diff options
Diffstat (limited to '')
| -rw-r--r-- | src/random-seed/random-seed.c | 326 |
1 files changed, 326 insertions, 0 deletions
diff --git a/src/random-seed/random-seed.c b/src/random-seed/random-seed.c new file mode 100644 index 0000000..82c29d0 --- /dev/null +++ b/src/random-seed/random-seed.c @@ -0,0 +1,326 @@ +/* SPDX-License-Identifier: LGPL-2.1-or-later */ + +#include <errno.h> +#include <fcntl.h> +#include <linux/random.h> +#include <sys/ioctl.h> +#if USE_SYS_RANDOM_H +# include <sys/random.h> +#endif +#include <sys/stat.h> +#include <sys/xattr.h> +#include <unistd.h> + +#include "sd-id128.h" + +#include "alloc-util.h" +#include "fd-util.h" +#include "fs-util.h" +#include "io-util.h" +#include "log.h" +#include "main-func.h" +#include "missing_random.h" +#include "missing_syscall.h" +#include "mkdir.h" +#include "parse-util.h" +#include "random-util.h" +#include "string-util.h" +#include "sync-util.h" +#include "sha256.h" +#include "util.h" +#include "xattr-util.h" + +typedef enum CreditEntropy { + CREDIT_ENTROPY_NO_WAY, + CREDIT_ENTROPY_YES_PLEASE, + CREDIT_ENTROPY_YES_FORCED, +} CreditEntropy; + +static CreditEntropy may_credit(int seed_fd) { + _cleanup_free_ char *creditable = NULL; + const char *e; + int r; + + assert(seed_fd >= 0); + + e = getenv("SYSTEMD_RANDOM_SEED_CREDIT"); + if (!e) { + log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is not set, not crediting entropy."); + return CREDIT_ENTROPY_NO_WAY; + } + if (streq(e, "force")) { + log_debug("$SYSTEMD_RANDOM_SEED_CREDIT is set to 'force', crediting entropy."); + return CREDIT_ENTROPY_YES_FORCED; + } + + r = parse_boolean(e); + if (r <= 0) { + if (r < 0) + log_warning_errno(r, "Failed to parse $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy: %m"); + else + log_debug("Crediting entropy is turned off via $SYSTEMD_RANDOM_SEED_CREDIT, not crediting entropy."); + + return CREDIT_ENTROPY_NO_WAY; + } + + /* Determine if the file is marked as creditable */ + r = fgetxattr_malloc(seed_fd, "user.random-seed-creditable", &creditable); + if (r < 0) { + if (ERRNO_IS_XATTR_ABSENT(r)) + log_debug_errno(r, "Seed file is not marked as creditable, not crediting."); + else + log_warning_errno(r, "Failed to read extended attribute, ignoring: %m"); + + return CREDIT_ENTROPY_NO_WAY; + } + + r = parse_boolean(creditable); + if (r <= 0) { + if (r < 0) + log_warning_errno(r, "Failed to parse user.random-seed-creditable extended attribute, ignoring: %s", creditable); + else + log_debug("Seed file is marked as not creditable, not crediting."); + + return CREDIT_ENTROPY_NO_WAY; + } + + /* Don't credit the random seed if we are in first-boot mode, because we are supposed to start from + * scratch. This is a safety precaution for cases where we people ship "golden" images with empty + * /etc but populated /var that contains a random seed. */ + if (access("/run/systemd/first-boot", F_OK) < 0) { + + if (errno != ENOENT) { + log_warning_errno(errno, "Failed to check whether we are in first-boot mode, not crediting entropy: %m"); + return CREDIT_ENTROPY_NO_WAY; + } + + /* If ENOENT all is good, we are not in first-boot mode. */ + } else { + log_debug("Not crediting entropy, since booted in first-boot mode."); + return CREDIT_ENTROPY_NO_WAY; + } + + return CREDIT_ENTROPY_YES_PLEASE; +} + +static int run(int argc, char *argv[]) { + bool read_seed_file, write_seed_file, synchronous, hashed_old_seed = false; + _cleanup_close_ int seed_fd = -1, random_fd = -1; + _cleanup_free_ void* buf = NULL; + struct sha256_ctx hash_state; + size_t buf_size; + struct stat st; + ssize_t k, l; + int r; + + log_setup(); + + if (argc != 2) + return log_error_errno(SYNTHETIC_ERRNO(EINVAL), + "This program requires one argument."); + + umask(0022); + + buf_size = random_pool_size(); + + r = mkdir_parents(RANDOM_SEED, 0755); + if (r < 0) + return log_error_errno(r, "Failed to create directory " RANDOM_SEED_DIR ": %m"); + + /* When we load the seed we read it and write it to the device and then immediately update the saved seed with + * new data, to make sure the next boot gets seeded differently. */ + + if (streq(argv[1], "load")) { + + seed_fd = open(RANDOM_SEED, O_RDWR|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600); + if (seed_fd < 0) { + int open_rw_error = -errno; + + write_seed_file = false; + + seed_fd = open(RANDOM_SEED, O_RDONLY|O_CLOEXEC|O_NOCTTY); + if (seed_fd < 0) { + bool missing = errno == ENOENT; + + log_full_errno(missing ? LOG_DEBUG : LOG_ERR, + open_rw_error, "Failed to open " RANDOM_SEED " for writing: %m"); + r = log_full_errno(missing ? LOG_DEBUG : LOG_ERR, + errno, "Failed to open " RANDOM_SEED " for reading: %m"); + return missing ? 0 : r; + } + } else + write_seed_file = true; + + random_fd = open("/dev/urandom", O_RDWR|O_CLOEXEC|O_NOCTTY, 0600); + if (random_fd < 0) + return log_error_errno(errno, "Failed to open /dev/urandom: %m"); + + read_seed_file = true; + synchronous = true; /* make this invocation a synchronous barrier for random pool initialization */ + + } else if (streq(argv[1], "save")) { + + random_fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY); + if (random_fd < 0) + return log_error_errno(errno, "Failed to open /dev/urandom: %m"); + + seed_fd = open(RANDOM_SEED, O_WRONLY|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600); + if (seed_fd < 0) + return log_error_errno(errno, "Failed to open " RANDOM_SEED ": %m"); + + read_seed_file = false; + write_seed_file = true; + synchronous = false; + } else + return log_error_errno(SYNTHETIC_ERRNO(EINVAL), + "Unknown verb '%s'.", argv[1]); + + if (fstat(seed_fd, &st) < 0) + return log_error_errno(errno, "Failed to stat() seed file " RANDOM_SEED ": %m"); + + /* If the seed file is larger than what we expect, then honour the existing size and save/restore as much as it says */ + if ((uint64_t) st.st_size > buf_size) + buf_size = MIN(st.st_size, RANDOM_POOL_SIZE_MAX); + + buf = malloc(buf_size); + if (!buf) + return log_oom(); + + if (read_seed_file) { + sd_id128_t mid; + + /* First, let's write the machine ID into /dev/urandom, not crediting entropy. Why? As an + * extra protection against "golden images" that are put together sloppily, i.e. images which + * are duplicated on multiple systems but where the random seed file is not properly + * reset. Frequently the machine ID is properly reset on those systems however (simply + * because it's easier to notice, if it isn't due to address clashes and so on, while random + * seed equivalence is generally not noticed easily), hence let's simply write the machined + * ID into the random pool too. */ + r = sd_id128_get_machine(&mid); + if (r < 0) + log_debug_errno(r, "Failed to get machine ID, ignoring: %m"); + else { + r = loop_write(random_fd, &mid, sizeof(mid), false); + if (r < 0) + log_debug_errno(r, "Failed to write machine ID to /dev/urandom, ignoring: %m"); + } + + k = loop_read(seed_fd, buf, buf_size, false); + if (k < 0) + log_error_errno(k, "Failed to read seed from " RANDOM_SEED ": %m"); + else if (k == 0) + log_debug("Seed file " RANDOM_SEED " not yet initialized, proceeding."); + else { + CreditEntropy lets_credit; + + /* If we're going to later write out a seed file, initialize a hash state with + * the contents of the seed file we just read, so that the new one can't regress + * in entropy. */ + if (write_seed_file) { + sha256_init_ctx(&hash_state); + sha256_process_bytes(&k, sizeof(k), &hash_state); /* Hash length to distinguish from new seed. */ + sha256_process_bytes(buf, k, &hash_state); + hashed_old_seed = true; + } + + (void) lseek(seed_fd, 0, SEEK_SET); + + lets_credit = may_credit(seed_fd); + + /* Before we credit or use the entropy, let's make sure to securely drop the + * creditable xattr from the file, so that we never credit the same random seed + * again. Note that further down we'll write a new seed again, and likely mark it as + * credible again, hence this is just paranoia to close the short time window between + * the time we upload the random seed into the kernel and download the new one from + * it. */ + + if (fremovexattr(seed_fd, "user.random-seed-creditable") < 0) { + if (!ERRNO_IS_XATTR_ABSENT(errno)) + log_warning_errno(errno, "Failed to remove extended attribute, ignoring: %m"); + + /* Otherwise, there was no creditable flag set, which is OK. */ + } else { + r = fsync_full(seed_fd); + if (r < 0) { + log_warning_errno(r, "Failed to synchronize seed to disk, not crediting entropy: %m"); + + if (lets_credit == CREDIT_ENTROPY_YES_PLEASE) + lets_credit = CREDIT_ENTROPY_NO_WAY; + } + } + + r = random_write_entropy(random_fd, buf, k, + IN_SET(lets_credit, CREDIT_ENTROPY_YES_PLEASE, CREDIT_ENTROPY_YES_FORCED)); + if (r < 0) + log_error_errno(r, "Failed to write seed to /dev/urandom: %m"); + } + } + + if (write_seed_file) { + bool getrandom_worked = false; + + /* This is just a safety measure. Given that we are root and most likely created the file + * ourselves the mode and owner should be correct anyway. */ + r = fchmod_and_chown(seed_fd, 0600, 0, 0); + if (r < 0) + return log_error_errno(r, "Failed to adjust seed file ownership and access mode: %m"); + + /* Let's make this whole job asynchronous, i.e. let's make ourselves a barrier for + * proper initialization of the random pool. */ + k = getrandom(buf, buf_size, GRND_NONBLOCK); + if (k < 0 && errno == EAGAIN && synchronous) { + log_notice("Kernel entropy pool is not initialized yet, waiting until it is."); + k = getrandom(buf, buf_size, 0); /* retry synchronously */ + } + if (k < 0) + log_debug_errno(errno, "Failed to read random data with getrandom(), falling back to /dev/urandom: %m"); + else if ((size_t) k < buf_size) + log_debug("Short read from getrandom(), falling back to /dev/urandom."); + else + getrandom_worked = true; + + if (!getrandom_worked) { + /* Retry with classic /dev/urandom */ + k = loop_read(random_fd, buf, buf_size, false); + if (k < 0) + return log_error_errno(k, "Failed to read new seed from /dev/urandom: %m"); + if (k == 0) + return log_error_errno(SYNTHETIC_ERRNO(EIO), + "Got EOF while reading from /dev/urandom."); + } + + /* If we previously read in a seed file, then hash the new seed into the old one, + * and replace the last 32 bytes of the seed with the hash output, so that the + * new seed file can't regress in entropy. */ + if (hashed_old_seed) { + uint8_t hash[SHA256_DIGEST_SIZE]; + sha256_process_bytes(&k, sizeof(k), &hash_state); /* Hash length to distinguish from old seed. */ + sha256_process_bytes(buf, k, &hash_state); + sha256_finish_ctx(&hash_state, hash); + l = MIN((size_t)k, sizeof(hash)); + memcpy((uint8_t *)buf + k - l, hash, l); + } + + r = loop_write(seed_fd, buf, (size_t) k, false); + if (r < 0) + return log_error_errno(r, "Failed to write new random seed file: %m"); + + if (ftruncate(seed_fd, k) < 0) + return log_error_errno(r, "Failed to truncate random seed file: %m"); + + r = fsync_full(seed_fd); + if (r < 0) + return log_error_errno(r, "Failed to synchronize seed file: %m"); + + /* If we got this random seed data from getrandom() the data is suitable for crediting + * entropy later on. Let's keep that in mind by setting an extended attribute. on the file */ + if (getrandom_worked) + if (fsetxattr(seed_fd, "user.random-seed-creditable", "1", 1, 0) < 0) + log_full_errno(ERRNO_IS_NOT_SUPPORTED(errno) ? LOG_DEBUG : LOG_WARNING, errno, + "Failed to mark seed file as creditable, ignoring: %m"); + } + + return 0; +} + +DEFINE_MAIN_FUNCTION(run); |