/* SPDX-License-Identifier: LGPL-2.1-or-later */
#include "nm-sd-adapt-shared.h"
#if defined(__i386__) || defined(__x86_64__)
#include <cpuid.h>
#endif
#include <elf.h>
#include <errno.h>
#include <fcntl.h>
#include <linux/random.h>
#include <pthread.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/time.h>
#if HAVE_SYS_AUXV_H
# include <sys/auxv.h>
#endif
#include "alloc-util.h"
#include "env-util.h"
#include "errno-util.h"
#include "fd-util.h"
#include "fileio.h"
#include "io-util.h"
#include "missing_random.h"
#include "missing_syscall.h"
#include "parse-util.h"
#include "random-util.h"
#include "siphash24.h"
#include "time-util.h"
static bool srand_called = false;
int rdrand(unsigned long *ret) {
/* So, you are a "security researcher", and you wonder why we bother with using raw RDRAND here,
* instead of sticking to /dev/urandom or getrandom()?
*
* Here's why: early boot. On Linux, during early boot the random pool that backs /dev/urandom and
* getrandom() is generally not initialized yet. It is very common that initialization of the random
* pool takes a longer time (up to many minutes), in particular on embedded devices that have no
* explicit hardware random generator, as well as in virtualized environments such as major cloud
* installations that do not provide virtio-rng or a similar mechanism.
*
* In such an environment using getrandom() synchronously means we'd block the entire system boot-up
* until the pool is initialized, i.e. *very* long. Using getrandom() asynchronously (GRND_NONBLOCK)
* would mean acquiring randomness during early boot would simply fail. Using /dev/urandom would mean
* generating many kmsg log messages about our use of it before the random pool is properly
* initialized. Neither of these outcomes is desirable.
*
* Thus, for very specific purposes we use RDRAND instead of either of these three options. RDRAND
* provides us quickly and relatively reliably with random values, without having to delay boot,
* without triggering warning messages in kmsg.
*
* Note that we use RDRAND only under very specific circumstances, when the requirements on the
* quality of the returned entropy permit it. Specifically, here are some cases where we *do* use
* RDRAND:
*
* • UUID generation: UUIDs are supposed to be universally unique but are not cryptographic
* key material. The quality and trust level of RDRAND should hence be OK: UUIDs should be
* generated in a way that is reliably unique, but they do not require ultimate trust into
* the entropy generator. systemd generates a number of UUIDs during early boot, including
* 'invocation IDs' for every unit spawned that identify the specific invocation of the
* service globally, and a number of others. Other alternatives for generating these UUIDs
* have been considered, but don't really work: for example, hashing uuids from a local
* system identifier combined with a counter falls flat because during early boot disk
* storage is not yet available (think: initrd) and thus a system-specific ID cannot be
* stored or retrieved yet.
*
* • Hash table seed generation: systemd uses many hash tables internally. Hash tables are
* generally assumed to have O(1) access complexity, but can deteriorate to prohibitive
* O(n) access complexity if an attacker manages to trigger a large number of hash
* collisions. Thus, systemd (as any software employing hash tables should) uses seeded
* hash functions for its hash tables, with a seed generated randomly. The hash tables
* systemd employs watch the fill level closely and reseed if necessary. This allows use of
* a low quality RNG initially, as long as it improves should a hash table be under attack:
* the attacker after all needs to trigger many collisions to exploit it for the purpose
* of DoS, but if doing so improves the seed the attack surface is reduced as the attack
* takes place.
*
* Some cases where we do NOT use RDRAND are:
*
* • Generation of cryptographic key material 🔑
*
* • Generation of cryptographic salt values 🧂
*
* This function returns:
*
* -EOPNOTSUPP → RDRAND is not available on this system 😔
* -EAGAIN → The operation failed this time, but is likely to work if you try again a few
* times ♻
* -EUCLEAN → We got some random value, but it looked strange, so we refused using it.
* This failure might or might not be temporary. 😕
*/
#if defined(__i386__) || defined(__x86_64__)
static int have_rdrand = -1;
unsigned long v;
uint8_t success;
if (have_rdrand < 0) {
uint32_t eax, ebx, ecx, edx;
/* Check if RDRAND is supported by the CPU */
if (__get_cpuid(1, &eax, &ebx, &ecx, &edx) == 0) {
have_rdrand = false;
return -EOPNOTSUPP;
}
/* Compat with old gcc where bit_RDRND didn't exist yet */
#ifndef bit_RDRND
#define bit_RDRND (1U << 30)
#endif
have_rdrand = !!(ecx & bit_RDRND);
if (have_rdrand > 0) {
/* Allow disabling use of RDRAND with SYSTEMD_RDRAND=0
If it is unset getenv_bool_secure will return a negative value. */
if (getenv_bool_secure("SYSTEMD_RDRAND") == 0) {
have_rdrand = false;
return -EOPNOTSUPP;
}
}
}
if (have_rdrand == 0)
return -EOPNOTSUPP;
asm volatile("rdrand %0;"
"setc %1"
: "=r" (v),
"=qm" (success));
msan_unpoison(&success, sizeof(success));
if (!success)
return -EAGAIN;
/* Apparently on some AMD CPUs RDRAND will sometimes (after a suspend/resume cycle?) report success
* via the carry flag but nonetheless return the same fixed value -1 in all cases. This appears to be
* a bad bug in the CPU or firmware. Let's deal with that and work-around this by explicitly checking
* for this special value (and also 0, just to be sure) and filtering it out. This is a work-around
* only however and something AMD really should fix properly. The Linux kernel should probably work
* around this issue by turning off RDRAND altogether on those CPUs. See:
* https://github.com/systemd/systemd/issues/11810 */
if (v == 0 || v == ULONG_MAX)
return log_debug_errno(SYNTHETIC_ERRNO(EUCLEAN),
"RDRAND returned suspicious value %lx, assuming bad hardware RNG, not using value.", v);
*ret = v;
return 0;
#else
return -EOPNOTSUPP;
#endif
}
int genuine_random_bytes(void *p, size_t n, RandomFlags flags) {
static int have_syscall = -1;
_cleanup_close_ int fd = -1;
bool got_some = false;
int r;
/* Gathers some high-quality randomness from the kernel (or potentially mid-quality randomness from
* the CPU if the RANDOM_ALLOW_RDRAND flag is set). This call won't block, unless the RANDOM_BLOCK
* flag is set. If RANDOM_MAY_FAIL is set, an error is returned if the random pool is not
* initialized. Otherwise it will always return some data from the kernel, regardless of whether the
* random pool is fully initialized or not. If RANDOM_EXTEND_WITH_PSEUDO is set, and some but not
* enough better quality randomness could be acquired, the rest is filled up with low quality
* randomness.
*
* Of course, when creating cryptographic key material you really shouldn't use RANDOM_ALLOW_DRDRAND
* or even RANDOM_EXTEND_WITH_PSEUDO.
*
* When generating UUIDs it's fine to use RANDOM_ALLOW_RDRAND but not OK to use
* RANDOM_EXTEND_WITH_PSEUDO. In fact RANDOM_EXTEND_WITH_PSEUDO is only really fine when invoked via
* an "all bets are off" wrapper, such as random_bytes(), see below. */
if (n == 0)
return 0;
if (FLAGS_SET(flags, RANDOM_ALLOW_RDRAND))
/* Try x86-64' RDRAND intrinsic if we have it. We only use it if high quality randomness is
* not required, as we don't trust it (who does?). Note that we only do a single iteration of
* RDRAND here, even though the Intel docs suggest calling this in a tight loop of 10
* invocations or so. That's because we don't really care about the quality here. We
* generally prefer using RDRAND if the caller allows us to, since this way we won't upset
* the kernel's random subsystem by accessing it before the pool is initialized (after all it
* will kmsg log about every attempt to do so)..*/
for (;;) {
unsigned long u;
size_t m;
if (rdrand(&u) < 0) {
if (got_some && FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
/* Fill in the remaining bytes using pseudo-random values */
pseudo_random_bytes(p, n);
return 0;
}
/* OK, this didn't work, let's go to getrandom() + /dev/urandom instead */
break;
}
m = MIN(sizeof(u), n);
memcpy(p, &u, m);
p = (uint8_t*) p + m;
n -= m;
if (n == 0)
return 0; /* Yay, success! */
got_some = true;
}
/* Use the getrandom() syscall unless we know we don't have it. */
if (have_syscall != 0 && !HAS_FEATURE_MEMORY_SANITIZER) {
for (;;) {
#if !HAVE_GETRANDOM
/* NetworkManager Note: systemd calls the syscall directly in this case. Don't add that workaround.
* If you don't compile against a libc that provides getrandom(), you don't get it. */
r = -1;
errno = ENOSYS;
#else
r = getrandom(p, n,
(FLAGS_SET(flags, RANDOM_BLOCK) ? 0 : GRND_NONBLOCK) |
(FLAGS_SET(flags, RANDOM_ALLOW_INSECURE) ? GRND_INSECURE : 0));
#endif
if (r > 0) {
have_syscall = true;
if ((size_t) r == n)
return 0; /* Yay, success! */
assert((size_t) r < n);
p = (uint8_t*) p + r;
n -= r;
if (FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
/* Fill in the remaining bytes using pseudo-random values */
pseudo_random_bytes(p, n);
return 0;
}
got_some = true;
/* Hmm, we didn't get enough good data but the caller insists on good data? Then try again */
if (FLAGS_SET(flags, RANDOM_BLOCK))
continue;
/* Fill in the rest with /dev/urandom */
break;
} else if (r == 0) {
have_syscall = true;
return -EIO;
} else if (ERRNO_IS_NOT_SUPPORTED(errno)) {
/* We lack the syscall, continue with reading from /dev/urandom. */
have_syscall = false;
break;
} else if (errno == EAGAIN) {
/* The kernel has no entropy whatsoever. Let's remember to use the syscall
* the next time again though.
*
* If RANDOM_MAY_FAIL is set, return an error so that random_bytes() can
* produce some pseudo-random bytes instead. Otherwise, fall back to
* /dev/urandom, which we know is empty, but the kernel will produce some
* bytes for us on a best-effort basis. */
have_syscall = true;
if (got_some && FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
/* Fill in the remaining bytes using pseudorandom values */
pseudo_random_bytes(p, n);
return 0;
}
if (FLAGS_SET(flags, RANDOM_MAY_FAIL))
return -ENODATA;
/* Use /dev/urandom instead */
break;
} else if (errno == EINVAL) {
/* Most likely: unknown flag. We know that GRND_INSECURE might cause this,
* hence try without. */
if (FLAGS_SET(flags, RANDOM_ALLOW_INSECURE)) {
flags = flags &~ RANDOM_ALLOW_INSECURE;
continue;
}
return -errno;
} else
return -errno;
}
}
fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
if (fd < 0)
return errno == ENOENT ? -ENOSYS : -errno;
return loop_read_exact(fd, p, n, true);
}
static void clear_srand_initialization(void) {
srand_called = false;
}
void initialize_srand(void) {
static bool pthread_atfork_registered = false;
unsigned x;
#if HAVE_SYS_AUXV_H
const void *auxv;
#endif
unsigned long k;
if (srand_called)
return;
#if HAVE_SYS_AUXV_H
/* The kernel provides us with 16 bytes of entropy in auxv, so let's try to make use of that to seed
* the pseudo-random generator. It's better than nothing... But let's first hash it to make it harder
* to recover the original value by watching any pseudo-random bits we generate. After all the
* AT_RANDOM data might be used by other stuff too (in particular: ASLR), and we probably shouldn't
* leak the seed for that. */
auxv = ULONG_TO_PTR(getauxval(AT_RANDOM));
if (auxv) {
static const uint8_t auxval_hash_key[16] = {
0x92, 0x6e, 0xfe, 0x1b, 0xcf, 0x00, 0x52, 0x9c, 0xcc, 0x42, 0xcf, 0xdc, 0x94, 0x1f, 0x81, 0x0f
};
x = (unsigned) siphash24(auxv, 16, auxval_hash_key);
} else
#endif
x = 0;
x ^= (unsigned) now(CLOCK_REALTIME);
x ^= (unsigned) gettid();
if (rdrand(&k) >= 0)
x ^= (unsigned) k;
srand(x);
srand_called = true;
if (!pthread_atfork_registered) {
(void) pthread_atfork(NULL, NULL, clear_srand_initialization);
pthread_atfork_registered = true;
}
}
/* INT_MAX gives us only 31 bits, so use 24 out of that. */
#if RAND_MAX >= INT_MAX
assert_cc(RAND_MAX >= 16777215);
# define RAND_STEP 3
#else
/* SHORT_INT_MAX or lower gives at most 15 bits, we just use 8 out of that. */
assert_cc(RAND_MAX >= 255);
# define RAND_STEP 1
#endif
void pseudo_random_bytes(void *p, size_t n) {
uint8_t *q;
/* This returns pseudo-random data using libc's rand() function. You probably never want to call this
* directly, because why would you use this if you can get better stuff cheaply? Use random_bytes()
* instead, see below: it will fall back to this function if there's nothing better to get, but only
* then. */
initialize_srand();
for (q = p; q < (uint8_t*) p + n; q += RAND_STEP) {
unsigned rr;
rr = (unsigned) rand();
#if RAND_STEP >= 3
if ((size_t) (q - (uint8_t*) p + 2) < n)
q[2] = rr >> 16;
#endif
#if RAND_STEP >= 2
if ((size_t) (q - (uint8_t*) p + 1) < n)
q[1] = rr >> 8;
#endif
q[0] = rr;
}
}
void random_bytes(void *p, size_t n) {
/* This returns high quality randomness if we can get it cheaply. If we can't because for some reason
* it is not available we'll try some crappy fallbacks.
*
* What this function will do:
*
* • This function will preferably use the CPU's RDRAND operation, if it is available, in
* order to return "mid-quality" random values cheaply.
*
* • Use getrandom() with GRND_NONBLOCK, to return high-quality random values if they are
* cheaply available.
*
* • This function will return pseudo-random data, generated via libc rand() if nothing
* better is available.
*
* • This function will work fine in early boot
*
* • This function will always succeed
*
* What this function won't do:
*
* • This function will never fail: it will give you randomness no matter what. It might not
* be high quality, but it will return some, possibly generated via libc's rand() call.
*
* • This function will never block: if the only way to get good randomness is a blocking,
* synchronous getrandom() we'll instead provide you with pseudo-random data.
*
* This function is hence great for things like seeding hash tables, generating random numeric UNIX
* user IDs (that are checked for collisions before use) and such.
*
* This function is hence not useful for generating UUIDs or cryptographic key material.
*/
if (genuine_random_bytes(p, n, RANDOM_EXTEND_WITH_PSEUDO|RANDOM_MAY_FAIL|RANDOM_ALLOW_RDRAND|RANDOM_ALLOW_INSECURE) >= 0)
return;
/* If for some reason some user made /dev/urandom unavailable to us, or the kernel has no entropy, use a PRNG instead. */
pseudo_random_bytes(p, n);
}
#if 0 /* NM_IGNORED */
size_t random_pool_size(void) {
_cleanup_free_ char *s = NULL;
int r;
/* Read pool size, if possible */
r = read_one_line_file("/proc/sys/kernel/random/poolsize", &s);
if (r < 0)
log_debug_errno(r, "Failed to read pool size from kernel: %m");
else {
unsigned sz;
r = safe_atou(s, &sz);
if (r < 0)
log_debug_errno(r, "Failed to parse pool size: %s", s);
else
/* poolsize is in bits on 2.6, but we want bytes */
return CLAMP(sz / 8, RANDOM_POOL_SIZE_MIN, RANDOM_POOL_SIZE_MAX);
}
/* Use the minimum as default, if we can't retrieve the correct value */
return RANDOM_POOL_SIZE_MIN;
}
int random_write_entropy(int fd, const void *seed, size_t size, bool credit) {
_cleanup_close_ int opened_fd = -1;
int r;
assert(seed || size == 0);
if (size == 0)
return 0;
if (fd < 0) {
opened_fd = open("/dev/urandom", O_WRONLY|O_CLOEXEC|O_NOCTTY);
if (opened_fd < 0)
return -errno;
fd = opened_fd;
}
if (credit) {
_cleanup_free_ struct rand_pool_info *info = NULL;
/* 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)
return -EOVERFLOW;
info = malloc(offsetof(struct rand_pool_info, buf) + size);
if (!info)
return -ENOMEM;
info->entropy_count = size * 8;
info->buf_size = size;
memcpy(info->buf, seed, size);
if (ioctl(fd, RNDADDENTROPY, info) < 0)
return -errno;
} else {
r = loop_write(fd, seed, size, false);
if (r < 0)
return r;
}
return 1;
}
#endif /* NM_IGNORED */