_msg_and_die("I/O error");

 //Length is not random, and taking its address spills variable to stack
 //		sha256_hash(hash, &seed_len, sizeof(seed_len));
 		sha256_hash(hash, req.buf, seed_len);

 		req.buf_size = seed_len;
 		seed_len *= 8;
 		req.entropy_count = credit ? seed_len : 0;
 		printf("Seeding %u bits %s crediting\n",
 				(unsigned)seed_len, credit ? "and" : "without");
 		fd = xopen("/dev/urandom", O_RDONLY);
 		xioctl(fd, RNDADDENTROPY, &req);
 		if (ENABLE_FEATURE_CLEAN_UP)
 			close(fd);
 	}
 }

 int seedrng_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
 int seedrng_main(int argc UNUSED_PARAM, char **argv)
 {
 	const char *seed_dir;
 	int fd, dfd;
 	int i;
 	unsigned opts;
 	uint8_t new_seed[MAX_SEED_LEN];
 	size_t new_seed_len;
 	bool new_seed_creditable;
 	struct timespec timestamp[2];
 	sha256_ctx_t hash;

 	enum {
 		OPT_n = (1 << 0), /* must be 1 */
 		OPT_d = (1 << 1),
 	};
 #if ENABLE_LONG_OPTS
 	static const char longopts[] ALIGN1 =
 		"skip-credit\0" No_argument       "n"
 		"seed-dir\0"    Required_argument "d"
 		;
 #endif

 	seed_dir = DEFAULT_SEED_DIR;
 	opts = getopt32long(argv, "nd:", longopts, &seed_dir);
 	umask(0077);
 	if (getuid() != 0)
 		bb_simple_error_msg_and_die(bb_msg_you_must_be_root);

 	if (mkdir(seed_dir, 0700) < 0 && errno != EEXIST)
 		bb_perror_msg_and_die("can't create directory '%s'", seed_dir);
 	dfd = xopen(seed_dir, O_DIRECTORY | O_RDONLY);
 	xfchdir(dfd);
 	/* Concurrent runs of this tool might feed the same data to RNG twice.
 	 * Avoid concurrent runs by taking a blocking lock on the directory.
 	 * Not checking for errors. Looking at manpage,
 	 * ENOLCK "The kernel ran out of memory for allocating lock records"
 	 * seems to be the only one which is possible - and if that happens,
 	 * machine is OOMing (much worse problem than inability to lock...).
 	 * Also, typically configured Linux machines do not fail GFP_KERNEL
 	 * allocations (they trigger memory reclaim instead).
 	 */
 	flock(dfd, LOCK_EX); /* blocks while another instance runs */

 	sha256_begin(&hash);
 //Hashing in a constant string doesn't add any entropy
 //	sha256_hash(&hash, "SeedRNG v1 Old+New Prefix", 25);
 	clock_gettime(CLOCK_REALTIME, &timestamp[0]);
 	clock_gettime(CLOCK_BOOTTIME, &timestamp[1]);
 	sha256_hash(&hash, timestamp, sizeof(timestamp));

 	for (i = 0; i <= 1; i++) {
 		seed_from_file_if_exists(
 			i == 0 ? NON_CREDITABLE_SEED_NAME : CREDITABLE_SEED_NAME,
 			dfd,
 			/*credit?*/ (opts ^ OPT_n) & i, /* 0, then 1 unless -n */
 			&hash);
 	}

 	new_seed_len = determine_optimal_seed_len();
 	new_seed_creditable = read_new_seed(new_seed, new_seed_len);
 //Length is not random, and taking its address spills variable to stack
 //	sha256_hash(&hash, &new_seed_len, sizeof(new_seed_len));
 	sha256_hash(&hash, new_seed, new_seed_len);
 	sha256_end(&hash, new_seed + new_seed_len - SHA256_OUTSIZE);

 	printf("Saving %u bits of %screditable seed for next boot\n",
 		(unsigned)new_seed_len * 8, new_seed_creditable ? "" : "non-");
 	fd = xopen3(NON_CREDITABLE_SEED_NAME, O_WRONLY | O_CREAT | O_TRUNC, 0400);
 	xwrite(fd, new_seed, new_seed_len);
 	if (new_seed_creditable) {
 		/* More paranoia when we create a file which we believe contains
 		 * genuine entropy: make sure disk is not full, quota isn't exceeded, etc:
 		 */
 		if (fsync(fd) < 0)
 			bb_perror_msg_and_die("can't write '%s'", NON_CREDITABLE_SEED_NAME);
 		xrename(NON_CREDITABLE_SEED_NAME, CREDITABLE_SEED_NAME);
 	}
 	return EXIT_SUCCESS;
 }
	// SPDX-License-Identifier: GPL-2.0 OR MIT
	/*
	* Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
	*
	* SeedRNG is a simple program made for seeding the Linux kernel random number
	* generator from seed files. It is is useful in light of the fact that the
	* Linux kernel RNG cannot be initialized from shell scripts, and new seeds
	* cannot be safely generated from boot time shell scripts either. It should
	* be run once at init time and once at shutdown time. It can be run at other
	* times on a timer as well. Whenever it is run, it writes existing seed files
	* into the RNG pool, and then creates a new seed file. If the RNG is
	* initialized at the time of creating a new seed file, then that new seed file
	* is marked as "creditable", which means it can be used to initialize the RNG.
	* Otherwise, it is marked as "non-creditable", in which case it is still used
	* to seed the RNG's pool, but will not initialize the RNG. In order to ensure
	* that entropy only ever stays the same or increases from one seed file to the
	* next, old seed values are hashed together with new seed values when writing
	* new seed files.
	*
	* This is based on code from <https://git.zx2c4.com/seedrng/about/>.
	*/
	//config:config SEEDRNG
	//config: bool "seedrng (1.3 kb)"
	//config: default y
	//config: help
	//config: Seed the kernel RNG from seed files, meant to be called
	//config: once during startup, once during shutdown, and optionally
	//config: at some periodic interval in between.

	//applet:IF_SEEDRNG(APPLET(seedrng, BB_DIR_USR_SBIN, BB_SUID_DROP))

	//kbuild:lib-$(CONFIG_SEEDRNG) += seedrng.o

	//usage:#define seedrng_trivial_usage
	//usage: "[-d DIR] [-n]"
	//usage:#define seedrng_full_usage "\n\n"
	//usage: "Seed the kernel RNG from seed files"
	//usage: "\n"
	//usage: "\n -d DIR Use seed files in DIR (default: /var/lib/seedrng)"
	//usage: "\n -n Do not credit randomness, even if creditable"

	#include "libbb.h"

	#include <linux/random.h>
	#include <sys/random.h>
	#include <sys/file.h>

	#ifndef GRND_INSECURE
	#define GRND_INSECURE 0x0004 /* Apparently some headers don't ship with this yet. */
	#endif

	#define DEFAULT_SEED_DIR "/var/lib/seedrng"
	#define CREDITABLE_SEED_NAME "seed.credit"
	#define NON_CREDITABLE_SEED_NAME "seed.no-credit"

	enum {
	MIN_SEED_LEN = SHA256_OUTSIZE,
	/* kernels < 5.18 could return short reads from getrandom()
	* if signal is pending and length is > 256.
	* Let's limit our reads to 256 bytes.
	*/
	MAX_SEED_LEN = 256,
	};

	static size_t determine_optimal_seed_len(void)
	{
	char poolsize_str[12];
	unsigned poolsize;
	int n;

	n = open_read_close("/proc/sys/kernel/random/poolsize", poolsize_str, sizeof(poolsize_str) - 1);
	if (n < 0) {
	bb_perror_msg("can't determine pool size, assuming %u bits", MIN_SEED_LEN * 8);
	return MIN_SEED_LEN;
	}
	poolsize_str[n] = '\0';
	poolsize = (bb_strtou(poolsize_str, NULL, 10) + 7) / 8;
	return MAX(MIN(poolsize, MAX_SEED_LEN), MIN_SEED_LEN);
	}

	static bool read_new_seed(uint8_t *seed, size_t len)
	{
	bool is_creditable;
	ssize_t ret;

	ret = getrandom(seed, len, GRND_NONBLOCK);
	if (ret == (ssize_t)len) {
	return true;
	}
	if (ret < 0 && errno == ENOSYS) {
	int fd = xopen("/dev/random", O_RDONLY);
	struct pollfd random_fd;
	random_fd.fd = fd;
	random_fd.events = POLLIN;
	is_creditable = poll(&random_fd, 1, 0) == 1;
	//This is racy. is_creditable can be set to true here, but other process
	//can consume "good" random data from /dev/urandom before we do it below.
	close(fd);
	} else {
	if (getrandom(seed, len, GRND_INSECURE) == (ssize_t)len)
	return false;
	is_creditable = false;
	}

	/* Either getrandom() is not implemented, or
	* getrandom(GRND_INSECURE) did not give us LEN bytes.
	* Fallback to reading /dev/urandom.
	*/
	errno = 0;
	if (open_read_close("/dev/urandom", seed, len) != (ssize_t)len)
	bb_perror_msg_and_die("can't read '%s'", "/dev/urandom");
	return is_creditable;
	}

	static void seed_from_file_if_exists(const char filename, int dfd, bool credit, sha256_ctx_t hash)
	{
	struct {
	int entropy_count;
	int buf_size;
	uint8_t buf[MAX_SEED_LEN];
	} req;
	ssize_t seed_len;

	seed_len = open_read_close(filename, req.buf, sizeof(req.buf));
	if (seed_len < 0) {
	if (errno != ENOENT)
	bb_perror_msg_and_die("can't read '%s'", filename);
	return;
	}
	xunlink(filename);
	if (seed_len != 0) {
	int fd;

	/* We are going to use this data to seed the RNG:
	* we believe it to genuinely containing entropy.
	* If this just-unlinked file survives
	* (if machine crashes before deletion is recorded on disk)
	* and we reuse it after reboot, this assumption
	* would be violated, and RNG may end up generating
	* the same data. fsync the directory
	* to make sure file is gone:
	*/
	if (fsync(dfd) != 0)
	bb_simple_perror_msg_and_die("I/O error");

	//Length is not random, and taking its address spills variable to stack
	// sha256_hash(hash, &seed_len, sizeof(seed_len));
	sha256_hash(hash, req.buf, seed_len);

	req.buf_size = seed_len;
	seed_len *= 8;
	req.entropy_count = credit ? seed_len : 0;
	printf("Seeding %u bits %s crediting\n",
	(unsigned)seed_len, credit ? "and" : "without");
	fd = xopen("/dev/urandom", O_RDONLY);
	xioctl(fd, RNDADDENTROPY, &req);
	if (ENABLE_FEATURE_CLEAN_UP)
	close(fd);
	}
	}

	int seedrng_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
	int seedrng_main(int argc UNUSED_PARAM, char **argv)
	{
	const char *seed_dir;
	int fd, dfd;
	int i;
	unsigned opts;
	uint8_t new_seed[MAX_SEED_LEN];
	size_t new_seed_len;
	bool new_seed_creditable;
	struct timespec timestamp[2];
	sha256_ctx_t hash;

	enum {
	OPT_n = (1 << 0), /* must be 1 */
	OPT_d = (1 << 1),
	};
	#if ENABLE_LONG_OPTS
	static const char longopts[] ALIGN1 =
	"skip-credit\0" No_argument "n"
	"seed-dir\0" Required_argument "d"
	;
	#endif

	seed_dir = DEFAULT_SEED_DIR;
	opts = getopt32long(argv, "nd:", longopts, &seed_dir);
	umask(0077);
	if (getuid() != 0)
	bb_simple_error_msg_and_die(bb_msg_you_must_be_root);

	if (mkdir(seed_dir, 0700) < 0 && errno != EEXIST)
	bb_perror_msg_and_die("can't create directory '%s'", seed_dir);
	dfd = xopen(seed_dir, O_DIRECTORY \| O_RDONLY);
	xfchdir(dfd);
	/* Concurrent runs of this tool might feed the same data to RNG twice.
	* Avoid concurrent runs by taking a blocking lock on the directory.
	* Not checking for errors. Looking at manpage,
	* ENOLCK "The kernel ran out of memory for allocating lock records"
	* seems to be the only one which is possible - and if that happens,
	* machine is OOMing (much worse problem than inability to lock...).
	* Also, typically configured Linux machines do not fail GFP_KERNEL
	* allocations (they trigger memory reclaim instead).
	*/
	flock(dfd, LOCK_EX); /* blocks while another instance runs */

	sha256_begin(&hash);
	//Hashing in a constant string doesn't add any entropy
	// sha256_hash(&hash, "SeedRNG v1 Old+New Prefix", 25);
	clock_gettime(CLOCK_REALTIME, &timestamp[0]);
	clock_gettime(CLOCK_BOOTTIME, &timestamp[1]);
	sha256_hash(&hash, timestamp, sizeof(timestamp));

	for (i = 0; i <= 1; i++) {
	seed_from_file_if_exists(
	i == 0 ? NON_CREDITABLE_SEED_NAME : CREDITABLE_SEED_NAME,
	dfd,
	/credit?/ (opts ^ OPT_n) & i, /* 0, then 1 unless -n */
	&hash);
	}

	new_seed_len = determine_optimal_seed_len();
	new_seed_creditable = read_new_seed(new_seed, new_seed_len);
	//Length is not random, and taking its address spills variable to stack
	// sha256_hash(&hash, &new_seed_len, sizeof(new_seed_len));
	sha256_hash(&hash, new_seed, new_seed_len);
	sha256_end(&hash, new_seed + new_seed_len - SHA256_OUTSIZE);

	printf("Saving %u bits of %screditable seed for next boot\n",
	(unsigned)new_seed_len * 8, new_seed_creditable ? "" : "non-");
	fd = xopen3(NON_CREDITABLE_SEED_NAME, O_WRONLY \| O_CREAT \| O_TRUNC, 0400);
	xwrite(fd, new_seed, new_seed_len);
	if (new_seed_creditable) {
	/* More paranoia when we create a file which we believe contains
	* genuine entropy: make sure disk is not full, quota isn't exceeded, etc:
	*/
	if (fsync(fd) < 0)
	bb_perror_msg_and_die("can't write '%s'", NON_CREDITABLE_SEED_NAME);
	xrename(NON_CREDITABLE_SEED_NAME, CREDITABLE_SEED_NAME);
	}
	return EXIT_SUCCESS;
	}