src/random-seed/random-seed.c

   1 /* SPDX-License-Identifier: LGPL-2.1-or-later */
   2
   3 #include <errno.h>
   4 #include <fcntl.h>
   5 #include <getopt.h>
   6 #include <linux/random.h>
   7 #include <sys/ioctl.h>
   8 #if USE_SYS_RANDOM_H
   9 #  include <sys/random.h>
  10 #endif
  11 #include <sys/stat.h>
  12 #include <sys/xattr.h>
  13 #include <unistd.h>
  14
  15 #include "sd-id128.h"
  16
  17 #include "alloc-util.h"
  18 #include "fd-util.h"
  19 #include "fs-util.h"
  20 #include "io-util.h"
  21 #include "log.h"
  22 #include "main-func.h"
  23 #include "missing_random.h"
  24 #include "missing_syscall.h"
  25 #include "mkdir.h"
  26 #include "parse-argument.h"
  27 #include "parse-util.h"
  28 #include "pretty-print.h"
  29 #include "random-util.h"
  30 #include "string-table.h"
  31 #include "string-util.h"
  32 #include "strv.h"
  33 #include "sync-util.h"
  34 #include "sha256.h"
  35 #include "terminal-util.h"
  36 #include "util.h"
  37 #include "xattr-util.h"
  38
  39 typedef enum SeedAction {
  40         ACTION_LOAD,
  41         ACTION_SAVE,
  42         _ACTION_MAX,
  43         _ACTION_INVALID = -EINVAL,
  44 } SeedAction;
  45
  46 typedef enum CreditEntropy {
  47         CREDIT_ENTROPY_NO_WAY,
  48         CREDIT_ENTROPY_YES_PLEASE,
  49         CREDIT_ENTROPY_YES_FORCED,
  50 } CreditEntropy;
  51
  52 static SeedAction arg_action = _ACTION_INVALID;
  53
  54 static CreditEntropy may_credit(int seed_fd) {
  55         _cleanup_free_ char *creditable = NULL;
  56         const char *e;
  57         int r;
  58
  59         assert(seed_fd >= 0);
  60
  61         e = getenv("SYSTEMD_RANDOM_SEED_CREDIT");
  62         if (!e) {
  63                 log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is not set, not crediting entropy.");
  64                 return CREDIT_ENTROPY_NO_WAY;
  65         }
  66         if (streq(e, "force")) {
  67                 log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is set to 'force', crediting entropy.");
  68                 return CREDIT_ENTROPY_YES_FORCED;
  69         }
  70
  71         r = parse_boolean(e);
  72         if (r <= 0) {
  73                 if (r < 0)
  74                         log_warning_errno(r, "Failed to parse $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy: %m");
  75                 else
  76                         log_debug("Crediting entropy is turned off via $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy.");
  77
  78                 return CREDIT_ENTROPY_NO_WAY;
  79         }
  80
  81         /* Determine if the file is marked as creditable */
  82         r = fgetxattr_malloc(seed_fd, "user.random-seed-creditable", &creditable);
  83         if (r < 0) {
  84                 if (ERRNO_IS_XATTR_ABSENT(r))
  85                         log_debug_errno(r, "Seed file is not marked as creditable, not crediting.");
  86                 else
  87                         log_warning_errno(r, "Failed to read extended attribute, ignoring: %m");
  88
  89                 return CREDIT_ENTROPY_NO_WAY;
  90         }
  91
  92         r = parse_boolean(creditable);
  93         if (r <= 0) {
  94                 if (r < 0)
  95                         log_warning_errno(r, "Failed to parse user.random-seed-creditable extended attribute, ignoring: %s", creditable);
  96                 else
  97                         log_debug("Seed file is marked as not creditable, not crediting.");
  98
  99                 return CREDIT_ENTROPY_NO_WAY;
 100         }
 101
 102         /* Don't credit the random seed if we are in first-boot mode, because we are supposed to start from
 103          * scratch. This is a safety precaution for cases where we people ship "golden" images with empty
 104          * /etc but populated /var that contains a random seed. */
 105         r = RET_NERRNO(access("/run/systemd/first-boot", F_OK));
 106         if (r == -ENOENT)
 107                 /* All is good, we are not in first-boot mode. */
 108                 return CREDIT_ENTROPY_YES_PLEASE;
 109         if (r < 0) {
 110                 log_warning_errno(r, "Failed to check whether we are in first-boot mode, not crediting entropy: %m");
 111                 return CREDIT_ENTROPY_NO_WAY;
 112         }
 113
 114         log_debug("Not crediting entropy, since booted in first-boot mode.");
 115         return CREDIT_ENTROPY_NO_WAY;
 116 }
 117
 118 static int random_seed_size(int seed_fd, size_t *ret_size) {
 119         struct stat st;
 120
 121         assert(ret_size);
 122         assert(seed_fd >= 0);
 123
 124         if (fstat(seed_fd, &st) < 0)
 125                 return log_error_errno(errno, "Failed to stat() seed file " RANDOM_SEED ": %m");
 126
 127         /* If the seed file is larger than what the kernel expects, then honour the existing size and
 128          * save/restore as much as it says */
 129
 130         *ret_size = CLAMP((uint64_t)st.st_size, random_pool_size(), RANDOM_POOL_SIZE_MAX);
 131         return 0;
 132 }
 133
 134 static int load_seed_file(
 135                 int seed_fd,
 136                 int urandom_fd,
 137                 size_t seed_size,
 138                 struct sha256_ctx **ret_hash_state) {
 139
 140         _cleanup_free_ void *buf = NULL;
 141         CreditEntropy lets_credit;
 142         sd_id128_t mid;
 143         ssize_t k;
 144         int r;
 145
 146         assert(seed_fd >= 0);
 147         assert(urandom_fd >= 0);
 148
 149         /* First, let's write the machine ID into /dev/urandom, not crediting entropy. Why? As an extra
 150          * protection against "golden images" that are put together sloppily, i.e. images which are
 151          * duplicated on multiple systems but where the random seed file is not properly reset. Frequently
 152          * the machine ID is properly reset on those systems however (simply because it's easier to notice,
 153          * if it isn't due to address clashes and so on, while random seed equivalence is generally not
 154          * noticed easily), hence let's simply write the machined ID into the random pool too. */
 155         r = sd_id128_get_machine(&mid);
 156         if (r < 0)
 157                 log_debug_errno(r, "Failed to get machine ID, ignoring: %m");
 158         else {
 159                 r = random_write_entropy(urandom_fd, &mid, sizeof(mid), /* credit= */ false);
 160                 if (r < 0)
 161                         log_debug_errno(r, "Failed to write machine ID to /dev/urandom, ignoring: %m");
 162         }
 163
 164         buf = malloc(seed_size);
 165         if (!buf)
 166                 return log_oom();
 167
 168         k = loop_read(seed_fd, buf, seed_size, false);
 169         if (k < 0) {
 170                 log_error_errno(k, "Failed to read seed from " RANDOM_SEED ": %m");
 171                 return 0;
 172         }
 173         if (k == 0) {
 174                 log_debug("Seed file " RANDOM_SEED " not yet initialized, proceeding.");
 175                 return 0;
 176         }
 177
 178         /* If we're going to later write out a seed file, initialize a hash state with the contents of the
 179          * seed file we just read, so that the new one can't regress in entropy. */
 180         if (ret_hash_state) {
 181                 struct sha256_ctx *hash_state;
 182
 183                 hash_state = malloc(sizeof(struct sha256_ctx));
 184                 if (!hash_state)
 185                         return log_oom();
 186
 187                 sha256_init_ctx(hash_state);
 188                 sha256_process_bytes(&k, sizeof(k), hash_state); /* Hash length to distinguish from new seed. */
 189                 sha256_process_bytes(buf, k, hash_state);
 190
 191                 *ret_hash_state = hash_state;
 192         }
 193
 194         (void) lseek(seed_fd, 0, SEEK_SET);
 195
 196         lets_credit = may_credit(seed_fd);
 197
 198         /* Before we credit or use the entropy, let's make sure to securely drop the creditable xattr from
 199          * the file, so that we never credit the same random seed again. Note that further down we'll write a
 200          * new seed again, and likely mark it as credible again, hence this is just paranoia to close the
 201          * short time window between the time we upload the random seed into the kernel and download the new
 202          * one from it. */
 203
 204         if (fremovexattr(seed_fd, "user.random-seed-creditable") < 0) {
 205                 if (!ERRNO_IS_XATTR_ABSENT(errno))
 206                         log_warning_errno(errno, "Failed to remove extended attribute, ignoring: %m");
 207
 208                 /* Otherwise, there was no creditable flag set, which is OK. */
 209         } else {
 210                 r = fsync_full(seed_fd);
 211                 if (r < 0) {
 212                         log_warning_errno(r, "Failed to synchronize seed to disk, not crediting entropy: %m");
 213
 214                         if (lets_credit == CREDIT_ENTROPY_YES_PLEASE)
 215                                 lets_credit = CREDIT_ENTROPY_NO_WAY;
 216                 }
 217         }
 218
 219         r = random_write_entropy(urandom_fd, buf, k,
 220                                  IN_SET(lets_credit, CREDIT_ENTROPY_YES_PLEASE, CREDIT_ENTROPY_YES_FORCED));
 221         if (r < 0)
 222                 log_error_errno(r, "Failed to write seed to /dev/urandom: %m");
 223
 224         return 0;
 225 }
 226
 227 static int save_seed_file(
 228                 int seed_fd,
 229                 int urandom_fd,
 230                 size_t seed_size,
 231                 bool synchronous,
 232                 struct sha256_ctx *hash_state) {
 233
 234         _cleanup_free_ void *buf = NULL;
 235         bool getrandom_worked = false;
 236         ssize_t k, l;
 237         int r;
 238
 239         assert(seed_fd >= 0);
 240         assert(urandom_fd >= 0);
 241
 242         /* This is just a safety measure. Given that we are root and most likely created the file ourselves
 243          * the mode and owner should be correct anyway. */
 244         r = fchmod_and_chown(seed_fd, 0600, 0, 0);
 245         if (r < 0)
 246                 return log_error_errno(r, "Failed to adjust seed file ownership and access mode: %m");
 247
 248         buf = malloc(seed_size);
 249         if (!buf)
 250                 return log_oom();
 251
 252         /* Let's make this whole job asynchronous, i.e. let's make ourselves a barrier for proper
 253          * initialization of the random pool. */
 254         k = getrandom(buf, seed_size, GRND_NONBLOCK);
 255         if (k < 0 && errno == EAGAIN && synchronous) {
 256                 log_notice("Kernel entropy pool is not initialized yet, waiting until it is.");
 257                 k = getrandom(buf, seed_size, 0); /* retry synchronously */
 258         }
 259         if (k < 0)
 260                 log_debug_errno(errno, "Failed to read random data with getrandom(), falling back to /dev/urandom: %m");
 261         else if ((size_t) k < seed_size)
 262                 log_debug("Short read from getrandom(), falling back to /dev/urandom.");
 263         else
 264                 getrandom_worked = true;
 265
 266         if (!getrandom_worked) {
 267                 /* Retry with classic /dev/urandom */
 268                 k = loop_read(urandom_fd, buf, seed_size, false);
 269                 if (k < 0)
 270                         return log_error_errno(k, "Failed to read new seed from /dev/urandom: %m");
 271                 if (k == 0)
 272                         return log_error_errno(SYNTHETIC_ERRNO(EIO), "Got EOF while reading from /dev/urandom.");
 273         }
 274
 275         /* If we previously read in a seed file, then hash the new seed into the old one, and replace the
 276          * last 32 bytes of the seed with the hash output, so that the new seed file can't regress in
 277          * entropy. */
 278         if (hash_state) {
 279                 uint8_t hash[SHA256_DIGEST_SIZE];
 280
 281                 sha256_process_bytes(&k, sizeof(k), hash_state); /* Hash length to distinguish from old seed. */
 282                 sha256_process_bytes(buf, k, hash_state);
 283                 sha256_finish_ctx(hash_state, hash);
 284                 l = MIN((size_t)k, sizeof(hash));
 285                 memcpy((uint8_t *)buf + k - l, hash, l);
 286         }
 287
 288         r = loop_write(seed_fd, buf, (size_t) k, false);
 289         if (r < 0)
 290                 return log_error_errno(r, "Failed to write new random seed file: %m");
 291
 292         if (ftruncate(seed_fd, k) < 0)
 293                 return log_error_errno(r, "Failed to truncate random seed file: %m");
 294
 295         r = fsync_full(seed_fd);
 296         if (r < 0)
 297                 return log_error_errno(r, "Failed to synchronize seed file: %m");
 298
 299         /* If we got this random seed data from getrandom() the data is suitable for crediting entropy later
 300          * on. Let's keep that in mind by setting an extended attribute. on the file */
 301         if (getrandom_worked)
 302                 if (fsetxattr(seed_fd, "user.random-seed-creditable", "1", 1, 0) < 0)
 303                         log_full_errno(ERRNO_IS_NOT_SUPPORTED(errno) ? LOG_DEBUG : LOG_WARNING, errno,
 304                                        "Failed to mark seed file as creditable, ignoring: %m");
 305         return 0;
 306 }
 307
 308 static int help(int argc, char *argv[], void *userdata) {
 309         _cleanup_free_ char *link = NULL;
 310         int r;
 311
 312         r = terminal_urlify_man("systemd-random-seed", "8", &link);
 313         if (r < 0)
 314                 return log_oom();
 315
 316         printf("%1$s [OPTIONS...] COMMAND\n"
 317                "\n%5$sLoad and save the system random seed at boot and shutdown.%6$s\n"
 318                "\n%3$sCommands:%4$s\n"
 319                "  load                Load a random seed saved on disk into the kernel entropy pool\n"
 320                "  save                Save a new random seed on disk\n"
 321                "\n%3$sOptions:%4$s\n"
 322                "  -h --help           Show this help\n"
 323                "     --version        Show package version\n"
 324                "\nSee the %2$s for details.\n",
 325                program_invocation_short_name,
 326                link,
 327                ansi_underline(),
 328                ansi_normal(),
 329                ansi_highlight(),
 330                ansi_normal());
 331
 332         return 0;
 333 }
 334
 335 static const char* const seed_action_table[_ACTION_MAX] = {
 336         [ACTION_LOAD] = "load",
 337         [ACTION_SAVE] = "save",
 338 };
 339
 340 DEFINE_PRIVATE_STRING_TABLE_LOOKUP_FROM_STRING(seed_action, SeedAction);
 341
 342 static int parse_argv(int argc, char *argv[]) {
 343         enum {
 344                 ARG_VERSION = 0x100,
 345         };
 346
 347         static const struct option options[] = {
 348                 { "help",    no_argument, NULL, 'h'         },
 349                 { "version", no_argument, NULL, ARG_VERSION },
 350         };
 351
 352         int c;
 353
 354         assert(argc >= 0);
 355         assert(argv);
 356
 357         while ((c = getopt_long(argc, argv, "h", options, NULL)) >= 0)
 358                 switch (c) {
 359                 case 'h':
 360                         return help(0, NULL, NULL);
 361                 case ARG_VERSION:
 362                         return version();
 363                 case '?':
 364                         return -EINVAL;
 365
 366                 default:
 367                         assert_not_reached();
 368                 }
 369
 370         if (optind + 1 != argc)
 371                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "This program requires one argument.");
 372
 373         arg_action = seed_action_from_string(argv[optind]);
 374         if (arg_action < 0)
 375                 return log_error_errno(arg_action, "Unknown action '%s'", argv[optind]);
 376
 377         return 1;
 378 }
 379
 380 static int run(int argc, char *argv[]) {
 381         _cleanup_free_ struct sha256_ctx *hash_state = NULL;
 382         _cleanup_close_ int seed_fd = -1, random_fd = -1;
 383         bool read_seed_file, write_seed_file, synchronous;
 384         size_t seed_size;
 385         int r;
 386
 387         log_setup();
 388
 389         r = parse_argv(argc, argv);
 390         if (r <= 0)
 391                 return r;
 392
 393         umask(0022);
 394
 395         r = mkdir_parents(RANDOM_SEED, 0755);
 396         if (r < 0)
 397                 return log_error_errno(r, "Failed to create directory " RANDOM_SEED_DIR ": %m");
 398
 399         /* When we load the seed we read it and write it to the device and then immediately update the saved
 400          * seed with new data, to make sure the next boot gets seeded differently. */
 401
 402         switch (arg_action) {
 403         case ACTION_LOAD:
 404                 seed_fd = open(RANDOM_SEED, O_RDWR|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
 405                 if (seed_fd < 0) {
 406                         int open_rw_error = -errno;
 407
 408                         write_seed_file = false;
 409
 410                         seed_fd = open(RANDOM_SEED, O_RDONLY|O_CLOEXEC|O_NOCTTY);
 411                         if (seed_fd < 0) {
 412                                 bool missing = errno == ENOENT;
 413
 414                                 log_full_errno(missing ? LOG_DEBUG : LOG_ERR,
 415                                                open_rw_error, "Failed to open " RANDOM_SEED " for writing: %m");
 416                                 r = log_full_errno(missing ? LOG_DEBUG : LOG_ERR,
 417                                                    errno, "Failed to open " RANDOM_SEED " for reading: %m");
 418                                 return missing ? 0 : r;
 419                         }
 420                 } else
 421                         write_seed_file = true;
 422
 423                 random_fd = open("/dev/urandom", O_RDWR|O_CLOEXEC|O_NOCTTY);
 424                 if (random_fd < 0)
 425                         return log_error_errno(errno, "Failed to open /dev/urandom: %m");
 426
 427                 read_seed_file = true;
 428                 synchronous = true; /* make this invocation a synchronous barrier for random pool initialization */
 429                 break;
 430
 431         case ACTION_SAVE:
 432                 random_fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
 433                 if (random_fd < 0)
 434                         return log_error_errno(errno, "Failed to open /dev/urandom: %m");
 435
 436                 seed_fd = open(RANDOM_SEED, O_WRONLY|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
 437                 if (seed_fd < 0)
 438                         return log_error_errno(errno, "Failed to open " RANDOM_SEED ": %m");
 439
 440                 read_seed_file = false;
 441                 write_seed_file = true;
 442                 synchronous = false;
 443                 break;
 444
 445         default:
 446                 assert_not_reached();
 447         }
 448
 449         r = random_seed_size(seed_fd, &seed_size);
 450         if (r < 0)
 451                 return r;
 452
 453         if (read_seed_file)
 454                 r = load_seed_file(seed_fd, random_fd, seed_size,
 455                                    write_seed_file ? &hash_state : NULL);
 456
 457         if (r >= 0 && write_seed_file)
 458                 r = save_seed_file(seed_fd, random_fd, seed_size, synchronous, hash_state);
 459
 460         return r;
 461 }
 462
 463 DEFINE_MAIN_FUNCTION(run);