1 /* SPDX-License-Identifier: LGPL-2.1-or-later */
5 #include <linux/random.h>
8 # include <sys/random.h>
11 #include <sys/xattr.h>
16 #include "alloc-util.h"
21 #include "main-func.h"
22 #include "missing_random.h"
23 #include "missing_syscall.h"
25 #include "parse-util.h"
26 #include "random-util.h"
27 #include "string-util.h"
28 #include "sync-util.h"
31 #include "xattr-util.h"
33 typedef enum CreditEntropy
{
34 CREDIT_ENTROPY_NO_WAY
,
35 CREDIT_ENTROPY_YES_PLEASE
,
36 CREDIT_ENTROPY_YES_FORCED
,
39 static CreditEntropy
may_credit(int seed_fd
) {
40 _cleanup_free_
char *creditable
= NULL
;
46 e
= getenv("SYSTEMD_RANDOM_SEED_CREDIT");
48 log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is not set, not crediting entropy.");
49 return CREDIT_ENTROPY_NO_WAY
;
51 if (streq(e
, "force")) {
52 log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is set to 'force', crediting entropy.");
53 return CREDIT_ENTROPY_YES_FORCED
;
59 log_warning_errno(r
, "Failed to parse $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy: %m");
61 log_debug("Crediting entropy is turned off via $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy.");
63 return CREDIT_ENTROPY_NO_WAY
;
66 /* Determine if the file is marked as creditable */
67 r
= fgetxattr_malloc(seed_fd
, "user.random-seed-creditable", &creditable
);
69 if (IN_SET(r
, -ENODATA
, -ENOSYS
, -EOPNOTSUPP
))
70 log_debug_errno(r
, "Seed file is not marked as creditable, not crediting.");
72 log_warning_errno(r
, "Failed to read extended attribute, ignoring: %m");
74 return CREDIT_ENTROPY_NO_WAY
;
77 r
= parse_boolean(creditable
);
80 log_warning_errno(r
, "Failed to parse user.random-seed-creditable extended attribute, ignoring: %s", creditable
);
82 log_debug("Seed file is marked as not creditable, not crediting.");
84 return CREDIT_ENTROPY_NO_WAY
;
87 /* Don't credit the random seed if we are in first-boot mode, because we are supposed to start from
88 * scratch. This is a safety precaution for cases where we people ship "golden" images with empty
89 * /etc but populated /var that contains a random seed. */
90 if (access("/run/systemd/first-boot", F_OK
) < 0) {
92 if (errno
!= ENOENT
) {
93 log_warning_errno(errno
, "Failed to check whether we are in first-boot mode, not crediting entropy: %m");
94 return CREDIT_ENTROPY_NO_WAY
;
97 /* If ENOENT all is good, we are not in first-boot mode. */
99 log_debug("Not crediting entropy, since booted in first-boot mode.");
100 return CREDIT_ENTROPY_NO_WAY
;
103 return CREDIT_ENTROPY_YES_PLEASE
;
106 static int run(int argc
, char *argv
[]) {
107 bool read_seed_file
, write_seed_file
, synchronous
, hashed_old_seed
= false;
108 _cleanup_close_
int seed_fd
= -1, random_fd
= -1;
109 _cleanup_free_
void* buf
= NULL
;
110 struct sha256_ctx hash_state
;
119 return log_error_errno(SYNTHETIC_ERRNO(EINVAL
),
120 "This program requires one argument.");
124 buf_size
= random_pool_size();
126 r
= mkdir_parents(RANDOM_SEED
, 0755);
128 return log_error_errno(r
, "Failed to create directory " RANDOM_SEED_DIR
": %m");
130 /* When we load the seed we read it and write it to the device and then immediately update the saved seed with
131 * new data, to make sure the next boot gets seeded differently. */
133 if (streq(argv
[1], "load")) {
135 seed_fd
= open(RANDOM_SEED
, O_RDWR
|O_CLOEXEC
|O_NOCTTY
|O_CREAT
, 0600);
137 int open_rw_error
= -errno
;
139 write_seed_file
= false;
141 seed_fd
= open(RANDOM_SEED
, O_RDONLY
|O_CLOEXEC
|O_NOCTTY
);
143 bool missing
= errno
== ENOENT
;
145 log_full_errno(missing
? LOG_DEBUG
: LOG_ERR
,
146 open_rw_error
, "Failed to open " RANDOM_SEED
" for writing: %m");
147 r
= log_full_errno(missing
? LOG_DEBUG
: LOG_ERR
,
148 errno
, "Failed to open " RANDOM_SEED
" for reading: %m");
149 return missing
? 0 : r
;
152 write_seed_file
= true;
154 random_fd
= open("/dev/urandom", O_RDWR
|O_CLOEXEC
|O_NOCTTY
, 0600);
156 return log_error_errno(errno
, "Failed to open /dev/urandom: %m");
158 read_seed_file
= true;
159 synchronous
= true; /* make this invocation a synchronous barrier for random pool initialization */
161 } else if (streq(argv
[1], "save")) {
163 random_fd
= open("/dev/urandom", O_RDONLY
|O_CLOEXEC
|O_NOCTTY
);
165 return log_error_errno(errno
, "Failed to open /dev/urandom: %m");
167 seed_fd
= open(RANDOM_SEED
, O_WRONLY
|O_CLOEXEC
|O_NOCTTY
|O_CREAT
, 0600);
169 return log_error_errno(errno
, "Failed to open " RANDOM_SEED
": %m");
171 read_seed_file
= false;
172 write_seed_file
= true;
175 return log_error_errno(SYNTHETIC_ERRNO(EINVAL
),
176 "Unknown verb '%s'.", argv
[1]);
178 if (fstat(seed_fd
, &st
) < 0)
179 return log_error_errno(errno
, "Failed to stat() seed file " RANDOM_SEED
": %m");
181 /* If the seed file is larger than what we expect, then honour the existing size and save/restore as much as it says */
182 if ((uint64_t) st
.st_size
> buf_size
)
183 buf_size
= MIN(st
.st_size
, RANDOM_POOL_SIZE_MAX
);
185 buf
= malloc(buf_size
);
189 if (read_seed_file
) {
192 /* First, let's write the machine ID into /dev/urandom, not crediting entropy. Why? As an
193 * extra protection against "golden images" that are put together sloppily, i.e. images which
194 * are duplicated on multiple systems but where the random seed file is not properly
195 * reset. Frequently the machine ID is properly reset on those systems however (simply
196 * because it's easier to notice, if it isn't due to address clashes and so on, while random
197 * seed equivalence is generally not noticed easily), hence let's simply write the machined
198 * ID into the random pool too. */
199 r
= sd_id128_get_machine(&mid
);
201 log_debug_errno(r
, "Failed to get machine ID, ignoring: %m");
203 r
= loop_write(random_fd
, &mid
, sizeof(mid
), false);
205 log_debug_errno(r
, "Failed to write machine ID to /dev/urandom, ignoring: %m");
208 k
= loop_read(seed_fd
, buf
, buf_size
, false);
210 log_error_errno(k
, "Failed to read seed from " RANDOM_SEED
": %m");
212 log_debug("Seed file " RANDOM_SEED
" not yet initialized, proceeding.");
214 CreditEntropy lets_credit
;
216 /* If we're going to later write out a seed file, initialize a hash state with
217 * the contents of the seed file we just read, so that the new one can't regress
219 if (write_seed_file
) {
220 sha256_init_ctx(&hash_state
);
221 sha256_process_bytes(&k
, sizeof(k
), &hash_state
); /* Hash length to distinguish from new seed. */
222 sha256_process_bytes(buf
, k
, &hash_state
);
223 hashed_old_seed
= true;
226 (void) lseek(seed_fd
, 0, SEEK_SET
);
228 lets_credit
= may_credit(seed_fd
);
230 /* Before we credit or use the entropy, let's make sure to securely drop the
231 * creditable xattr from the file, so that we never credit the same random seed
232 * again. Note that further down we'll write a new seed again, and likely mark it as
233 * credible again, hence this is just paranoia to close the short time window between
234 * the time we upload the random seed into the kernel and download the new one from
237 if (fremovexattr(seed_fd
, "user.random-seed-creditable") < 0) {
238 if (!IN_SET(errno
, ENODATA
, ENOSYS
, EOPNOTSUPP
))
239 log_warning_errno(errno
, "Failed to remove extended attribute, ignoring: %m");
241 /* Otherwise, there was no creditable flag set, which is OK. */
243 r
= fsync_full(seed_fd
);
245 log_warning_errno(r
, "Failed to synchronize seed to disk, not crediting entropy: %m");
247 if (lets_credit
== CREDIT_ENTROPY_YES_PLEASE
)
248 lets_credit
= CREDIT_ENTROPY_NO_WAY
;
252 r
= random_write_entropy(random_fd
, buf
, k
,
253 IN_SET(lets_credit
, CREDIT_ENTROPY_YES_PLEASE
, CREDIT_ENTROPY_YES_FORCED
));
255 log_error_errno(r
, "Failed to write seed to /dev/urandom: %m");
259 if (write_seed_file
) {
260 bool getrandom_worked
= false;
262 /* This is just a safety measure. Given that we are root and most likely created the file
263 * ourselves the mode and owner should be correct anyway. */
264 r
= fchmod_and_chown(seed_fd
, 0600, 0, 0);
266 return log_error_errno(r
, "Failed to adjust seed file ownership and access mode: %m");
268 /* Let's make this whole job asynchronous, i.e. let's make ourselves a barrier for
269 * proper initialization of the random pool. */
270 k
= getrandom(buf
, buf_size
, GRND_NONBLOCK
);
271 if (k
< 0 && errno
== EAGAIN
&& synchronous
) {
272 log_notice("Kernel entropy pool is not initialized yet, waiting until it is.");
273 k
= getrandom(buf
, buf_size
, 0); /* retry synchronously */
276 log_debug_errno(errno
, "Failed to read random data with getrandom(), falling back to /dev/urandom: %m");
277 else if ((size_t) k
< buf_size
)
278 log_debug("Short read from getrandom(), falling back to /dev/urandom.");
280 getrandom_worked
= true;
282 if (!getrandom_worked
) {
283 /* Retry with classic /dev/urandom */
284 k
= loop_read(random_fd
, buf
, buf_size
, false);
286 return log_error_errno(k
, "Failed to read new seed from /dev/urandom: %m");
288 return log_error_errno(SYNTHETIC_ERRNO(EIO
),
289 "Got EOF while reading from /dev/urandom.");
292 /* If we previously read in a seed file, then hash the new seed into the old one,
293 * and replace the last 32 bytes of the seed with the hash output, so that the
294 * new seed file can't regress in entropy. */
295 if (hashed_old_seed
) {
296 uint8_t hash
[SHA256_DIGEST_SIZE
];
297 sha256_process_bytes(&k
, sizeof(k
), &hash_state
); /* Hash length to distinguish from old seed. */
298 sha256_process_bytes(buf
, k
, &hash_state
);
299 sha256_finish_ctx(&hash_state
, hash
);
300 l
= MIN((size_t)k
, sizeof(hash
));
301 memcpy((uint8_t *)buf
+ k
- l
, hash
, l
);
304 r
= loop_write(seed_fd
, buf
, (size_t) k
, false);
306 return log_error_errno(r
, "Failed to write new random seed file: %m");
308 if (ftruncate(seed_fd
, k
) < 0)
309 return log_error_errno(r
, "Failed to truncate random seed file: %m");
311 r
= fsync_full(seed_fd
);
313 return log_error_errno(r
, "Failed to synchronize seed file: %m");
315 /* If we got this random seed data from getrandom() the data is suitable for crediting
316 * entropy later on. Let's keep that in mind by setting an extended attribute. on the file */
317 if (getrandom_worked
)
318 if (fsetxattr(seed_fd
, "user.random-seed-creditable", "1", 1, 0) < 0)
319 log_full_errno(ERRNO_IS_NOT_SUPPORTED(errno
) ? LOG_DEBUG
: LOG_WARNING
, errno
,
320 "Failed to mark seed file as creditable, ignoring: %m");
326 DEFINE_MAIN_FUNCTION(run
);