1 /* SPDX-License-Identifier: LGPL-2.1+ */
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_syscall.h"
24 #include "parse-util.h"
25 #include "random-util.h"
26 #include "string-util.h"
28 #include "xattr-util.h"
30 typedef enum CreditEntropy {
31 CREDIT_ENTROPY_NO_WAY,
32 CREDIT_ENTROPY_YES_PLEASE,
33 CREDIT_ENTROPY_YES_FORCED,
36 static CreditEntropy may_credit(int seed_fd) {
37 _cleanup_free_ char *creditable = NULL;
43 e = getenv("SYSTEMD_RANDOM_SEED_CREDIT");
45 log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is not set, not crediting entropy.");
46 return CREDIT_ENTROPY_NO_WAY;
48 if (streq(e, "force")) {
49 log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is set to 'force', crediting entropy.");
50 return CREDIT_ENTROPY_YES_FORCED;
56 log_warning_errno(r, "Failed to parse $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy: %m");
58 log_debug("Crediting entropy is turned off via $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy.");
60 return CREDIT_ENTROPY_NO_WAY;
63 /* Determine if the file is marked as creditable */
64 r = fgetxattr_malloc(seed_fd, "user.random-seed-creditable", &creditable);
66 if (IN_SET(r, -ENODATA, -ENOSYS, -EOPNOTSUPP))
67 log_debug_errno(r, "Seed file is not marked as creditable, not crediting.");
69 log_warning_errno(r, "Failed to read extended attribute, ignoring: %m");
71 return CREDIT_ENTROPY_NO_WAY;
74 r = parse_boolean(creditable);
77 log_warning_errno(r, "Failed to parse user.random-seed-creditable extended attribute, ignoring: %s", creditable);
79 log_debug("Seed file is marked as not creditable, not crediting.");
81 return CREDIT_ENTROPY_NO_WAY;
84 /* Don't credit the random seed if we are in first-boot mode, because we are supposed to start from
85 * scratch. This is a safety precaution for cases where we people ship "golden" images with empty
86 * /etc but populated /var that contains a random seed. */
87 if (access("/run/systemd/first-boot", F_OK) < 0) {
89 if (errno != ENOENT) {
90 log_warning_errno(errno, "Failed to check whether we are in first-boot mode, not crediting entropy: %m");
91 return CREDIT_ENTROPY_NO_WAY;
94 /* If ENOENT all is good, we are not in first-boot mode. */
96 log_debug("Not crediting entropy, since booted in first-boot mode.");
97 return CREDIT_ENTROPY_NO_WAY;
100 return CREDIT_ENTROPY_YES_PLEASE;
103 static int run(int argc, char *argv[]) {
104 _cleanup_close_ int seed_fd = -1, random_fd = -1;
105 bool read_seed_file, write_seed_file, synchronous;
106 _cleanup_free_ void* buf = NULL;
115 return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
116 "This program requires one argument.");
120 buf_size = random_pool_size();
122 r = mkdir_parents(RANDOM_SEED, 0755);
124 return log_error_errno(r, "Failed to create directory " RANDOM_SEED_DIR ": %m");
126 /* When we load the seed we read it and write it to the device and then immediately update the saved seed with
127 * new data, to make sure the next boot gets seeded differently. */
129 if (streq(argv[1], "load")) {
131 seed_fd = open(RANDOM_SEED, O_RDWR|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
133 int open_rw_error = -errno;
135 write_seed_file = false;
137 seed_fd = open(RANDOM_SEED, O_RDONLY|O_CLOEXEC|O_NOCTTY);
139 bool missing = errno == ENOENT;
141 log_full_errno(missing ? LOG_DEBUG : LOG_ERR,
142 open_rw_error, "Failed to open " RANDOM_SEED " for writing: %m");
143 r = log_full_errno(missing ? LOG_DEBUG : LOG_ERR,
144 errno, "Failed to open " RANDOM_SEED " for reading: %m");
145 return missing ? 0 : r;
148 write_seed_file = true;
150 random_fd = open("/dev/urandom", O_RDWR|O_CLOEXEC|O_NOCTTY, 0600);
152 return log_error_errno(errno, "Failed to open /dev/urandom: %m");
154 read_seed_file = true;
155 synchronous = true; /* make this invocation a synchronous barrier for random pool initialization */
157 } else if (streq(argv[1], "save")) {
159 random_fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
161 return log_error_errno(errno, "Failed to open /dev/urandom: %m");
163 seed_fd = open(RANDOM_SEED, O_WRONLY|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
165 return log_error_errno(errno, "Failed to open " RANDOM_SEED ": %m");
167 read_seed_file = false;
168 write_seed_file = true;
171 return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
172 "Unknown verb '%s'.", argv[1]);
174 if (fstat(seed_fd, &st) < 0)
175 return log_error_errno(errno, "Failed to stat() seed file " RANDOM_SEED ": %m");
177 /* If the seed file is larger than what we expect, then honour the existing size and save/restore as much as it says */
178 if ((uint64_t) st.st_size > buf_size)
179 buf_size = MIN(st.st_size, RANDOM_POOL_SIZE_MAX);
181 buf = malloc(buf_size);
185 if (read_seed_file) {
188 /* First, let's write the machine ID into /dev/urandom, not crediting entropy. Why? As an
189 * extra protection against "golden images" that are put together sloppily, i.e. images which
190 * are duplicated on multiple systems but where the random seed file is not properly
191 * reset. Frequently the machine ID is properly reset on those systems however (simply
192 * because it's easier to notice, if it isn't due to address clashes and so on, while random
193 * seed equivalence is generally not noticed easily), hence let's simply write the machined
194 * ID into the random pool too. */
195 r = sd_id128_get_machine(&mid);
197 log_debug_errno(r, "Failed to get machine ID, ignoring: %m");
199 r = loop_write(random_fd, &mid, sizeof(mid), false);
201 log_debug_errno(r, "Failed to write machine ID to /dev/urandom, ignoring: %m");
204 k = loop_read(seed_fd, buf, buf_size, false);
206 log_error_errno(k, "Failed to read seed from " RANDOM_SEED ": %m");
208 log_debug("Seed file " RANDOM_SEED " not yet initialized, proceeding.");
210 CreditEntropy lets_credit;
212 (void) lseek(seed_fd, 0, SEEK_SET);
214 lets_credit = may_credit(seed_fd);
216 /* Before we credit or use the entropy, let's make sure to securely drop the
217 * creditable xattr from the file, so that we never credit the same random seed
218 * again. Note that further down we'll write a new seed again, and likely mark it as
219 * credible again, hence this is just paranoia to close the short time window between
220 * the time we upload the random seed into the kernel and download the new one from
223 if (fremovexattr(seed_fd, "user.random-seed-creditable") < 0) {
224 if (!IN_SET(errno, ENODATA, ENOSYS, EOPNOTSUPP))
225 log_warning_errno(errno, "Failed to remove extended attribute, ignoring: %m");
227 /* Otherwise, there was no creditable flag set, which is OK. */
229 r = fsync_full(seed_fd);
231 log_warning_errno(r, "Failed to synchronize seed to disk, not crediting entropy: %m");
233 if (lets_credit == CREDIT_ENTROPY_YES_PLEASE)
234 lets_credit = CREDIT_ENTROPY_NO_WAY;
238 if (IN_SET(lets_credit, CREDIT_ENTROPY_YES_PLEASE, CREDIT_ENTROPY_YES_FORCED)) {
239 _cleanup_free_ struct rand_pool_info *info = NULL;
241 info = malloc(offsetof(struct rand_pool_info, buf) + k);
245 info->entropy_count = k * 8;
247 memcpy(info->buf, buf, k);
249 if (ioctl(random_fd, RNDADDENTROPY, info) < 0)
250 return log_warning_errno(errno, "Failed to credit entropy, ignoring: %m");
252 r = loop_write(random_fd, buf, (size_t) k, false);
254 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.");
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: %m");
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 r = loop_write(seed_fd, buf, (size_t) k, false);
294 return log_error_errno(r, "Failed to write new random seed file: %m");
296 if (ftruncate(seed_fd, k) < 0)
297 return log_error_errno(r, "Failed to truncate random seed file: %m");
299 r = fsync_full(seed_fd);
301 return log_error_errno(r, "Failed to synchronize seed file: %m");
303 /* If we got this random seed data from getrandom() the data is suitable for crediting
304 * entropy later on. Let's keep that in mind by setting an extended attribute. on the file */
305 if (getrandom_worked)
306 if (fsetxattr(seed_fd, "user.random-seed-creditable", "1", 1, 0) < 0)
307 log_full_errno(IN_SET(errno, ENOSYS, EOPNOTSUPP) ? LOG_DEBUG : LOG_WARNING, errno,
308 "Failed to mark seed file as creditable, ignoring: %m");
314 DEFINE_MAIN_FUNCTION(run);