/* SPDX-License-Identifier: LGPL-2.1+ */ #if defined(__i386__) || defined(__x86_64__) #include #endif #include #include #include #include #include #include #include #include #include #include #include #if HAVE_SYS_AUXV_H # include #endif #include "alloc-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) 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 (;;) { r = getrandom(p, n, (FLAGS_SET(flags, RANDOM_BLOCK) ? 0 : GRND_NONBLOCK) | (FLAGS_SET(flags, RANDOM_ALLOW_INSECURE) ? GRND_INSECURE : 0)); 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); } 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) { int r; assert(fd >= 0); assert(seed && size > 0); 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 0; }