diff options
Diffstat (limited to 'src/basic/random-util.c')
| -rw-r--r-- | src/basic/random-util.c | 115 |
1 files changed, 111 insertions, 4 deletions
diff --git a/src/basic/random-util.c b/src/basic/random-util.c index 0561f0cb22..3af6f271f0 100644 --- a/src/basic/random-util.c +++ b/src/basic/random-util.c @@ -33,6 +33,66 @@ 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; @@ -90,10 +150,20 @@ int genuine_random_bytes(void *p, size_t n, RandomFlags flags) { 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; @@ -255,6 +325,11 @@ void initialize_srand(void) { 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) { @@ -276,6 +351,38 @@ void pseudo_random_bytes(void *p, size_t n) { 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; |