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 "siphash24.h"
  32 #include "time-util.h"
  33
  34 int rdrand(unsigned long *ret) {
  35
  36 #if defined(__i386__) || defined(__x86_64__)
  37         static int have_rdrand = -1;
  38         uint8_t success;
  39
  40         if (have_rdrand < 0) {
  41                 uint32_t eax, ebx, ecx, edx;
  42
  43                 /* Check if RDRAND is supported by the CPU */
  44                 if (__get_cpuid(1, &eax, &ebx, &ecx, &edx) == 0) {
  45                         have_rdrand = false;
  46                         return -EOPNOTSUPP;
  47                 }
  48
  49 /* Compat with old gcc where bit_RDRND didn't exist yet */
  50 #ifndef bit_RDRND
  51 #define bit_RDRND (1U << 30)
  52 #endif
  53
  54                 have_rdrand = !!(ecx & bit_RDRND);
  55         }
  56
  57         if (have_rdrand == 0)
  58                 return -EOPNOTSUPP;
  59
  60         asm volatile("rdrand %0;"
  61                      "setc %1"
  62                      : "=r" (*ret),
  63                        "=qm" (success));
  64         msan_unpoison(&success, sizeof(sucess));
  65         if (!success)
  66                 return -EAGAIN;
  67
  68         return 0;
  69 #else
  70         return -EOPNOTSUPP;
  71 #endif
  72 }
  73
  74 int genuine_random_bytes(void *p, size_t n, RandomFlags flags) {
  75         static int have_syscall = -1;
  76         _cleanup_close_ int fd = -1;
  77         bool got_some = false;
  78         int r;
  79
  80         /* Gathers some randomness from the kernel (or the CPU if the RANDOM_ALLOW_RDRAND flag is set). This
  81          * call won't block, unless the RANDOM_BLOCK flag is set. If RANDOM_MAY_FAIL is set, an error is
  82          * returned if the random pool is not initialized. Otherwise it will always return some data from the
  83          * kernel, regardless of whether the random pool is fully initialized or not. */
  84
  85         if (n == 0)
  86                 return 0;
  87
  88         if (FLAGS_SET(flags, RANDOM_ALLOW_RDRAND))
  89                 /* Try x86-64' RDRAND intrinsic if we have it. We only use it if high quality randomness is
  90                  * not required, as we don't trust it (who does?). Note that we only do a single iteration of
  91                  * RDRAND here, even though the Intel docs suggest calling this in a tight loop of 10
  92                  * invocations or so. That's because we don't really care about the quality here. We
  93                  * generally prefer using RDRAND if the caller allows us to, since this way we won't upset
  94                  * the kernel's random subsystem by accessing it before the pool is initialized (after all it
  95                  * will kmsg log about every attempt to do so)..*/
  96                 for (;;) {
  97                         unsigned long u;
  98                         size_t m;
  99
 100                         if (rdrand(&u) < 0) {
 101                                 if (got_some && FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
 102                                         /* Fill in the remaining bytes using pseudo-random values */
 103                                         pseudo_random_bytes(p, n);
 104                                         return 0;
 105                                 }
 106
 107                                 /* OK, this didn't work, let's go to getrandom() + /dev/urandom instead */
 108                                 break;
 109                         }
 110
 111                         m = MIN(sizeof(u), n);
 112                         memcpy(p, &u, m);
 113
 114                         p = (uint8_t*) p + m;
 115                         n -= m;
 116
 117                         if (n == 0)
 118                                 return 0; /* Yay, success! */
 119
 120                         got_some = true;
 121                 }
 122
 123         /* Use the getrandom() syscall unless we know we don't have it. */
 124         if (have_syscall != 0 && !HAS_FEATURE_MEMORY_SANITIZER) {
 125
 126                 for (;;) {
 127                         r = getrandom(p, n, FLAGS_SET(flags, RANDOM_BLOCK) ? 0 : GRND_NONBLOCK);
 128                         if (r > 0) {
 129                                 have_syscall = true;
 130
 131                                 if ((size_t) r == n)
 132                                         return 0; /* Yay, success! */
 133
 134                                 assert((size_t) r < n);
 135                                 p = (uint8_t*) p + r;
 136                                 n -= r;
 137
 138                                 if (FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
 139                                         /* Fill in the remaining bytes using pseudo-random values */
 140                                         pseudo_random_bytes(p, n);
 141                                         return 0;
 142                                 }
 143
 144                                 got_some = true;
 145
 146                                 /* Hmm, we didn't get enough good data but the caller insists on good data? Then try again */
 147                                 if (FLAGS_SET(flags, RANDOM_BLOCK))
 148                                         continue;
 149
 150                                 /* Fill in the rest with /dev/urandom */
 151                                 break;
 152
 153                         } else if (r == 0) {
 154                                 have_syscall = true;
 155                                 return -EIO;
 156
 157                         } else if (errno == ENOSYS) {
 158                                 /* We lack the syscall, continue with reading from /dev/urandom. */
 159                                 have_syscall = false;
 160                                 break;
 161
 162                         } else if (errno == EAGAIN) {
 163                                 /* The kernel has no entropy whatsoever. Let's remember to use the syscall
 164                                  * the next time again though.
 165                                  *
 166                                  * If RANDOM_MAY_FAIL is set, return an error so that random_bytes() can
 167                                  * produce some pseudo-random bytes instead. Otherwise, fall back to
 168                                  * /dev/urandom, which we know is empty, but the kernel will produce some
 169                                  * bytes for us on a best-effort basis. */
 170                                 have_syscall = true;
 171
 172                                 if (got_some && FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
 173                                         /* Fill in the remaining bytes using pseudorandom values */
 174                                         pseudo_random_bytes(p, n);
 175                                         return 0;
 176                                 }
 177
 178                                 if (FLAGS_SET(flags, RANDOM_MAY_FAIL))
 179                                         return -ENODATA;
 180
 181                                 /* Use /dev/urandom instead */
 182                                 break;
 183                         } else
 184                                 return -errno;
 185                 }
 186         }
 187
 188         fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
 189         if (fd < 0)
 190                 return errno == ENOENT ? -ENOSYS : -errno;
 191
 192         return loop_read_exact(fd, p, n, true);
 193 }
 194
 195 void initialize_srand(void) {
 196         static bool srand_called = false;
 197         unsigned x;
 198 #if HAVE_SYS_AUXV_H
 199         const void *auxv;
 200 #endif
 201         unsigned long k;
 202
 203         if (srand_called)
 204                 return;
 205
 206 #if HAVE_SYS_AUXV_H
 207         /* The kernel provides us with 16 bytes of entropy in auxv, so let's try to make use of that to seed
 208          * the pseudo-random generator. It's better than nothing... But let's first hash it to make it harder
 209          * to recover the original value by watching any pseudo-random bits we generate. After all the
 210          * AT_RANDOM data might be used by other stuff too (in particular: ASLR), and we probably shouldn't
 211          * leak the seed for that. */
 212
 213         auxv = ULONG_TO_PTR(getauxval(AT_RANDOM));
 214         if (auxv) {
 215                 static const uint8_t auxval_hash_key[16] = {
 216                         0x92, 0x6e, 0xfe, 0x1b, 0xcf, 0x00, 0x52, 0x9c, 0xcc, 0x42, 0xcf, 0xdc, 0x94, 0x1f, 0x81, 0x0f
 217                 };
 218
 219                 x = (unsigned) siphash24(auxv, 16, auxval_hash_key);
 220         } else
 221 #endif
 222                 x = 0;
 223
 224         x ^= (unsigned) now(CLOCK_REALTIME);
 225         x ^= (unsigned) gettid();
 226
 227         if (rdrand(&k) >= 0)
 228                 x ^= (unsigned) k;
 229
 230         srand(x);
 231         srand_called = true;
 232 }
 233
 234 /* INT_MAX gives us only 31 bits, so use 24 out of that. */
 235 #if RAND_MAX >= INT_MAX
 236 #  define RAND_STEP 3
 237 #else
 238 /* SHORT_INT_MAX or lower gives at most 15 bits, we just just 8 out of that. */
 239 #  define RAND_STEP 1
 240 #endif
 241
 242 void pseudo_random_bytes(void *p, size_t n) {
 243         uint8_t *q;
 244
 245         initialize_srand();
 246
 247         for (q = p; q < (uint8_t*) p + n; q += RAND_STEP) {
 248                 unsigned rr;
 249
 250                 rr = (unsigned) rand();
 251
 252 #if RAND_STEP >= 3
 253                 if ((size_t) (q - (uint8_t*) p + 2) < n)
 254                         q[2] = rr >> 16;
 255 #endif
 256 #if RAND_STEP >= 2
 257                 if ((size_t) (q - (uint8_t*) p + 1) < n)
 258                         q[1] = rr >> 8;
 259 #endif
 260                 q[0] = rr;
 261         }
 262 }
 263
 264 void random_bytes(void *p, size_t n) {
 265
 266         if (genuine_random_bytes(p, n, RANDOM_EXTEND_WITH_PSEUDO|RANDOM_MAY_FAIL|RANDOM_ALLOW_RDRAND) >= 0)
 267                 return;
 268
 269         /* If for some reason some user made /dev/urandom unavailable to us, or the kernel has no entropy, use a PRNG instead. */
 270         pseudo_random_bytes(p, n);
 271 }