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