Use seedrng for seeding the random number generator

The RNG can't actually be seeded from a shell script, due to the
reliance on ioctls. For this reason, the seedrng project provides a
basic script meant to be copy and pasted into projects like OpenRC and
tweaked as needed: https://git.zx2c4.com/seedrng/about/

This commit imports it into OpenRC and wires up /etc/init.d/urandom to
call it. It shouldn't be called by other things on the system, so it
lives in rc_sbindir.

Closes #506.
Closes #507.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

3 files changed, 467 insertions, 2 deletions

diff --git a/src/rc/Makefile b/src/rc/Makefile
index fd796d92..62539f13 100644
--- a/src/rc/Makefile
+++ b/src/rc/Makefile
@@ -15,7 +15,7 @@ endif
 
 ifeq (${OS},Linux)
 SRCS+=		kill_all.c openrc-init.c openrc-shutdown.c rc-sysvinit.c broadcast.c \
-			rc-wtmp.c
+			rc-wtmp.c seedrng.c
 endif
 
 CLEANFILES=	version.h rc-selinux.o
@@ -47,6 +47,7 @@ RC_SBINPROGS=	mark_service_starting mark_service_started \
 
 ifeq (${OS},Linux)
 RC_BINPROGS+= kill_all
+RC_SBINPROGS+= seedrng
 SBINPROGS+= openrc-init openrc-shutdown
 endif
 
@@ -180,3 +181,6 @@ shell_var: shell_var.o
 
 swclock: swclock.o _usage.o rc-misc.o
 	${CC} ${LOCAL_CFLAGS} ${LOCAL_LDFLAGS} ${CFLAGS} ${LDFLAGS} -o $@ $^ ${LDADD}
+
+seedrng: seedrng.o
+	${CC} ${LOCAL_CFLAGS} ${LOCAL_LDFLAGS} ${CFLAGS} ${LDFLAGS} -o $@ $^ ${LDADD}
diff --git a/src/rc/meson.build b/src/rc/meson.build
index 8fdf3ac3..b724c51f 100644
--- a/src/rc/meson.build
+++ b/src/rc/meson.build
@@ -268,7 +268,15 @@ if os == 'Linux'
     link_with: [libeinfo,librc],
     install: true,
     install_dir: rc_bindir)
-  endif
+
+  executable('seedrng',
+    ['seedrng.c'],
+    c_args : cc_branding_flags,
+    include_directories: [incdir, einfo_incdir, rc_incdir],
+    link_with: [libeinfo, librc],
+    install: true,
+    install_dir: rc_sbindir)
+endif
 
 executable('shell_var',
   ['shell_var.c'],
diff --git a/src/rc/seedrng.c b/src/rc/seedrng.c
new file mode 100644
index 00000000..c1f94145
--- /dev/null
+++ b/src/rc/seedrng.c
@@ -0,0 +1,453 @@
+/*
+ * seedrng.c
+ * Seed kernel RNG from seed file, based on code from:
+ * https://git.zx2c4.com/seedrng/about/
+ */
+
+/*
+ * Copyright (c) 2022 The OpenRC Authors.
+ * See the Authors file at the top-level directory of this distribution and
+ * https://github.com/OpenRC/openrc/blob/HEAD/AUTHORS
+ *
+ * This file is part of OpenRC. It is subject to the license terms in
+ * the LICENSE file found in the top-level directory of this
+ * distribution and at https://github.com/OpenRC/openrc/blob/HEAD/LICENSE
+ * This file may not be copied, modified, propagated, or distributed
+ *    except according to the terms contained in the LICENSE file.
+ */
+
+#include <linux/random.h>
+#include <sys/random.h>
+#include <sys/ioctl.h>
+#include <sys/file.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <fcntl.h>
+#include <poll.h>
+#include <unistd.h>
+#include <time.h>
+#include <errno.h>
+#include <endian.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <string.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "rc.h"
+#include "einfo.h"
+#include "helpers.h"
+
+#ifndef GRND_INSECURE
+#define GRND_INSECURE 0x0004 /* Apparently some headers don't ship with this yet. */
+#endif
+
+static const char *SEED_DIR;
+static const char *LOCK_FILE;
+static char *CREDITABLE_SEED;
+static char *NON_CREDITABLE_SEED;
+
+enum blake2s_lengths {
+	BLAKE2S_BLOCK_LEN = 64,
+	BLAKE2S_HASH_LEN = 32,
+	BLAKE2S_KEY_LEN = 32
+};
+
+enum seedrng_lengths {
+	MAX_SEED_LEN = 512,
+	MIN_SEED_LEN = BLAKE2S_HASH_LEN
+};
+
+struct blake2s_state {
+	uint32_t h[8];
+	uint32_t t[2];
+	uint32_t f[2];
+	uint8_t buf[BLAKE2S_BLOCK_LEN];
+	unsigned int buflen;
+	unsigned int outlen;
+};
+
+#define le32_to_cpup(a) le32toh(*(a))
+#define cpu_to_le32(a) htole32(a)
+#ifndef ARRAY_SIZE
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+#endif
+#ifndef DIV_ROUND_UP
+#define DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))
+#endif
+
+static inline void cpu_to_le32_array(uint32_t *buf, unsigned int words)
+{
+        while (words--) {
+		*buf = cpu_to_le32(*buf);
+		++buf;
+	}
+}
+
+static inline void le32_to_cpu_array(uint32_t *buf, unsigned int words)
+{
+        while (words--) {
+		*buf = le32_to_cpup(buf);
+		++buf;
+        }
+}
+
+static inline uint32_t ror32(uint32_t word, unsigned int shift)
+{
+	return (word >> (shift & 31)) | (word << ((-shift) & 31));
+}
+
+static const uint32_t blake2s_iv[8] = {
+	0x6A09E667UL, 0xBB67AE85UL, 0x3C6EF372UL, 0xA54FF53AUL,
+	0x510E527FUL, 0x9B05688CUL, 0x1F83D9ABUL, 0x5BE0CD19UL
+};
+
+static const uint8_t blake2s_sigma[10][16] = {
+	{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 },
+	{ 14, 10, 4, 8, 9, 15, 13, 6, 1, 12, 0, 2, 11, 7, 5, 3 },
+	{ 11, 8, 12, 0, 5, 2, 15, 13, 10, 14, 3, 6, 7, 1, 9, 4 },
+	{ 7, 9, 3, 1, 13, 12, 11, 14, 2, 6, 5, 10, 4, 0, 15, 8 },
+	{ 9, 0, 5, 7, 2, 4, 10, 15, 14, 1, 11, 12, 6, 8, 3, 13 },
+	{ 2, 12, 6, 10, 0, 11, 8, 3, 4, 13, 7, 5, 15, 14, 1, 9 },
+	{ 12, 5, 1, 15, 14, 13, 4, 10, 0, 7, 6, 3, 9, 2, 8, 11 },
+	{ 13, 11, 7, 14, 12, 1, 3, 9, 5, 0, 15, 4, 8, 6, 2, 10 },
+	{ 6, 15, 14, 9, 11, 3, 0, 8, 12, 2, 13, 7, 1, 4, 10, 5 },
+	{ 10, 2, 8, 4, 7, 6, 1, 5, 15, 11, 9, 14, 3, 12, 13, 0 },
+};
+
+static void blake2s_set_lastblock(struct blake2s_state *state)
+{
+	state->f[0] = -1;
+}
+
+static void blake2s_increment_counter(struct blake2s_state *state, const uint32_t inc)
+{
+	state->t[0] += inc;
+	state->t[1] += (state->t[0] < inc);
+}
+
+static void blake2s_init_param(struct blake2s_state *state, const uint32_t param)
+{
+	int i;
+
+	memset(state, 0, sizeof(*state));
+	for (i = 0; i < 8; ++i)
+		state->h[i] = blake2s_iv[i];
+	state->h[0] ^= param;
+}
+
+static void blake2s_init(struct blake2s_state *state, const size_t outlen)
+{
+	blake2s_init_param(state, 0x01010000 | outlen);
+	state->outlen = outlen;
+}
+
+static void blake2s_compress(struct blake2s_state *state, const uint8_t *block, size_t nblocks, const uint32_t inc)
+{
+	uint32_t m[16];
+	uint32_t v[16];
+	int i;
+
+	while (nblocks > 0) {
+		blake2s_increment_counter(state, inc);
+		memcpy(m, block, BLAKE2S_BLOCK_LEN);
+		le32_to_cpu_array(m, ARRAY_SIZE(m));
+		memcpy(v, state->h, 32);
+		v[ 8] = blake2s_iv[0];
+		v[ 9] = blake2s_iv[1];
+		v[10] = blake2s_iv[2];
+		v[11] = blake2s_iv[3];
+		v[12] = blake2s_iv[4] ^ state->t[0];
+		v[13] = blake2s_iv[5] ^ state->t[1];
+		v[14] = blake2s_iv[6] ^ state->f[0];
+		v[15] = blake2s_iv[7] ^ state->f[1];
+
+#define G(r, i, a, b, c, d) do { \
+	a += b + m[blake2s_sigma[r][2 * i + 0]]; \
+	d = ror32(d ^ a, 16); \
+	c += d; \
+	b = ror32(b ^ c, 12); \
+	a += b + m[blake2s_sigma[r][2 * i + 1]]; \
+	d = ror32(d ^ a, 8); \
+	c += d; \
+	b = ror32(b ^ c, 7); \
+} while (0)
+
+#define ROUND(r) do { \
+	G(r, 0, v[0], v[ 4], v[ 8], v[12]); \
+	G(r, 1, v[1], v[ 5], v[ 9], v[13]); \
+	G(r, 2, v[2], v[ 6], v[10], v[14]); \
+	G(r, 3, v[3], v[ 7], v[11], v[15]); \
+	G(r, 4, v[0], v[ 5], v[10], v[15]); \
+	G(r, 5, v[1], v[ 6], v[11], v[12]); \
+	G(r, 6, v[2], v[ 7], v[ 8], v[13]); \
+	G(r, 7, v[3], v[ 4], v[ 9], v[14]); \
+} while (0)
+		ROUND(0);
+		ROUND(1);
+		ROUND(2);
+		ROUND(3);
+		ROUND(4);
+		ROUND(5);
+		ROUND(6);
+		ROUND(7);
+		ROUND(8);
+		ROUND(9);
+
+#undef G
+#undef ROUND
+
+		for (i = 0; i < 8; ++i)
+			state->h[i] ^= v[i] ^ v[i + 8];
+
+		block += BLAKE2S_BLOCK_LEN;
+		--nblocks;
+	}
+}
+
+static void blake2s_update(struct blake2s_state *state, const void *inp, size_t inlen)
+{
+	const size_t fill = BLAKE2S_BLOCK_LEN - state->buflen;
+	const uint8_t *in = inp;
+
+	if (!inlen)
+		return;
+	if (inlen > fill) {
+		memcpy(state->buf + state->buflen, in, fill);
+		blake2s_compress(state, state->buf, 1, BLAKE2S_BLOCK_LEN);
+		state->buflen = 0;
+		in += fill;
+		inlen -= fill;
+	}
+	if (inlen > BLAKE2S_BLOCK_LEN) {
+		const size_t nblocks = DIV_ROUND_UP(inlen, BLAKE2S_BLOCK_LEN);
+		blake2s_compress(state, in, nblocks - 1, BLAKE2S_BLOCK_LEN);
+		in += BLAKE2S_BLOCK_LEN * (nblocks - 1);
+		inlen -= BLAKE2S_BLOCK_LEN * (nblocks - 1);
+	}
+	memcpy(state->buf + state->buflen, in, inlen);
+	state->buflen += inlen;
+}
+
+static void blake2s_final(struct blake2s_state *state, uint8_t *out)
+{
+	blake2s_set_lastblock(state);
+	memset(state->buf + state->buflen, 0, BLAKE2S_BLOCK_LEN - state->buflen);
+	blake2s_compress(state, state->buf, 1, state->buflen);
+	cpu_to_le32_array(state->h, ARRAY_SIZE(state->h));
+	memcpy(out, state->h, state->outlen);
+}
+
+static size_t determine_optimal_seed_len(void)
+{
+	size_t ret = 0;
+	char poolsize_str[11] = { 0 };
+	int fd = open("/proc/sys/kernel/random/poolsize", O_RDONLY);
+
+	if (fd < 0 || read(fd, poolsize_str, sizeof(poolsize_str) - 1) < 0) {
+		ewarn("Unable to determine pool size, falling back to %u bits: %s", MIN_SEED_LEN * 8, strerror(errno));
+		ret = MIN_SEED_LEN;
+	} else
+		ret = DIV_ROUND_UP(strtoul(poolsize_str, NULL, 10), 8);
+	if (fd >= 0)
+		close(fd);
+	if (ret < MIN_SEED_LEN)
+		ret = MIN_SEED_LEN;
+	else if (ret > MAX_SEED_LEN)
+		ret = MAX_SEED_LEN;
+	return ret;
+}
+
+static int read_new_seed(uint8_t *seed, size_t len, bool *is_creditable)
+{
+	ssize_t ret;
+	int urandom_fd;
+
+	*is_creditable = false;
+	ret = getrandom(seed, len, GRND_NONBLOCK);
+	if (ret == (ssize_t)len) {
+		*is_creditable = true;
+		return 0;
+	}
+	if (ret == -1 && errno == ENOSYS) {
+		struct pollfd random_fd = {
+			.fd = open("/dev/random", O_RDONLY),
+			.events = POLLIN
+		};
+		if (random_fd.fd < 0)
+			return -errno;
+		*is_creditable = poll(&random_fd, 1, 0) == 1;
+		close(random_fd.fd);
+	} else if (getrandom(seed, len, GRND_INSECURE) == (ssize_t)len)
+		return 0;
+	urandom_fd = open("/dev/urandom", O_RDONLY);
+	if (urandom_fd < 0)
+		return -errno;
+	ret = read(urandom_fd, seed, len);
+	if (ret == (ssize_t)len)
+		ret = 0;
+	else
+		ret = -errno ? -errno : -EIO;
+	close(urandom_fd);
+	return ret;
+}
+
+static int seed_rng(uint8_t *seed, size_t len, bool credit)
+{
+	struct {
+		int entropy_count;
+		int buf_size;
+		uint8_t buffer[MAX_SEED_LEN];
+	} req = {
+		.entropy_count = credit ? len * 8 : 0,
+		.buf_size = len
+	};
+	int random_fd, ret;
+
+	if (len > sizeof(req.buffer))
+		return -EFBIG;
+	memcpy(req.buffer, seed, len);
+
+	random_fd = open("/dev/random", O_RDWR);
+	if (random_fd < 0)
+		return -errno;
+	ret = ioctl(random_fd, RNDADDENTROPY, &req);
+	if (ret)
+		ret = -errno ? -errno : -EIO;
+	close(random_fd);
+	return ret;
+}
+
+static int seed_from_file_if_exists(const char *filename, bool credit, struct blake2s_state *hash)
+{
+	uint8_t seed[MAX_SEED_LEN];
+	ssize_t seed_len;
+	int fd, dfd, ret = 0;
+
+	fd = open(filename, O_RDONLY);
+	if (fd < 0 && errno == ENOENT)
+		return 0;
+	else if (fd < 0) {
+		ret = -errno;
+		eerror("Unable to open seed file: %s", strerror(errno));
+		return ret;
+	}
+	dfd = open(SEED_DIR, O_DIRECTORY | O_RDONLY);
+	if (dfd < 0) {
+		ret = -errno;
+		close(fd);
+		eerror("Unable to open seed directory: %s", strerror(errno));
+		return ret;
+	}
+	seed_len = read(fd, seed, sizeof(seed));
+	if (seed_len < 0) {
+		ret = -errno;
+		eerror("Unable to read seed file: %s", strerror(errno));
+	}
+	close(fd);
+	if (ret) {
+		close(dfd);
+		return ret;
+	}
+	if ((unlink(filename) < 0 || fsync(dfd) < 0) && seed_len) {
+		ret = -errno;
+		eerror("Unable to remove seed after reading, so not seeding: %s", strerror(errno));
+	}
+	close(dfd);
+	if (ret)
+		return ret;
+	if (!seed_len)
+		return 0;
+
+	blake2s_update(hash, &seed_len, sizeof(seed_len));
+	blake2s_update(hash, seed, seed_len);
+
+	einfo("Seeding %zd bits %s crediting", seed_len * 8, credit ? "and" : "without");
+	ret = seed_rng(seed, seed_len, credit);
+	if (ret < 0)
+		eerror("Unable to seed: %s", strerror(-ret));
+	return ret;
+}
+
+static void populate_global_paths(void)
+{
+	SEED_DIR = getenv("SEEDRNG_SEED_DIR");
+	if (!SEED_DIR || !*SEED_DIR)
+		SEED_DIR = "/var/lib/seedrng";
+	LOCK_FILE = getenv("SEEDRNG_LOCK_FILE");
+	if (!LOCK_FILE || !*LOCK_FILE)
+		LOCK_FILE = "/var/run/seedrng.lock";
+	xasprintf(&CREDITABLE_SEED, "%s/seed.credit", SEED_DIR);
+	xasprintf(&NON_CREDITABLE_SEED, "%s/seed.no-credit", SEED_DIR);
+}
+
+int main(int argc _unused, char *argv[] _unused)
+{
+	static const char seedrng_prefix[] = "SeedRNG v1 Old+New Prefix";
+	static const char seedrng_failure[] = "SeedRNG v1 No New Seed Failure";
+	int ret, fd, lock, program_ret = 0;
+	uint8_t new_seed[MAX_SEED_LEN];
+	size_t new_seed_len;
+	bool new_seed_creditable;
+	struct timespec realtime = { 0 }, boottime = { 0 };
+	struct blake2s_state hash;
+
+	umask(0077);
+	if (getuid())
+		eerrorx("This rc helper program requires root");
+
+	populate_global_paths();
+	blake2s_init(&hash, BLAKE2S_HASH_LEN);
+	blake2s_update(&hash, seedrng_prefix, strlen(seedrng_prefix));
+	clock_gettime(CLOCK_REALTIME, &realtime);
+	clock_gettime(CLOCK_BOOTTIME, &boottime);
+	blake2s_update(&hash, &realtime, sizeof(realtime));
+	blake2s_update(&hash, &boottime, sizeof(boottime));
+
+	if (mkdir(SEED_DIR, 0700) < 0 && errno != EEXIST)
+		eerrorx("Unable to create \"%s\" directory: %s", SEED_DIR, strerror(errno));
+
+	lock = open(LOCK_FILE, O_WRONLY | O_CREAT, 0000);
+	if (lock < 0 || flock(lock, LOCK_EX) < 0)
+		eerrorx("Unable to open lock file: %s", strerror(errno));
+
+	ret = seed_from_file_if_exists(NON_CREDITABLE_SEED, false, &hash);
+	if (ret < 0)
+		program_ret |= 1 << 1;
+	ret = seed_from_file_if_exists(CREDITABLE_SEED, !rc_yesno(getenv("SEEDRNG_SKIP_CREDIT")), &hash);
+	if (ret < 0)
+		program_ret |= 1 << 2;
+
+	new_seed_len = determine_optimal_seed_len();
+	ret = read_new_seed(new_seed, new_seed_len, &new_seed_creditable);
+	if (ret < 0) {
+		eerror("Unable to read new seed: %s", strerror(-ret));
+		new_seed_len = BLAKE2S_HASH_LEN;
+		strncpy((char *)new_seed, seedrng_failure, new_seed_len);
+		program_ret |= 1 << 3;
+	}
+	blake2s_update(&hash, &new_seed_len, sizeof(new_seed_len));
+	blake2s_update(&hash, new_seed, new_seed_len);
+	blake2s_final(&hash, new_seed + new_seed_len - BLAKE2S_HASH_LEN);
+
+	einfo("Saving %zu bits of %s seed for next boot", new_seed_len * 8, new_seed_creditable ? "creditable" : "non-creditable");
+	fd = open(NON_CREDITABLE_SEED, O_WRONLY | O_CREAT | O_TRUNC, 0400);
+	if (fd < 0) {
+		eerror("Unable to open seed file for writing: %s", strerror(errno));
+		program_ret |= 1 << 4;
+		goto out;
+	}
+	if (write(fd, new_seed, new_seed_len) != (ssize_t)new_seed_len || fsync(fd) < 0) {
+		eerror("Unable to write seed file: %s", strerror(errno));
+		program_ret |= 1 << 5;
+		goto out;
+	}
+	if (new_seed_creditable && rename(NON_CREDITABLE_SEED, CREDITABLE_SEED) < 0) {
+		ewarn("Unable to make new seed creditable: %s", strerror(errno));
+		program_ret |= 1 << 6;
+	}
+out:
+	close(fd);
+	close(lock);
+	return program_ret;
+}