// 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;
}