urandom-seed: 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 OpenWRT
and tweaked as needed: <https://git.zx2c4.com/seedrng/about/>.

This commit imports it into the urandom-seed package and wires up the
init scripts to call it. This also is a significant improvement over the
current init script, which does not robustly handle cleaning up of seeds
and syncing to prevent reuse. Additionally, the existing script creates
a new seed immediately after writing an old one, which means that the
amount of entropy might actually regress, due to failing to credit the
old seed.

Closes: https://github.com/openwrt/openwrt/issues/9570
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz> [fixed missing INSTALL_DIR]

5 files changed, 442 insertions, 35 deletions

diff --git a/package/system/urandom-seed/Makefile b/package/system/urandom-seed/Makefile
index 7c5524a9db..0c8d77f445 100644
--- a/package/system/urandom-seed/Makefile
+++ b/package/system/urandom-seed/Makefile
@@ -9,7 +9,6 @@ include $(INCLUDE_DIR)/package.mk
 define Package/urandom-seed
   SECTION:=base
   CATEGORY:=Base system
-  DEPENDS:=+getrandom
   TITLE:=/etc/urandom.seed handling for OpenWrt
   URL:=https://openwrt.org/
 endef
@@ -19,11 +18,15 @@ define Build/Prepare
 endef
 
 define Build/Compile/Default
+	$(TARGET_CC) $(TARGET_CFLAGS) $(TARGET_CPPFLAGS) $(TARGET_LDFLAGS) \
+		-std=gnu99 -o $(PKG_BUILD_DIR)/seedrng seedrng.c
 endef
 Build/Compile = $(Build/Compile/Default)
 
 define Package/urandom-seed/install
 	$(CP) ./files/* $(1)/
+	$(INSTALL_DIR) $(1)/sbin
+	$(CP) $(PKG_BUILD_DIR)/seedrng $(1)/sbin/
 endef
 
 $(eval $(call BuildPackage,urandom-seed))
diff --git a/package/system/urandom-seed/files/etc/init.d/urandom_seed b/package/system/urandom-seed/files/etc/init.d/urandom_seed
index 17d9c13400..d6e81c6079 100755
--- a/package/system/urandom-seed/files/etc/init.d/urandom_seed
+++ b/package/system/urandom-seed/files/etc/init.d/urandom_seed
@@ -5,7 +5,7 @@ USE_PROCD=1
 
 start_service() {
     procd_open_instance "urandom_seed"
-    procd_set_param command "/sbin/urandom_seed"
+    procd_set_param command "/sbin/seedrng"
     procd_set_param stdout 1
     procd_set_param stderr 1
     procd_close_instance
diff --git a/package/system/urandom-seed/files/lib/preinit/81_urandom_seed b/package/system/urandom-seed/files/lib/preinit/81_urandom_seed
index 2adc6c47f0..b3014daeaf 100644
--- a/package/system/urandom-seed/files/lib/preinit/81_urandom_seed
+++ b/package/system/urandom-seed/files/lib/preinit/81_urandom_seed
@@ -2,21 +2,11 @@ log_urandom_seed() {
     echo "urandom-seed: $1" > /dev/kmsg
 }
 
-_do_urandom_seed() {
-    [ -f "$1" ] || { log_urandom_seed "Seed file not found ($1)"; return; }
-    [ -O "$1" -a -G "$1" -a ! -x "$1" ] || { log_urandom_seed "Wrong owner / permissions for $1"; return; }
-
-    log_urandom_seed "Seeding with $1"
-    cat "$1" > /dev/urandom
-}
-
 do_urandom_seed() {
     [ -c /dev/urandom ] || { log_urandom_seed "Something is wrong with /dev/urandom"; return; }
-
-    _do_urandom_seed "/etc/urandom.seed"
-
-    SEED="$(uci -q get system.@system[0].urandom_seed)"
-    [ "${SEED:0:1}" = "/" -a "$SEED" != "/etc/urandom.seed" ] && _do_urandom_seed "$SEED"
+    seedrng 2>&1 | while read -r line; do
+        log_urandom_seed "$line"
+    done
 }
 
 boot_hook_add preinit_main do_urandom_seed
diff --git a/package/system/urandom-seed/files/sbin/urandom_seed b/package/system/urandom-seed/files/sbin/urandom_seed
deleted file mode 100755
index 7043e8af4e..0000000000
--- a/package/system/urandom-seed/files/sbin/urandom_seed
+++ /dev/null
@@ -1,20 +0,0 @@
-#!/bin/sh
-set -e
-
-trap '[ "$?" -eq 0 ] || echo "An error occured" >&2' EXIT
-
-save() {
-    touch "$1.tmp"
-    chown root:root "$1.tmp"
-    chmod 600 "$1.tmp"
-    getrandom 512 > "$1.tmp"
-    mv "$1.tmp" "$1"
-    echo "Seed saved ($1)"
-}
-
-SEED="$(uci -q get system.@system[0].urandom_seed || true)"
-[ "${SEED:0:1}" = "/" ] && save "$SEED"
-
-SEED=/etc/urandom.seed
-[ ! -f $SEED ] && save "$SEED"
-true
diff --git a/package/system/urandom-seed/seedrng.c b/package/system/urandom-seed/seedrng.c
new file mode 100644
index 0000000000..9a2cb10f55
--- /dev/null
+++ b/package/system/urandom-seed/seedrng.c
@@ -0,0 +1,434 @@
+// SPDX-License-Identifier: (GPL-2.0 OR MIT OR Apache-2.0)
+/*
+ * Copyright (C) 2022 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+#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>
+
+#define SEED_DIR "/etc/seedrng"
+#define CREDITABLE_SEED SEED_DIR "/seed.credit"
+#define NON_CREDITABLE_SEED SEED_DIR "/seed.no-credit"
+#define LOCK_FILE "/tmp/run/seedrng.lock"
+
+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) {
+		fprintf(stderr, "WARNING: Unable to determine pool size, falling back to %u bits: %s\n", 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;
+	} else if (ret < 0 && 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;
+		fprintf(stderr, "ERROR: Unable to open seed file: %s\n", strerror(errno));
+		return ret;
+	}
+	dfd = open(SEED_DIR, O_DIRECTORY | O_RDONLY);
+	if (dfd < 0) {
+		ret = -errno;
+		close(fd);
+		fprintf(stderr, "ERROR: Unable to open seed directory: %s\n", strerror(errno));
+		return ret;
+	}
+	seed_len = read(fd, seed, sizeof(seed));
+	if (seed_len < 0) {
+		ret = -errno;
+		fprintf(stderr, "ERROR: Unable to read seed file: %s\n", strerror(errno));
+	}
+	close(fd);
+	if (ret) {
+		close(dfd);
+		return ret;
+	}
+	if ((unlink(filename) < 0 || fsync(dfd) < 0) && seed_len) {
+		ret = -errno;
+		fprintf(stderr, "ERROR: Unable to remove seed after reading, so not seeding: %s\n", 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);
+
+	fprintf(stdout, "Seeding %zd bits %s crediting\n", seed_len * 8, credit ? "and" : "without");
+	ret = seed_rng(seed, seed_len, credit);
+	if (ret < 0)
+		fprintf(stderr, "ERROR: Unable to seed: %s\n", strerror(-ret));
+	return ret;
+}
+
+static bool skip_credit(void)
+{
+	const char *skip = getenv("SEEDRNG_SKIP_CREDIT");
+	return skip && (!strcmp(skip, "1") || !strcasecmp(skip, "true") ||
+			!strcasecmp(skip, "yes") || !strcasecmp(skip, "y"));
+}
+
+int main(int argc __attribute__((unused)), char *argv[] __attribute__((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 = -1, 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()) {
+		fprintf(stderr, "ERROR: This program requires root\n");
+		return 1;
+	}
+
+	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) {
+		fprintf(stderr, "ERROR: Unable to create \"%s\" directory: %s\n", SEED_DIR, strerror(errno));
+		return 1;
+	}
+
+	lock = open(LOCK_FILE, O_WRONLY | O_CREAT, 0000);
+	if (lock < 0 || flock(lock, LOCK_EX) < 0) {
+		fprintf(stderr, "ERROR: Unable to open lock file: %s\n", strerror(errno));
+		program_ret = 1;
+		goto out;
+	}
+
+	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, !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) {
+		fprintf(stderr, "ERROR: Unable to read new seed: %s\n", 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);
+
+	fprintf(stdout, "Saving %zu bits of %s seed for next boot\n", 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) {
+		fprintf(stderr, "ERROR: Unable to open seed file for writing: %s\n", strerror(errno));
+		program_ret |= 1 << 4;
+		goto out;
+	}
+	if (write(fd, new_seed, new_seed_len) != (ssize_t)new_seed_len || fsync(fd) < 0) {
+		fprintf(stderr, "ERROR: Unable to write seed file: %s\n", strerror(errno));
+		program_ret |= 1 << 5;
+		goto out;
+	}
+	if (new_seed_creditable && rename(NON_CREDITABLE_SEED, CREDITABLE_SEED) < 0) {
+		fprintf(stderr, "WARNING: Unable to make new seed creditable: %s\n", strerror(errno));
+		program_ret |= 1 << 6;
+	}
+out:
+	if (fd >= 0)
+		close(fd);
+	if (lock >= 0)
+		close(lock);
+	return program_ret;
+}