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Diffstat (limited to 'src/basic/random-util.c')
| -rw-r--r-- | src/basic/random-util.c | 485 |
1 files changed, 485 insertions, 0 deletions
diff --git a/src/basic/random-util.c b/src/basic/random-util.c new file mode 100644 index 0000000..c8c34a2 --- /dev/null +++ b/src/basic/random-util.c @@ -0,0 +1,485 @@ +/* SPDX-License-Identifier: LGPL-2.1-or-later */ + +#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 (;;) { + 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; +} |