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/* SPDX-License-Identifier: LGPL-2.1+ */
#include <fcntl.h>
#include <linux/random.h>
#include <sys/ioctl.h>
#include <unistd.h>
#include "alloc-util.h"
#include "chattr-util.h"
#include "efi-random.h"
#include "efivars.h"
#include "fd-util.h"
#include "fs-util.h"
#include "strv.h"
/* If a random seed was passed by the boot loader in the LoaderRandomSeed EFI variable, let's credit it to
* the kernel's random pool, but only once per boot. If this is run very early during initialization we can
* instantly boot up with a filled random pool.
*
* This makes no judgement on the entropy passed, it's the job of the boot loader to only pass us a seed that
* is suitably validated. */
static void lock_down_efi_variables(void) {
const char *p;
int r;
/* Paranoia: let's restrict access modes of these a bit, so that unprivileged users can't use them to
* identify the system or gain too much insight into what we might have credited to the entropy
* pool. */
FOREACH_STRING(p,
"/sys/firmware/efi/efivars/LoaderRandomSeed-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f",
"/sys/firmware/efi/efivars/LoaderSystemToken-4a67b082-0a4c-41cf-b6c7-440b29bb8c4f") {
r = chattr_path(p, 0, FS_IMMUTABLE_FL, NULL);
if (r == -ENOENT)
continue;
if (r < 0)
log_warning_errno(r, "Failed to drop FS_IMMUTABLE_FL from %s, ignoring: %m", p);
if (chmod(p, 0600) < 0)
log_warning_errno(errno, "Failed to reduce access mode of %s, ignoring: %m", p);
}
}
int efi_take_random_seed(void) {
_cleanup_free_ struct rand_pool_info *info = NULL;
_cleanup_free_ void *value = NULL;
_cleanup_close_ int random_fd = -1;
size_t size;
int r;
/* Paranoia comes first. */
lock_down_efi_variables();
if (access("/run/systemd/efi-random-seed-taken", F_OK) < 0) {
if (errno != ENOENT) {
log_warning_errno(errno, "Failed to determine whether we already used the random seed token, not using it.");
return 0;
}
/* ENOENT means we haven't used it yet. */
} else {
log_debug("EFI random seed already used, not using again.");
return 0;
}
r = efi_get_variable(EFI_VENDOR_LOADER, "LoaderRandomSeed", NULL, &value, &size);
if (r == -EOPNOTSUPP) {
log_debug_errno(r, "System lacks EFI support, not initializing random seed from EFI variable.");
return 0;
}
if (r == -ENOENT) {
log_debug_errno(r, "Boot loader did not pass LoaderRandomSeed EFI variable, not crediting any entropy.");
return 0;
}
if (r < 0)
return log_warning_errno(r, "Failed to read LoaderRandomSeed EFI variable, ignoring: %m");
if (size == 0)
return log_warning_errno(SYNTHETIC_ERRNO(EINVAL), "Random seed passed from boot loader has zero size? Ignoring.");
/* The kernel API only accepts "int" as entropy count (which is in bits), let's avoid any chance for
* confusion here. */
if (size > INT_MAX / 8)
size = INT_MAX / 8;
random_fd = open("/dev/urandom", O_WRONLY|O_CLOEXEC|O_NOCTTY);
if (random_fd < 0)
return log_warning_errno(errno, "Failed to open /dev/urandom for writing, ignoring: %m");
/* Before we use the seed, let's mark it as used, so that we never credit it twice. Also, it's a nice
* way to let users known that we successfully acquired entropy from the boot laoder. */
r = touch("/run/systemd/efi-random-seed-taken");
if (r < 0)
return log_warning_errno(r, "Unable to mark EFI random seed as used, not using it: %m");
info = malloc(offsetof(struct rand_pool_info, buf) + size);
if (!info)
return log_oom();
info->entropy_count = size * 8;
info->buf_size = size;
memcpy(info->buf, value, size);
if (ioctl(random_fd, RNDADDENTROPY, info) < 0)
return log_warning_errno(errno, "Failed to credit entropy, ignoring: %m");
log_info("Successfully credited entropy passed from boot loader.");
return 1;
}
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