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 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;
bool got_some = false;
int r;
- /* Gathers some randomness from the kernel (or 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. */
+ /* 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;
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) {
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) >= 0)
return;
projects / thirdparty / systemd.git / blobdiff