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 "build.h"
  19 #include "chase-symlinks.h"
  20 #include "efi-loader.h"
  21 #include "fd-util.h"
  22 #include "find-esp.h"
  23 #include "fs-util.h"
  24 #include "io-util.h"
  25 #include "log.h"
  26 #include "main-func.h"
  27 #include "missing_random.h"
  28 #include "missing_syscall.h"
  29 #include "mkdir.h"
  30 #include "parse-argument.h"
  31 #include "parse-util.h"
  32 #include "path-util.h"
  33 #include "pretty-print.h"
  34 #include "random-util.h"
  35 #include "string-table.h"
  36 #include "string-util.h"
  37 #include "strv.h"
  38 #include "sync-util.h"
  39 #include "sha256.h"
  40 #include "terminal-util.h"
  41 #include "xattr-util.h"
  42
  43 typedef enum SeedAction {
  44         ACTION_LOAD,
  45         ACTION_SAVE,
  46         _ACTION_MAX,
  47         _ACTION_INVALID = -EINVAL,
  48 } SeedAction;
  49
  50 typedef enum CreditEntropy {
  51         CREDIT_ENTROPY_NO_WAY,
  52         CREDIT_ENTROPY_YES_PLEASE,
  53         CREDIT_ENTROPY_YES_FORCED,
  54 } CreditEntropy;
  55
  56 static SeedAction arg_action = _ACTION_INVALID;
  57
  58 static CreditEntropy may_credit(int seed_fd) {
  59         _cleanup_free_ char *creditable = NULL;
  60         const char *e;
  61         int r;
  62
  63         assert(seed_fd >= 0);
  64
  65         e = getenv("SYSTEMD_RANDOM_SEED_CREDIT");
  66         if (!e) {
  67                 log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is not set, not crediting entropy.");
  68                 return CREDIT_ENTROPY_NO_WAY;
  69         }
  70         if (streq(e, "force")) {
  71                 log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is set to 'force', crediting entropy.");
  72                 return CREDIT_ENTROPY_YES_FORCED;
  73         }
  74
  75         r = parse_boolean(e);
  76         if (r <= 0) {
  77                 if (r < 0)
  78                         log_warning_errno(r, "Failed to parse $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy: %m");
  79                 else
  80                         log_debug("Crediting entropy is turned off via $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy.");
  81
  82                 return CREDIT_ENTROPY_NO_WAY;
  83         }
  84
  85         /* Determine if the file is marked as creditable */
  86         r = fgetxattr_malloc(seed_fd, "user.random-seed-creditable", &creditable);
  87         if (r < 0) {
  88                 if (ERRNO_IS_XATTR_ABSENT(r))
  89                         log_debug_errno(r, "Seed file is not marked as creditable, not crediting.");
  90                 else
  91                         log_warning_errno(r, "Failed to read extended attribute, ignoring: %m");
  92
  93                 return CREDIT_ENTROPY_NO_WAY;
  94         }
  95
  96         r = parse_boolean(creditable);
  97         if (r <= 0) {
  98                 if (r < 0)
  99                         log_warning_errno(r, "Failed to parse user.random-seed-creditable extended attribute, ignoring: %s", creditable);
 100                 else
 101                         log_debug("Seed file is marked as not creditable, not crediting.");
 102
 103                 return CREDIT_ENTROPY_NO_WAY;
 104         }
 105
 106         /* Don't credit the random seed if we are in first-boot mode, because we are supposed to start from
 107          * scratch. This is a safety precaution for cases where we people ship "golden" images with empty
 108          * /etc but populated /var that contains a random seed. */
 109         r = RET_NERRNO(access("/run/systemd/first-boot", F_OK));
 110         if (r == -ENOENT)
 111                 /* All is good, we are not in first-boot mode. */
 112                 return CREDIT_ENTROPY_YES_PLEASE;
 113         if (r < 0) {
 114                 log_warning_errno(r, "Failed to check whether we are in first-boot mode, not crediting entropy: %m");
 115                 return CREDIT_ENTROPY_NO_WAY;
 116         }
 117
 118         log_debug("Not crediting entropy, since booted in first-boot mode.");
 119         return CREDIT_ENTROPY_NO_WAY;
 120 }
 121
 122 static int random_seed_size(int seed_fd, size_t *ret_size) {
 123         struct stat st;
 124
 125         assert(ret_size);
 126         assert(seed_fd >= 0);
 127
 128         if (fstat(seed_fd, &st) < 0)
 129                 return log_error_errno(errno, "Failed to stat() seed file " RANDOM_SEED ": %m");
 130
 131         /* If the seed file is larger than what the kernel expects, then honour the existing size and
 132          * save/restore as much as it says */
 133
 134         *ret_size = CLAMP((uint64_t)st.st_size, random_pool_size(), RANDOM_POOL_SIZE_MAX);
 135         return 0;
 136 }
 137
 138 static void load_machine_id(int urandom_fd) {
 139         sd_id128_t mid;
 140         int r;
 141
 142         assert(urandom_fd >= 0);
 143
 144         /* As an extra protection against "golden images" that are put together sloppily, i.e. images which
 145          * are duplicated on multiple systems but where the random seed file is not properly
 146          * reset. Frequently the machine ID is properly reset on those systems however (simply because it's
 147          * easier to notice, if it isn't due to address clashes and so on, while random seed equivalence is
 148          * generally not noticed easily), hence let's simply write the machined ID into the random pool
 149          * too. */
 150         r = sd_id128_get_machine(&mid);
 151         if (r < 0)
 152                 return (void) log_debug_errno(r, "Failed to get machine ID, ignoring: %m");
 153
 154         r = random_write_entropy(urandom_fd, &mid, sizeof(mid), /* credit= */ false);
 155         if (r < 0)
 156                 log_debug_errno(r, "Failed to write machine ID to /dev/urandom, ignoring: %m");
 157 }
 158
 159 static int load_seed_file(
 160                 int seed_fd,
 161                 int urandom_fd,
 162                 size_t seed_size,
 163                 struct sha256_ctx **ret_hash_state) {
 164
 165         _cleanup_free_ void *buf = NULL;
 166         CreditEntropy lets_credit;
 167         ssize_t k;
 168         int r;
 169
 170         assert(seed_fd >= 0);
 171         assert(urandom_fd >= 0);
 172
 173         buf = malloc(seed_size);
 174         if (!buf)
 175                 return log_oom();
 176
 177         k = loop_read(seed_fd, buf, seed_size, false);
 178         if (k < 0) {
 179                 log_warning_errno(k, "Failed to read seed from " RANDOM_SEED ": %m");
 180                 return 0;
 181         }
 182         if (k == 0) {
 183                 log_debug("Seed file " RANDOM_SEED " not yet initialized, proceeding.");
 184                 return 0;
 185         }
 186
 187         /* If we're going to later write out a seed file, initialize a hash state with the contents of the
 188          * seed file we just read, so that the new one can't regress in entropy. */
 189         if (ret_hash_state) {
 190                 struct sha256_ctx *hash_state;
 191
 192                 hash_state = new(struct sha256_ctx, 1);
 193                 if (!hash_state)
 194                         return log_oom();
 195
 196                 sha256_init_ctx(hash_state);
 197                 sha256_process_bytes_and_size(buf, k, hash_state); /* Hash with length to distinguish from new seed. */
 198
 199                 *ret_hash_state = hash_state;
 200         }
 201
 202         (void) lseek(seed_fd, 0, SEEK_SET);
 203
 204         lets_credit = may_credit(seed_fd);
 205
 206         /* Before we credit or use the entropy, let's make sure to securely drop the creditable xattr from
 207          * the file, so that we never credit the same random seed again. Note that further down we'll write a
 208          * new seed again, and likely mark it as credible again, hence this is just paranoia to close the
 209          * short time window between the time we upload the random seed into the kernel and download the new
 210          * one from it. */
 211
 212         if (fremovexattr(seed_fd, "user.random-seed-creditable") < 0) {
 213                 if (!ERRNO_IS_XATTR_ABSENT(errno))
 214                         log_warning_errno(errno, "Failed to remove extended attribute, ignoring: %m");
 215
 216                 /* Otherwise, there was no creditable flag set, which is OK. */
 217         } else {
 218                 r = fsync_full(seed_fd);
 219                 if (r < 0) {
 220                         log_warning_errno(r, "Failed to synchronize seed to disk, not crediting entropy: %m");
 221
 222                         if (lets_credit == CREDIT_ENTROPY_YES_PLEASE)
 223                                 lets_credit = CREDIT_ENTROPY_NO_WAY;
 224                 }
 225         }
 226
 227         r = random_write_entropy(urandom_fd, buf, k,
 228                                  IN_SET(lets_credit, CREDIT_ENTROPY_YES_PLEASE, CREDIT_ENTROPY_YES_FORCED));
 229         if (r < 0)
 230                 log_warning_errno(r, "Failed to write seed to /dev/urandom: %m");
 231
 232         return 0;
 233 }
 234
 235 static int save_seed_file(
 236                 int seed_fd,
 237                 int urandom_fd,
 238                 size_t seed_size,
 239                 bool synchronous,
 240                 struct sha256_ctx *hash_state) {
 241
 242         _cleanup_free_ void *buf = NULL;
 243         bool getrandom_worked = false;
 244         ssize_t k, l;
 245         int r;
 246
 247         assert(seed_fd >= 0);
 248         assert(urandom_fd >= 0);
 249
 250         /* This is just a safety measure. Given that we are root and most likely created the file ourselves
 251          * the mode and owner should be correct anyway. */
 252         r = fchmod_and_chown(seed_fd, 0600, 0, 0);
 253         if (r < 0)
 254                 return log_error_errno(r, "Failed to adjust seed file ownership and access mode: %m");
 255
 256         buf = malloc(seed_size);
 257         if (!buf)
 258                 return log_oom();
 259
 260         k = getrandom(buf, seed_size, GRND_NONBLOCK);
 261         if (k < 0 && errno == EAGAIN && synchronous) {
 262                 /* If we're asked to make ourselves a barrier for proper initialization of the random pool
 263                  * make this whole job synchronous by asking getrandom() to wait until the requested number
 264                  * of random bytes is available. */
 265                 log_notice("Kernel entropy pool is not initialized yet, waiting until it is.");
 266                 k = getrandom(buf, seed_size, 0);
 267         }
 268         if (k < 0)
 269                 log_debug_errno(errno, "Failed to read random data with getrandom(), falling back to /dev/urandom: %m");
 270         else if ((size_t) k < seed_size)
 271                 log_debug("Short read from getrandom(), falling back to /dev/urandom.");
 272         else
 273                 getrandom_worked = true;
 274
 275         if (!getrandom_worked) {
 276                 /* Retry with classic /dev/urandom */
 277                 k = loop_read(urandom_fd, buf, seed_size, false);
 278                 if (k < 0)
 279                         return log_error_errno(k, "Failed to read new seed from /dev/urandom: %m");
 280                 if (k == 0)
 281                         return log_error_errno(SYNTHETIC_ERRNO(EIO), "Got EOF while reading from /dev/urandom.");
 282         }
 283
 284         /* If we previously read in a seed file, then hash the new seed into the old one, and replace the
 285          * last 32 bytes of the seed with the hash output, so that the new seed file can't regress in
 286          * entropy. */
 287         if (hash_state) {
 288                 uint8_t hash[SHA256_DIGEST_SIZE];
 289
 290                 sha256_process_bytes_and_size(buf, k, hash_state); /* Hash with length to distinguish from old seed. */
 291                 sha256_finish_ctx(hash_state, hash);
 292                 l = MIN((size_t)k, sizeof(hash));
 293                 memcpy((uint8_t *)buf + k - l, hash, l);
 294         }
 295
 296         r = loop_write(seed_fd, buf, (size_t) k, false);
 297         if (r < 0)
 298                 return log_error_errno(r, "Failed to write new random seed file: %m");
 299
 300         if (ftruncate(seed_fd, k) < 0)
 301                 return log_error_errno(r, "Failed to truncate random seed file: %m");
 302
 303         r = fsync_full(seed_fd);
 304         if (r < 0)
 305                 return log_error_errno(r, "Failed to synchronize seed file: %m");
 306
 307         /* If we got this random seed data from getrandom() the data is suitable for crediting entropy later
 308          * on. Let's keep that in mind by setting an extended attribute. on the file */
 309         if (getrandom_worked)
 310                 if (fsetxattr(seed_fd, "user.random-seed-creditable", "1", 1, 0) < 0)
 311                         log_full_errno(ERRNO_IS_NOT_SUPPORTED(errno) ? LOG_DEBUG : LOG_WARNING, errno,
 312                                        "Failed to mark seed file as creditable, ignoring: %m");
 313         return 0;
 314 }
 315
 316 static int refresh_boot_seed(void) {
 317         uint8_t buffer[RANDOM_EFI_SEED_SIZE];
 318         struct sha256_ctx hash_state;
 319         _cleanup_free_ void *seed_file_bytes = NULL;
 320         _cleanup_free_ char *esp_path = NULL;
 321         _cleanup_close_ int seed_fd = -EBADF, dir_fd = -EBADF;
 322         size_t len;
 323         ssize_t n;
 324         int r;
 325
 326         assert_cc(RANDOM_EFI_SEED_SIZE == SHA256_DIGEST_SIZE);
 327
 328         r = find_esp_and_warn(NULL, NULL, /* unprivileged_mode= */ false, &esp_path,
 329                               NULL, NULL, NULL, NULL, NULL);
 330         if (r < 0) {
 331                 if (r == -ENOKEY) {
 332                         log_debug_errno(r, "Couldn't find any ESP, so not updating ESP random seed.");
 333                         return 0;
 334                 }
 335                 return r; /* find_esp_and_warn() already logged */
 336         }
 337
 338         r = chase_symlinks("/loader", esp_path, CHASE_PREFIX_ROOT|CHASE_PROHIBIT_SYMLINKS, NULL, &dir_fd);
 339         if (r < 0) {
 340                 if (r == -ENOENT) {
 341                         log_debug_errno(r, "Couldn't find ESP loader directory, so not updating ESP random seed.");
 342                         return 0;
 343                 }
 344                 return log_error_errno(r, "Failed to open ESP loader directory: %m");
 345         }
 346         seed_fd = openat(dir_fd, "random-seed", O_NOFOLLOW|O_RDWR|O_CLOEXEC|O_NOCTTY);
 347         if (seed_fd < 0 && errno == ENOENT) {
 348                 uint64_t features;
 349                 r = efi_loader_get_features(&features);
 350                 if (r == 0 && FLAGS_SET(features, EFI_LOADER_FEATURE_RANDOM_SEED))
 351                         seed_fd = openat(dir_fd, "random-seed", O_CREAT|O_EXCL|O_RDWR|O_CLOEXEC|O_NOCTTY, 0600);
 352                 else {
 353                         log_debug_errno(seed_fd, "Couldn't find ESP random seed, and not booted with systemd-boot, so not updating ESP random seed.");
 354                         return 0;
 355                 }
 356         }
 357         if (seed_fd < 0)
 358                 return log_error_errno(errno, "Failed to open EFI seed path: %m");
 359         r = random_seed_size(seed_fd, &len);
 360         if (r < 0)
 361                 return log_error_errno(r, "Failed to determine EFI seed path length: %m");
 362         seed_file_bytes = malloc(len);
 363         if (!seed_file_bytes)
 364                 return log_oom();
 365         n = loop_read(seed_fd, seed_file_bytes, len, false);
 366         if (n < 0)
 367                 return log_error_errno(n, "Failed to read EFI seed file: %m");
 368
 369         /* Hash the old seed in so that we never regress in entropy. */
 370         sha256_init_ctx(&hash_state);
 371         sha256_process_bytes_and_size(seed_file_bytes, n, &hash_state);
 372
 373         /* We're doing this opportunistically, so if the seeding dance before didn't manage to initialize the
 374          * RNG, there's no point in doing it here. Secondly, getrandom(GRND_NONBLOCK) has been around longer
 375          * than EFI seeding anyway, so there's no point in having non-getrandom() fallbacks here. So if this
 376          * fails, just return early to cut our losses. */
 377         n = getrandom(buffer, sizeof(buffer), GRND_NONBLOCK);
 378         if (n < 0) {
 379                 if (errno == EAGAIN) {
 380                         log_debug_errno(errno, "Random pool not initialized yet, so skipping EFI seed update");
 381                         return 0;
 382                 }
 383                 if (errno == ENOSYS) {
 384                         log_debug_errno(errno, "getrandom() not available, so skipping EFI seed update");
 385                         return 0;
 386                 }
 387                 return log_error_errno(errno, "Failed to generate random bytes for EFI seed: %m");
 388         }
 389         assert(n == sizeof(buffer));
 390
 391         /* Hash the new seed into the state containing the old one to generate our final seed. */
 392         sha256_process_bytes_and_size(buffer, n, &hash_state);
 393         sha256_finish_ctx(&hash_state, buffer);
 394
 395         if (lseek(seed_fd, 0, SEEK_SET) < 0)
 396                 return log_error_errno(errno, "Failed to seek to beginning of EFI seed file: %m");
 397         r = loop_write(seed_fd, buffer, sizeof(buffer), false);
 398         if (r < 0)
 399                 return log_error_errno(r, "Failed to write new EFI seed file: %m");
 400         if (ftruncate(seed_fd, sizeof(buffer)) < 0)
 401                 return log_error_errno(errno, "Failed to truncate EFI seed file: %m");
 402         r = fsync_full(seed_fd);
 403         if (r < 0)
 404                 return log_error_errno(r, "Failed to fsync EFI seed file: %m");
 405
 406         log_debug("Updated random seed in ESP");
 407         return 0;
 408 }
 409
 410 static int help(int argc, char *argv[], void *userdata) {
 411         _cleanup_free_ char *link = NULL;
 412         int r;
 413
 414         r = terminal_urlify_man("systemd-random-seed", "8", &link);
 415         if (r < 0)
 416                 return log_oom();
 417
 418         printf("%1$s [OPTIONS...] COMMAND\n"
 419                "\n%5$sLoad and save the system random seed at boot and shutdown.%6$s\n"
 420                "\n%3$sCommands:%4$s\n"
 421                "  load                Load a random seed saved on disk into the kernel entropy pool\n"
 422                "  save                Save a new random seed on disk\n"
 423                "\n%3$sOptions:%4$s\n"
 424                "  -h --help           Show this help\n"
 425                "     --version        Show package version\n"
 426                "\nSee the %2$s for details.\n",
 427                program_invocation_short_name,
 428                link,
 429                ansi_underline(),
 430                ansi_normal(),
 431                ansi_highlight(),
 432                ansi_normal());
 433
 434         return 0;
 435 }
 436
 437 static const char* const seed_action_table[_ACTION_MAX] = {
 438         [ACTION_LOAD] = "load",
 439         [ACTION_SAVE] = "save",
 440 };
 441
 442 DEFINE_PRIVATE_STRING_TABLE_LOOKUP_FROM_STRING(seed_action, SeedAction);
 443
 444 static int parse_argv(int argc, char *argv[]) {
 445         enum {
 446                 ARG_VERSION = 0x100,
 447         };
 448
 449         static const struct option options[] = {
 450                 { "help",    no_argument, NULL, 'h'         },
 451                 { "version", no_argument, NULL, ARG_VERSION },
 452         };
 453
 454         int c;
 455
 456         assert(argc >= 0);
 457         assert(argv);
 458
 459         while ((c = getopt_long(argc, argv, "h", options, NULL)) >= 0)
 460                 switch (c) {
 461                 case 'h':
 462                         return help(0, NULL, NULL);
 463                 case ARG_VERSION:
 464                         return version();
 465                 case '?':
 466                         return -EINVAL;
 467
 468                 default:
 469                         assert_not_reached();
 470                 }
 471
 472         if (optind + 1 != argc)
 473                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "This program requires one argument.");
 474
 475         arg_action = seed_action_from_string(argv[optind]);
 476         if (arg_action < 0)
 477                 return log_error_errno(arg_action, "Unknown action '%s'", argv[optind]);
 478
 479         return 1;
 480 }
 481
 482 static int run(int argc, char *argv[]) {
 483         _cleanup_free_ struct sha256_ctx *hash_state = NULL;
 484         _cleanup_close_ int seed_fd = -EBADF, random_fd = -EBADF;
 485         bool read_seed_file, write_seed_file, synchronous;
 486         size_t seed_size;
 487         int r;
 488
 489         log_setup();
 490
 491         r = parse_argv(argc, argv);
 492         if (r <= 0)
 493                 return r;
 494
 495         umask(0022);
 496
 497         r = mkdir_parents(RANDOM_SEED, 0755);
 498         if (r < 0)
 499                 return log_error_errno(r, "Failed to create directory " RANDOM_SEED_DIR ": %m");
 500
 501         random_fd = open("/dev/urandom", O_RDWR|O_CLOEXEC|O_NOCTTY);
 502         if (random_fd < 0)
 503                 return log_error_errno(errno, "Failed to open /dev/urandom: %m");
 504
 505         /* When we load the seed we read it and write it to the device and then immediately update the saved
 506          * seed with new data, to make sure the next boot gets seeded differently. */
 507
 508         switch (arg_action) {
 509         case ACTION_LOAD:
 510                 /* First, let's write the machine ID into /dev/urandom, not crediting entropy. See
 511                  * load_machine_id() for an explanation why. */
 512                 load_machine_id(random_fd);
 513
 514                 seed_fd = open(RANDOM_SEED, O_RDWR|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
 515                 if (seed_fd < 0) {
 516                         int open_rw_error = -errno;
 517
 518                         write_seed_file = false;
 519
 520                         seed_fd = open(RANDOM_SEED, O_RDONLY|O_CLOEXEC|O_NOCTTY);
 521                         if (seed_fd < 0) {
 522                                 bool missing = errno == ENOENT;
 523                                 int level = missing ? LOG_DEBUG : LOG_ERR;
 524
 525                                 log_full_errno(level, open_rw_error, "Failed to open " RANDOM_SEED " for writing: %m");
 526                                 log_full_errno(level, errno, "Failed to open " RANDOM_SEED " for reading: %m");
 527                                 r = -errno;
 528
 529                                 (void) refresh_boot_seed();
 530                                 return missing ? 0 : r;
 531                         }
 532                 } else
 533                         write_seed_file = true;
 534
 535                 read_seed_file = true;
 536                 synchronous = true; /* make this invocation a synchronous barrier for random pool initialization */
 537                 break;
 538
 539         case ACTION_SAVE:
 540                 (void) refresh_boot_seed();
 541                 seed_fd = open(RANDOM_SEED, O_WRONLY|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
 542                 if (seed_fd < 0)
 543                         return log_error_errno(errno, "Failed to open " RANDOM_SEED ": %m");
 544
 545                 read_seed_file = false;
 546                 write_seed_file = true;
 547                 synchronous = false;
 548                 break;
 549
 550         default:
 551                 assert_not_reached();
 552         }
 553
 554         r = random_seed_size(seed_fd, &seed_size);
 555         if (r < 0)
 556                 return r;
 557
 558         if (read_seed_file) {
 559                 r = load_seed_file(seed_fd, random_fd, seed_size,
 560                                    write_seed_file ? &hash_state : NULL);
 561                 (void) refresh_boot_seed();
 562         }
 563
 564         if (r >= 0 && write_seed_file)
 565                 r = save_seed_file(seed_fd, random_fd, seed_size, synchronous, hash_state);
 566
 567         return r;
 568 }
 569
 570 DEFINE_MAIN_FUNCTION(run);