// SPDX-License-Identifier: GPL-2.0 OR MIT /* * Copyright (C) 2022 Jason A. Donenfeld . 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 . */ //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 #include /* Fix up some libc (e.g. glibc <= 2.24) not having getrandom() */ #if defined HAVE_GETRANDOM #include #else /* No getrandom */ #include static ssize_t getrandom(void *buffer, size_t length, unsigned flags) { # if defined(__NR_getrandom) return syscall(__NR_getrandom, buffer, length, flags); # else errno = ENOSYS; return -1; # endif } #endif /* Apparently some headers don't ship with this yet. */ #ifndef GRND_NONBLOCK #define GRND_NONBLOCK 0x0001 #endif #ifndef GRND_INSECURE #define GRND_INSECURE 0x0004 #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, ×tamp[0]); clock_gettime(CLOCK_BOOTTIME, ×tamp[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; }