seedrng: it's not a part of util-linux, move to miscutils

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 239 insertions, 0 deletions

diff --git a/miscutils/seedrng.c b/miscutils/seedrng.c
new file mode 100644
index 000000000..8c81835f6
--- /dev/null
+++ b/miscutils/seedrng.c
@@ -0,0 +1,239 @@
+// 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
+		 * (e.g. if machine crashes _right now_)
+		 * and we reuse it after reboot, this assumption
+		 * would be violated. Fsync the directory to
+		 * make sure file is gone:
+		 */
+		fsync(dfd);
+
+//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;
+	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 likely - 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); /* would block while another copy runs */
+
+	sha256_begin(&hash);
+	sha256_hash(&hash, "SeedRNG v1 Old+New Prefix", 25);
+	clock_gettime(CLOCK_REALTIME, &timestamp);
+	sha256_hash(&hash, &timestamp, sizeof(timestamp));
+	clock_gettime(CLOCK_BOOTTIME, &timestamp);
+	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;
+}