src/basic/random-util.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2
   3 #if defined(__i386__) || defined(__x86_64__)
   4 #include <cpuid.h>
   5 #endif
   6
   7 #include <elf.h>
   8 #include <errno.h>
   9 #include <fcntl.h>
  10 #include <stdbool.h>
  11 #include <stdint.h>
  12 #include <stdlib.h>
  13 #include <string.h>
  14 #include <sys/time.h>
  15
  16 #if HAVE_SYS_AUXV_H
  17 #  include <sys/auxv.h>
  18 #endif
  19
  20 #if USE_SYS_RANDOM_H
  21 #  include <sys/random.h>
  22 #else
  23 #  include <linux/random.h>
  24 #endif
  25
  26 #include "alloc-util.h"
  27 #include "fd-util.h"
  28 #include "io-util.h"
  29 #include "missing.h"
  30 #include "random-util.h"
  31 #include "time-util.h"
  32
  33 int rdrand(unsigned long *ret) {
  34
  35 #if defined(__i386__) || defined(__x86_64__)
  36         static int have_rdrand = -1;
  37         unsigned char err;
  38
  39         if (have_rdrand < 0) {
  40                 uint32_t eax, ebx, ecx, edx;
  41
  42                 /* Check if RDRAND is supported by the CPU */
  43                 if (__get_cpuid(1, &eax, &ebx, &ecx, &edx) == 0) {
  44                         have_rdrand = false;
  45                         return -EOPNOTSUPP;
  46                 }
  47
  48                 have_rdrand = !!(ecx & (1U << 30));
  49         }
  50
  51         if (have_rdrand == 0)
  52                 return -EOPNOTSUPP;
  53
  54         asm volatile("rdrand %0;"
  55                      "setc %1"
  56                      : "=r" (*ret),
  57                        "=qm" (err));
  58         msan_unpoison(&err, sizeof(err));
  59         if (!err)
  60                 return -EAGAIN;
  61
  62         return 0;
  63 #else
  64         return -EOPNOTSUPP;
  65 #endif
  66 }
  67
  68 int genuine_random_bytes(void *p, size_t n, RandomFlags flags) {
  69         static int have_syscall = -1;
  70         _cleanup_close_ int fd = -1;
  71         bool got_some = false;
  72         int r;
  73
  74         /* Gathers some randomness from the kernel (or the CPU if the RANDOM_ALLOW_RDRAND flag is set). This call won't
  75          * block, unless the RANDOM_BLOCK flag is set. If RANDOM_DONT_DRAIN is set, an error is returned if the random
  76          * pool is not initialized. Otherwise it will always return some data from the kernel, regardless of whether
  77          * the random pool is fully initialized or not. */
  78
  79         if (n == 0)
  80                 return 0;
  81
  82         if (FLAGS_SET(flags, RANDOM_ALLOW_RDRAND))
  83                 /* Try x86-64' RDRAND intrinsic if we have it. We only use it if high quality randomness is not
  84                  * required, as we don't trust it (who does?). Note that we only do a single iteration of RDRAND here,
  85                  * even though the Intel docs suggest calling this in a tight loop of 10 invocations or so. That's
  86                  * because we don't really care about the quality here. We generally prefer using RDRAND if the caller
  87                  * allows us too, since this way we won't drain the kernel randomness pool if we don't need it, as the
  88                  * pool's entropy is scarce. */
  89                 for (;;) {
  90                         unsigned long u;
  91                         size_t m;
  92
  93                         if (rdrand(&u) < 0) {
  94                                 if (got_some && FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
  95                                         /* Fill in the remaining bytes using pseudo-random values */
  96                                         pseudo_random_bytes(p, n);
  97                                         return 0;
  98                                 }
  99
 100                                 /* OK, this didn't work, let's go to getrandom() + /dev/urandom instead */
 101                                 break;
 102                         }
 103
 104                         m = MIN(sizeof(u), n);
 105                         memcpy(p, &u, m);
 106
 107                         p = (uint8_t*) p + m;
 108                         n -= m;
 109
 110                         if (n == 0)
 111                                 return 0; /* Yay, success! */
 112
 113                         got_some = true;
 114                 }
 115
 116         /* Use the getrandom() syscall unless we know we don't have it. */
 117         if (have_syscall != 0 && !HAS_FEATURE_MEMORY_SANITIZER) {
 118
 119                 for (;;) {
 120                         r = getrandom(p, n, FLAGS_SET(flags, RANDOM_BLOCK) ? 0 : GRND_NONBLOCK);
 121                         if (r > 0) {
 122                                 have_syscall = true;
 123
 124                                 if ((size_t) r == n)
 125                                         return 0; /* Yay, success! */
 126
 127                                 assert((size_t) r < n);
 128                                 p = (uint8_t*) p + r;
 129                                 n -= r;
 130
 131                                 if (FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
 132                                         /* Fill in the remaining bytes using pseudo-random values */
 133                                         pseudo_random_bytes(p, n);
 134                                         return 0;
 135                                 }
 136
 137                                 got_some = true;
 138
 139                                 /* Hmm, we didn't get enough good data but the caller insists on good data? Then try again */
 140                                 if (FLAGS_SET(flags, RANDOM_BLOCK))
 141                                         continue;
 142
 143                                 /* Fill in the rest with /dev/urandom */
 144                                 break;
 145
 146                         } else if (r == 0) {
 147                                 have_syscall = true;
 148                                 return -EIO;
 149
 150                         } else if (errno == ENOSYS) {
 151                                 /* We lack the syscall, continue with reading from /dev/urandom. */
 152                                 have_syscall = false;
 153                                 break;
 154
 155                         } else if (errno == EAGAIN) {
 156                                 /* The kernel has no entropy whatsoever. Let's remember to use the syscall the next
 157                                  * time again though.
 158                                  *
 159                                  * If RANDOM_DONT_DRAIN is set, return an error so that random_bytes() can produce some
 160                                  * pseudo-random bytes instead. Otherwise, fall back to /dev/urandom, which we know is empty,
 161                                  * but the kernel will produce some bytes for us on a best-effort basis. */
 162                                 have_syscall = true;
 163
 164                                 if (got_some && FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
 165                                         /* Fill in the remaining bytes using pseudorandom values */
 166                                         pseudo_random_bytes(p, n);
 167                                         return 0;
 168                                 }
 169
 170                                 if (FLAGS_SET(flags, RANDOM_DONT_DRAIN))
 171                                         return -ENODATA;
 172
 173                                 /* Use /dev/urandom instead */
 174                                 break;
 175                         } else
 176                                 return -errno;
 177                 }
 178         }
 179
 180         fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
 181         if (fd < 0)
 182                 return errno == ENOENT ? -ENOSYS : -errno;
 183
 184         return loop_read_exact(fd, p, n, true);
 185 }
 186
 187 void initialize_srand(void) {
 188         static bool srand_called = false;
 189         unsigned x;
 190 #if HAVE_SYS_AUXV_H
 191         const void *auxv;
 192 #endif
 193         unsigned long k;
 194
 195         if (srand_called)
 196                 return;
 197
 198 #if HAVE_SYS_AUXV_H
 199         /* The kernel provides us with 16 bytes of entropy in auxv, so let's
 200          * try to make use of that to seed the pseudo-random generator. It's
 201          * better than nothing... */
 202
 203         auxv = (const void*) getauxval(AT_RANDOM);
 204         if (auxv) {
 205                 assert_cc(sizeof(x) <= 16);
 206                 memcpy(&x, auxv, sizeof(x));
 207         } else
 208 #endif
 209                 x = 0;
 210
 211         x ^= (unsigned) now(CLOCK_REALTIME);
 212         x ^= (unsigned) gettid();
 213
 214         if (rdrand(&k) >= 0)
 215                 x ^= (unsigned) k;
 216
 217         srand(x);
 218         srand_called = true;
 219 }
 220
 221 /* INT_MAX gives us only 31 bits, so use 24 out of that. */
 222 #if RAND_MAX >= INT_MAX
 223 #  define RAND_STEP 3
 224 #else
 225 /* SHORT_INT_MAX or lower gives at most 15 bits, we just just 8 out of that. */
 226 #  define RAND_STEP 1
 227 #endif
 228
 229 void pseudo_random_bytes(void *p, size_t n) {
 230         uint8_t *q;
 231
 232         initialize_srand();
 233
 234         for (q = p; q < (uint8_t*) p + n; q += RAND_STEP) {
 235                 unsigned rr;
 236
 237                 rr = (unsigned) rand();
 238
 239 #if RAND_STEP >= 3
 240                 if ((size_t) (q - (uint8_t*) p + 2) < n)
 241                         q[2] = rr >> 16;
 242 #endif
 243 #if RAND_STEP >= 2
 244                 if ((size_t) (q - (uint8_t*) p + 1) < n)
 245                         q[1] = rr >> 8;
 246 #endif
 247                 q[0] = rr;
 248         }
 249 }
 250
 251 void random_bytes(void *p, size_t n) {
 252
 253         if (genuine_random_bytes(p, n, RANDOM_EXTEND_WITH_PSEUDO|RANDOM_DONT_DRAIN|RANDOM_ALLOW_RDRAND) >= 0)
 254                 return;
 255
 256         /* If for some reason some user made /dev/urandom unavailable to us, or the kernel has no entropy, use a PRNG instead. */
 257         pseudo_random_bytes(p, n);
 258 }