1 /* SPDX-License-Identifier: LGPL-2.1-or-later */
6 #include <linux/random.h>
9 # include <sys/random.h>
12 #include <sys/xattr.h>
17 #include "alloc-util.h"
22 #include "main-func.h"
23 #include "missing_random.h"
24 #include "missing_syscall.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"
33 #include "sync-util.h"
35 #include "terminal-util.h"
37 #include "xattr-util.h"
39 typedef enum SeedAction
{
43 _ACTION_INVALID
= -EINVAL
,
46 typedef enum CreditEntropy
{
47 CREDIT_ENTROPY_NO_WAY
,
48 CREDIT_ENTROPY_YES_PLEASE
,
49 CREDIT_ENTROPY_YES_FORCED
,
52 static SeedAction arg_action
= _ACTION_INVALID
;
54 static CreditEntropy
may_credit(int seed_fd
) {
55 _cleanup_free_
char *creditable
= NULL
;
61 e
= getenv("SYSTEMD_RANDOM_SEED_CREDIT");
63 log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is not set, not crediting entropy.");
64 return CREDIT_ENTROPY_NO_WAY
;
66 if (streq(e
, "force")) {
67 log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is set to 'force', crediting entropy.");
68 return CREDIT_ENTROPY_YES_FORCED
;
74 log_warning_errno(r
, "Failed to parse $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy: %m");
76 log_debug("Crediting entropy is turned off via $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy.");
78 return CREDIT_ENTROPY_NO_WAY
;
81 /* Determine if the file is marked as creditable */
82 r
= fgetxattr_malloc(seed_fd
, "user.random-seed-creditable", &creditable
);
84 if (ERRNO_IS_XATTR_ABSENT(r
))
85 log_debug_errno(r
, "Seed file is not marked as creditable, not crediting.");
87 log_warning_errno(r
, "Failed to read extended attribute, ignoring: %m");
89 return CREDIT_ENTROPY_NO_WAY
;
92 r
= parse_boolean(creditable
);
95 log_warning_errno(r
, "Failed to parse user.random-seed-creditable extended attribute, ignoring: %s", creditable
);
97 log_debug("Seed file is marked as not creditable, not crediting.");
99 return CREDIT_ENTROPY_NO_WAY
;
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
));
107 /* All is good, we are not in first-boot mode. */
108 return CREDIT_ENTROPY_YES_PLEASE
;
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
;
114 log_debug("Not crediting entropy, since booted in first-boot mode.");
115 return CREDIT_ENTROPY_NO_WAY
;
118 static int random_seed_size(int seed_fd
, size_t *ret_size
) {
122 assert(seed_fd
>= 0);
124 if (fstat(seed_fd
, &st
) < 0)
125 return log_error_errno(errno
, "Failed to stat() seed file " RANDOM_SEED
": %m");
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 */
130 *ret_size
= CLAMP((uint64_t)st
.st_size
, random_pool_size(), RANDOM_POOL_SIZE_MAX
);
134 static int help(int argc
, char *argv
[], void *userdata
) {
135 _cleanup_free_
char *link
= NULL
;
138 r
= terminal_urlify_man("systemd-random-seed", "8", &link
);
142 printf("%1$s [OPTIONS...] COMMAND\n"
143 "\n%5$sLoad and save the system random seed at boot and shutdown.%6$s\n"
144 "\n%3$sCommands:%4$s\n"
145 " load Load a random seed saved on disk into the kernel entropy pool\n"
146 " save Save a new random seed on disk\n"
147 "\n%3$sOptions:%4$s\n"
148 " -h --help Show this help\n"
149 " --version Show package version\n"
150 "\nSee the %2$s for details.\n",
151 program_invocation_short_name
,
161 static const char* const seed_action_table
[_ACTION_MAX
] = {
162 [ACTION_LOAD
] = "load",
163 [ACTION_SAVE
] = "save",
166 DEFINE_PRIVATE_STRING_TABLE_LOOKUP_FROM_STRING(seed_action
, SeedAction
);
168 static int parse_argv(int argc
, char *argv
[]) {
173 static const struct option options
[] = {
174 { "help", no_argument
, NULL
, 'h' },
175 { "version", no_argument
, NULL
, ARG_VERSION
},
183 while ((c
= getopt_long(argc
, argv
, "h", options
, NULL
)) >= 0)
186 return help(0, NULL
, NULL
);
193 assert_not_reached();
196 if (optind
+ 1 != argc
)
197 return log_error_errno(SYNTHETIC_ERRNO(EINVAL
), "This program requires one argument.");
199 arg_action
= seed_action_from_string(argv
[optind
]);
201 return log_error_errno(arg_action
, "Unknown action '%s'", argv
[optind
]);
206 static int run(int argc
, char *argv
[]) {
207 bool read_seed_file
, write_seed_file
, synchronous
, hashed_old_seed
= false;
208 _cleanup_close_
int seed_fd
= -1, random_fd
= -1;
209 _cleanup_free_
void* buf
= NULL
;
210 struct sha256_ctx hash_state
;
217 r
= parse_argv(argc
, argv
);
223 r
= mkdir_parents(RANDOM_SEED
, 0755);
225 return log_error_errno(r
, "Failed to create directory " RANDOM_SEED_DIR
": %m");
227 /* When we load the seed we read it and write it to the device and then immediately update the saved
228 * seed with new data, to make sure the next boot gets seeded differently. */
230 switch (arg_action
) {
232 seed_fd
= open(RANDOM_SEED
, O_RDWR
|O_CLOEXEC
|O_NOCTTY
|O_CREAT
, 0600);
234 int open_rw_error
= -errno
;
236 write_seed_file
= false;
238 seed_fd
= open(RANDOM_SEED
, O_RDONLY
|O_CLOEXEC
|O_NOCTTY
);
240 bool missing
= errno
== ENOENT
;
242 log_full_errno(missing
? LOG_DEBUG
: LOG_ERR
,
243 open_rw_error
, "Failed to open " RANDOM_SEED
" for writing: %m");
244 r
= log_full_errno(missing
? LOG_DEBUG
: LOG_ERR
,
245 errno
, "Failed to open " RANDOM_SEED
" for reading: %m");
246 return missing
? 0 : r
;
249 write_seed_file
= true;
251 random_fd
= open("/dev/urandom", O_RDWR
|O_CLOEXEC
|O_NOCTTY
);
253 return log_error_errno(errno
, "Failed to open /dev/urandom: %m");
255 read_seed_file
= true;
256 synchronous
= true; /* make this invocation a synchronous barrier for random pool initialization */
260 random_fd
= open("/dev/urandom", O_RDONLY
|O_CLOEXEC
|O_NOCTTY
);
262 return log_error_errno(errno
, "Failed to open /dev/urandom: %m");
264 seed_fd
= open(RANDOM_SEED
, O_WRONLY
|O_CLOEXEC
|O_NOCTTY
|O_CREAT
, 0600);
266 return log_error_errno(errno
, "Failed to open " RANDOM_SEED
": %m");
268 read_seed_file
= false;
269 write_seed_file
= true;
274 assert_not_reached();
277 r
= random_seed_size(seed_fd
, &buf_size
);
281 buf
= malloc(buf_size
);
285 if (read_seed_file
) {
288 /* First, let's write the machine ID into /dev/urandom, not crediting entropy. Why? As an
289 * extra protection against "golden images" that are put together sloppily, i.e. images which
290 * are duplicated on multiple systems but where the random seed file is not properly
291 * reset. Frequently the machine ID is properly reset on those systems however (simply
292 * because it's easier to notice, if it isn't due to address clashes and so on, while random
293 * seed equivalence is generally not noticed easily), hence let's simply write the machined
294 * ID into the random pool too. */
295 r
= sd_id128_get_machine(&mid
);
297 log_debug_errno(r
, "Failed to get machine ID, ignoring: %m");
299 r
= random_write_entropy(random_fd
, &mid
, sizeof(mid
), /* credit= */ false);
301 log_debug_errno(r
, "Failed to write machine ID to /dev/urandom, ignoring: %m");
304 k
= loop_read(seed_fd
, buf
, buf_size
, false);
306 log_error_errno(k
, "Failed to read seed from " RANDOM_SEED
": %m");
308 log_debug("Seed file " RANDOM_SEED
" not yet initialized, proceeding.");
310 CreditEntropy lets_credit
;
312 /* If we're going to later write out a seed file, initialize a hash state with
313 * the contents of the seed file we just read, so that the new one can't regress
315 if (write_seed_file
) {
316 sha256_init_ctx(&hash_state
);
317 sha256_process_bytes(&k
, sizeof(k
), &hash_state
); /* Hash length to distinguish from new seed. */
318 sha256_process_bytes(buf
, k
, &hash_state
);
319 hashed_old_seed
= true;
322 (void) lseek(seed_fd
, 0, SEEK_SET
);
324 lets_credit
= may_credit(seed_fd
);
326 /* Before we credit or use the entropy, let's make sure to securely drop the
327 * creditable xattr from the file, so that we never credit the same random seed
328 * again. Note that further down we'll write a new seed again, and likely mark it as
329 * credible again, hence this is just paranoia to close the short time window between
330 * the time we upload the random seed into the kernel and download the new one from
333 if (fremovexattr(seed_fd
, "user.random-seed-creditable") < 0) {
334 if (!ERRNO_IS_XATTR_ABSENT(errno
))
335 log_warning_errno(errno
, "Failed to remove extended attribute, ignoring: %m");
337 /* Otherwise, there was no creditable flag set, which is OK. */
339 r
= fsync_full(seed_fd
);
341 log_warning_errno(r
, "Failed to synchronize seed to disk, not crediting entropy: %m");
343 if (lets_credit
== CREDIT_ENTROPY_YES_PLEASE
)
344 lets_credit
= CREDIT_ENTROPY_NO_WAY
;
348 r
= random_write_entropy(random_fd
, buf
, k
,
349 IN_SET(lets_credit
, CREDIT_ENTROPY_YES_PLEASE
, CREDIT_ENTROPY_YES_FORCED
));
351 log_error_errno(r
, "Failed to write seed to /dev/urandom: %m");
355 if (write_seed_file
) {
356 bool getrandom_worked
= false;
358 /* This is just a safety measure. Given that we are root and most likely created the file
359 * ourselves the mode and owner should be correct anyway. */
360 r
= fchmod_and_chown(seed_fd
, 0600, 0, 0);
362 return log_error_errno(r
, "Failed to adjust seed file ownership and access mode: %m");
364 /* Let's make this whole job asynchronous, i.e. let's make ourselves a barrier for
365 * proper initialization of the random pool. */
366 k
= getrandom(buf
, buf_size
, GRND_NONBLOCK
);
367 if (k
< 0 && errno
== EAGAIN
&& synchronous
) {
368 log_notice("Kernel entropy pool is not initialized yet, waiting until it is.");
369 k
= getrandom(buf
, buf_size
, 0); /* retry synchronously */
372 log_debug_errno(errno
, "Failed to read random data with getrandom(), falling back to /dev/urandom: %m");
373 else if ((size_t) k
< buf_size
)
374 log_debug("Short read from getrandom(), falling back to /dev/urandom.");
376 getrandom_worked
= true;
378 if (!getrandom_worked
) {
379 /* Retry with classic /dev/urandom */
380 k
= loop_read(random_fd
, buf
, buf_size
, false);
382 return log_error_errno(k
, "Failed to read new seed from /dev/urandom: %m");
384 return log_error_errno(SYNTHETIC_ERRNO(EIO
),
385 "Got EOF while reading from /dev/urandom.");
388 /* If we previously read in a seed file, then hash the new seed into the old one,
389 * and replace the last 32 bytes of the seed with the hash output, so that the
390 * new seed file can't regress in entropy. */
391 if (hashed_old_seed
) {
392 uint8_t hash
[SHA256_DIGEST_SIZE
];
393 sha256_process_bytes(&k
, sizeof(k
), &hash_state
); /* Hash length to distinguish from old seed. */
394 sha256_process_bytes(buf
, k
, &hash_state
);
395 sha256_finish_ctx(&hash_state
, hash
);
396 l
= MIN((size_t)k
, sizeof(hash
));
397 memcpy((uint8_t *)buf
+ k
- l
, hash
, l
);
400 r
= loop_write(seed_fd
, buf
, (size_t) k
, false);
402 return log_error_errno(r
, "Failed to write new random seed file: %m");
404 if (ftruncate(seed_fd
, k
) < 0)
405 return log_error_errno(r
, "Failed to truncate random seed file: %m");
407 r
= fsync_full(seed_fd
);
409 return log_error_errno(r
, "Failed to synchronize seed file: %m");
411 /* If we got this random seed data from getrandom() the data is suitable for crediting
412 * entropy later on. Let's keep that in mind by setting an extended attribute. on the file */
413 if (getrandom_worked
)
414 if (fsetxattr(seed_fd
, "user.random-seed-creditable", "1", 1, 0) < 0)
415 log_full_errno(ERRNO_IS_NOT_SUPPORTED(errno
) ? LOG_DEBUG
: LOG_WARNING
, errno
,
416 "Failed to mark seed file as creditable, ignoring: %m");
422 DEFINE_MAIN_FUNCTION(run
);