diff options
Diffstat (limited to 'src/random-seed')
| -rw-r--r-- | src/random-seed/meson.build | 9 | ||||
| -rw-r--r-- | src/random-seed/random-seed.c | 457 |
2 files changed, 466 insertions, 0 deletions
diff --git a/src/random-seed/meson.build b/src/random-seed/meson.build new file mode 100644 index 0000000..daa2eef --- /dev/null +++ b/src/random-seed/meson.build @@ -0,0 +1,9 @@ +# SPDX-License-Identifier: LGPL-2.1-or-later + +executables += [ + libexec_template + { + 'name' : 'systemd-random-seed', + 'conditions' : ['ENABLE_RANDOMSEED'], + 'sources' : files('random-seed.c'), + }, +] diff --git a/src/random-seed/random-seed.c b/src/random-seed/random-seed.c new file mode 100644 index 0000000..bad18ad --- /dev/null +++ b/src/random-seed/random-seed.c @@ -0,0 +1,457 @@ +/* SPDX-License-Identifier: LGPL-2.1-or-later */ + +#include <errno.h> +#include <fcntl.h> +#include <getopt.h> +#include <linux/random.h> +#include <sys/ioctl.h> +#if USE_SYS_RANDOM_H +# include <sys/random.h> +#endif +#include <sys/stat.h> +#include <sys/xattr.h> +#include <unistd.h> + +#include "sd-id128.h" + +#include "alloc-util.h" +#include "build.h" +#include "fd-util.h" +#include "fs-util.h" +#include "io-util.h" +#include "log.h" +#include "main-func.h" +#include "missing_random.h" +#include "missing_syscall.h" +#include "mkdir.h" +#include "parse-util.h" +#include "pretty-print.h" +#include "random-util.h" +#include "string-table.h" +#include "string-util.h" +#include "sync-util.h" +#include "sha256.h" +#include "xattr-util.h" + +typedef enum SeedAction { + ACTION_LOAD, + ACTION_SAVE, + _ACTION_MAX, + _ACTION_INVALID = -EINVAL, +} SeedAction; + +typedef enum CreditEntropy { + CREDIT_ENTROPY_NO_WAY, + CREDIT_ENTROPY_YES_PLEASE, + CREDIT_ENTROPY_YES_FORCED, +} CreditEntropy; + +static SeedAction arg_action = _ACTION_INVALID; + +static CreditEntropy may_credit(int seed_fd) { + const char *e; + int r; + + assert(seed_fd >= 0); + + e = getenv("SYSTEMD_RANDOM_SEED_CREDIT"); + if (!e) { + log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is not set, not crediting entropy."); + return CREDIT_ENTROPY_NO_WAY; + } + if (streq(e, "force")) { + log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is set to 'force', crediting entropy."); + return CREDIT_ENTROPY_YES_FORCED; + } + + r = parse_boolean(e); + if (r <= 0) { + if (r < 0) + log_warning_errno(r, "Failed to parse $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy: %m"); + else + log_debug("Crediting entropy is turned off via $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy."); + + return CREDIT_ENTROPY_NO_WAY; + } + + /* Determine if the file is marked as creditable */ + r = getxattr_at_bool(seed_fd, /* path= */ NULL, "user.random-seed-creditable", /* flags= */ 0); + if (r < 0) { + if (ERRNO_IS_XATTR_ABSENT(r)) + log_debug_errno(r, "Seed file is not marked as creditable, not crediting."); + else + log_warning_errno(r, "Failed to read extended attribute, ignoring: %m"); + + return CREDIT_ENTROPY_NO_WAY; + } + if (r == 0) { + log_debug("Seed file is marked as not creditable, not crediting."); + return CREDIT_ENTROPY_NO_WAY; + } + + /* Don't credit the random seed if we are in first-boot mode, because we are supposed to start from + * scratch. This is a safety precaution for cases where people ship "golden" images with empty + * /etc but populated /var that contains a random seed. */ + r = RET_NERRNO(access("/run/systemd/first-boot", F_OK)); + if (r == -ENOENT) + /* All is good, we are not in first-boot mode. */ + return CREDIT_ENTROPY_YES_PLEASE; + if (r < 0) { + log_warning_errno(r, "Failed to check whether we are in first-boot mode, not crediting entropy: %m"); + return CREDIT_ENTROPY_NO_WAY; + } + + log_debug("Not crediting entropy, since booted in first-boot mode."); + return CREDIT_ENTROPY_NO_WAY; +} + +static int random_seed_size(int seed_fd, size_t *ret_size) { + struct stat st; + + assert(ret_size); + assert(seed_fd >= 0); + + if (fstat(seed_fd, &st) < 0) + return log_error_errno(errno, "Failed to stat() seed file " RANDOM_SEED ": %m"); + + /* If the seed file is larger than what the kernel expects, then honour the existing size and + * save/restore as much as it says */ + + *ret_size = CLAMP((uint64_t)st.st_size, random_pool_size(), RANDOM_POOL_SIZE_MAX); + return 0; +} + +static void load_machine_id(int urandom_fd) { + sd_id128_t mid; + int r; + + assert(urandom_fd >= 0); + + /* As an extra protection against "golden images" that are put together sloppily, i.e. images which + * are duplicated on multiple systems but where the random seed file is not properly + * reset. Frequently the machine ID is properly reset on those systems however (simply because it's + * easier to notice, if it isn't due to address clashes and so on, while random seed equivalence is + * generally not noticed easily), hence let's simply write the machined ID into the random pool + * too. */ + r = sd_id128_get_machine(&mid); + if (r < 0) + return (void) log_debug_errno(r, "Failed to get machine ID, ignoring: %m"); + + r = random_write_entropy(urandom_fd, &mid, sizeof(mid), /* credit= */ false); + if (r < 0) + log_debug_errno(r, "Failed to write machine ID to /dev/urandom, ignoring: %m"); +} + +static int load_seed_file( + int seed_fd, + int urandom_fd, + size_t seed_size, + struct sha256_ctx **ret_hash_state) { + + _cleanup_free_ void *buf = NULL; + CreditEntropy lets_credit; + ssize_t k; + int r; + + assert(seed_fd >= 0); + assert(urandom_fd >= 0); + + buf = malloc(seed_size); + if (!buf) + return log_oom(); + + k = loop_read(seed_fd, buf, seed_size, false); + if (k < 0) { + log_warning_errno(k, "Failed to read seed from " RANDOM_SEED ": %m"); + return 0; + } + if (k == 0) { + log_debug("Seed file " RANDOM_SEED " not yet initialized, proceeding."); + return 0; + } + + /* If we're going to later write out a seed file, initialize a hash state with the contents of the + * seed file we just read, so that the new one can't regress in entropy. */ + if (ret_hash_state) { + struct sha256_ctx *hash_state; + + hash_state = new(struct sha256_ctx, 1); + if (!hash_state) + return log_oom(); + + sha256_init_ctx(hash_state); + sha256_process_bytes_and_size(buf, k, hash_state); /* Hash with length to distinguish from new seed. */ + + *ret_hash_state = hash_state; + } + + (void) lseek(seed_fd, 0, SEEK_SET); + + lets_credit = may_credit(seed_fd); + + /* Before we credit or use the entropy, let's make sure to securely drop the creditable xattr from + * the file, so that we never credit the same random seed again. Note that further down we'll write a + * new seed again, and likely mark it as credible again, hence this is just paranoia to close the + * short time window between the time we upload the random seed into the kernel and download the new + * one from it. */ + + if (fremovexattr(seed_fd, "user.random-seed-creditable") < 0) { + if (!ERRNO_IS_XATTR_ABSENT(errno)) + log_warning_errno(errno, "Failed to remove extended attribute, ignoring: %m"); + + /* Otherwise, there was no creditable flag set, which is OK. */ + } else { + r = fsync_full(seed_fd); + if (r < 0) { + log_warning_errno(r, "Failed to synchronize seed to disk, not crediting entropy: %m"); + + if (lets_credit == CREDIT_ENTROPY_YES_PLEASE) + lets_credit = CREDIT_ENTROPY_NO_WAY; + } + } + + r = random_write_entropy(urandom_fd, buf, k, + IN_SET(lets_credit, CREDIT_ENTROPY_YES_PLEASE, CREDIT_ENTROPY_YES_FORCED)); + if (r < 0) + log_warning_errno(r, "Failed to write seed to /dev/urandom: %m"); + + return 0; +} + +static int save_seed_file( + int seed_fd, + int urandom_fd, + size_t seed_size, + bool synchronous, + struct sha256_ctx *hash_state) { + + _cleanup_free_ void *buf = NULL; + bool getrandom_worked = false; + ssize_t k, l; + int r; + + assert(seed_fd >= 0); + assert(urandom_fd >= 0); + + /* This is just a safety measure. Given that we are root and most likely created the file ourselves + * the mode and owner should be correct anyway. */ + r = fchmod_and_chown(seed_fd, 0600, 0, 0); + if (r < 0) + return log_error_errno(r, "Failed to adjust seed file ownership and access mode: %m"); + + buf = malloc(seed_size); + if (!buf) + return log_oom(); + + k = getrandom(buf, seed_size, GRND_NONBLOCK); + if (k < 0 && errno == EAGAIN && synchronous) { + /* If we're asked to make ourselves a barrier for proper initialization of the random pool + * make this whole job synchronous by asking getrandom() to wait until the requested number + * of random bytes is available. */ + log_notice("Kernel entropy pool is not initialized yet, waiting until it is."); + k = getrandom(buf, seed_size, 0); + } + if (k < 0) + log_debug_errno(errno, "Failed to read random data with getrandom(), falling back to /dev/urandom: %m"); + else if ((size_t) k < seed_size) + log_debug("Short read from getrandom(), falling back to /dev/urandom."); + else + getrandom_worked = true; + + if (!getrandom_worked) { + /* Retry with classic /dev/urandom */ + k = loop_read(urandom_fd, buf, seed_size, false); + if (k < 0) + return log_error_errno(k, "Failed to read new seed from /dev/urandom: %m"); + if (k == 0) + return log_error_errno(SYNTHETIC_ERRNO(EIO), "Got EOF while reading from /dev/urandom."); + } + + /* If we previously read in a seed file, then hash the new seed into the old one, and replace the + * last 32 bytes of the seed with the hash output, so that the new seed file can't regress in + * entropy. */ + if (hash_state) { + uint8_t hash[SHA256_DIGEST_SIZE]; + + sha256_process_bytes_and_size(buf, k, hash_state); /* Hash with length to distinguish from old seed. */ + sha256_finish_ctx(hash_state, hash); + l = MIN((size_t)k, sizeof(hash)); + memcpy((uint8_t *)buf + k - l, hash, l); + } + + r = loop_write(seed_fd, buf, (size_t) k); + if (r < 0) + return log_error_errno(r, "Failed to write new random seed file: %m"); + + if (ftruncate(seed_fd, k) < 0) + return log_error_errno(r, "Failed to truncate random seed file: %m"); + + r = fsync_full(seed_fd); + if (r < 0) + return log_error_errno(r, "Failed to synchronize seed file: %m"); + + /* If we got this random seed data from getrandom() the data is suitable for crediting entropy later + * on. Let's keep that in mind by setting an extended attribute. on the file */ + if (getrandom_worked) + if (fsetxattr(seed_fd, "user.random-seed-creditable", "1", 1, 0) < 0) + log_full_errno(ERRNO_IS_NOT_SUPPORTED(errno) ? LOG_DEBUG : LOG_WARNING, errno, + "Failed to mark seed file as creditable, ignoring: %m"); + return 0; +} + +static int help(int argc, char *argv[], void *userdata) { + _cleanup_free_ char *link = NULL; + int r; + + r = terminal_urlify_man("systemd-random-seed", "8", &link); + if (r < 0) + return log_oom(); + + printf("%1$s [OPTIONS...] COMMAND\n" + "\n%5$sLoad and save the system random seed at boot and shutdown.%6$s\n" + "\n%3$sCommands:%4$s\n" + " load Load a random seed saved on disk into the kernel entropy pool\n" + " save Save a new random seed on disk\n" + "\n%3$sOptions:%4$s\n" + " -h --help Show this help\n" + " --version Show package version\n" + "\nSee the %2$s for details.\n", + program_invocation_short_name, + link, + ansi_underline(), + ansi_normal(), + ansi_highlight(), + ansi_normal()); + + return 0; +} + +static const char* const seed_action_table[_ACTION_MAX] = { + [ACTION_LOAD] = "load", + [ACTION_SAVE] = "save", +}; + +DEFINE_PRIVATE_STRING_TABLE_LOOKUP_FROM_STRING(seed_action, SeedAction); + +static int parse_argv(int argc, char *argv[]) { + enum { + ARG_VERSION = 0x100, + }; + + static const struct option options[] = { + { "help", no_argument, NULL, 'h' }, + { "version", no_argument, NULL, ARG_VERSION }, + {} + }; + + int c; + + assert(argc >= 0); + assert(argv); + + while ((c = getopt_long(argc, argv, "h", options, NULL)) >= 0) + switch (c) { + case 'h': + return help(0, NULL, NULL); + case ARG_VERSION: + return version(); + case '?': + return -EINVAL; + + default: + assert_not_reached(); + } + + if (optind + 1 != argc) + return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "This program requires one argument."); + + arg_action = seed_action_from_string(argv[optind]); + if (arg_action < 0) + return log_error_errno(arg_action, "Unknown action '%s'", argv[optind]); + + return 1; +} + +static int run(int argc, char *argv[]) { + _cleanup_free_ struct sha256_ctx *hash_state = NULL; + _cleanup_close_ int seed_fd = -EBADF, random_fd = -EBADF; + bool read_seed_file, write_seed_file, synchronous; + size_t seed_size; + int r; + + log_setup(); + + r = parse_argv(argc, argv); + if (r <= 0) + return r; + + umask(0022); + + r = mkdir_parents(RANDOM_SEED, 0755); + if (r < 0) + return log_error_errno(r, "Failed to create directory " RANDOM_SEED_DIR ": %m"); + + random_fd = open("/dev/urandom", O_RDWR|O_CLOEXEC|O_NOCTTY); + if (random_fd < 0) + return log_error_errno(errno, "Failed to open /dev/urandom: %m"); + + /* When we load the seed we read it and write it to the device and then immediately update the saved + * seed with new data, to make sure the next boot gets seeded differently. */ + + switch (arg_action) { + case ACTION_LOAD: + /* First, let's write the machine ID into /dev/urandom, not crediting entropy. See + * load_machine_id() for an explanation why. */ + load_machine_id(random_fd); + + seed_fd = open(RANDOM_SEED, O_RDWR|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600); + if (seed_fd < 0) { + int open_rw_error = -errno; + + write_seed_file = false; + + seed_fd = open(RANDOM_SEED, O_RDONLY|O_CLOEXEC|O_NOCTTY); + if (seed_fd < 0) { + bool missing = errno == ENOENT; + int level = missing ? LOG_DEBUG : LOG_ERR; + + log_full_errno(level, open_rw_error, "Failed to open " RANDOM_SEED " for writing: %m"); + log_full_errno(level, errno, "Failed to open " RANDOM_SEED " for reading: %m"); + return missing ? 0 : -errno; + } + } else + write_seed_file = true; + + read_seed_file = true; + synchronous = true; /* make this invocation a synchronous barrier for random pool initialization */ + break; + + case ACTION_SAVE: + seed_fd = open(RANDOM_SEED, O_WRONLY|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600); + if (seed_fd < 0) + return log_error_errno(errno, "Failed to open " RANDOM_SEED ": %m"); + + read_seed_file = false; + write_seed_file = true; + synchronous = false; + break; + + default: + assert_not_reached(); + } + + r = random_seed_size(seed_fd, &seed_size); + if (r < 0) + return r; + + if (read_seed_file) + r = load_seed_file(seed_fd, random_fd, seed_size, + write_seed_file ? &hash_state : NULL); + + if (r >= 0 && write_seed_file) + r = save_seed_file(seed_fd, random_fd, seed_size, synchronous, hash_state); + + return r; +} + +DEFINE_MAIN_FUNCTION(run); |