1 /* SPDX-License-Identifier: LGPL-2.1+ */
3 #if defined(__i386__) || defined(__x86_64__)
17 # include <sys/auxv.h>
21 # include <sys/random.h>
23 # include <linux/random.h>
29 #include "random-util.h"
30 #include "time-util.h"
32 #if HAS_FEATURE_MEMORY_SANITIZER
33 #include <sanitizer/msan_interface.h>
36 int rdrand(unsigned long *ret
) {
38 #if defined(__i386__) || defined(__x86_64__)
39 static int have_rdrand
= -1;
42 if (have_rdrand
< 0) {
43 uint32_t eax
, ebx
, ecx
, edx
;
45 /* Check if RDRAND is supported by the CPU */
46 if (__get_cpuid(1, &eax
, &ebx
, &ecx
, &edx
) == 0) {
51 have_rdrand
= !!(ecx
& (1U << 30));
57 asm volatile("rdrand %0;"
62 #if HAS_FEATURE_MEMORY_SANITIZER
63 __msan_unpoison(&err
, sizeof(err
));
75 int genuine_random_bytes(void *p
, size_t n
, RandomFlags flags
) {
76 static int have_syscall
= -1;
77 _cleanup_close_
int fd
= -1;
78 bool got_some
= false;
81 /* Gathers some randomness from the kernel (or the CPU if the RANDOM_ALLOW_RDRAND flag is set). This call won't
82 * block, unless the RANDOM_BLOCK flag is set. If RANDOM_DONT_DRAIN is set, an error is returned if the random
83 * pool is not initialized. Otherwise it will always return some data from the kernel, regardless of whether
84 * the random pool is fully initialized or not. */
89 if (FLAGS_SET(flags
, RANDOM_ALLOW_RDRAND
))
90 /* Try x86-64' RDRAND intrinsic if we have it. We only use it if high quality randomness is not
91 * required, as we don't trust it (who does?). Note that we only do a single iteration of RDRAND here,
92 * even though the Intel docs suggest calling this in a tight loop of 10 invocations or so. That's
93 * because we don't really care about the quality here. We generally prefer using RDRAND if the caller
94 * allows us too, since this way we won't drain the kernel randomness pool if we don't need it, as the
95 * pool's entropy is scarce. */
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
);
107 /* OK, this didn't work, let's go to getrandom() + /dev/urandom instead */
111 m
= MIN(sizeof(u
), n
);
114 p
= (uint8_t*) p
+ m
;
118 return 0; /* Yay, success! */
123 /* Use the getrandom() syscall unless we know we don't have it. */
124 if (have_syscall
!= 0 && !HAS_FEATURE_MEMORY_SANITIZER
) {
127 r
= getrandom(p
, n
, FLAGS_SET(flags
, RANDOM_BLOCK
) ? 0 : GRND_NONBLOCK
);
132 return 0; /* Yay, success! */
134 assert((size_t) r
< n
);
135 p
= (uint8_t*) p
+ r
;
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
);
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
))
150 /* Fill in the rest with /dev/urandom */
157 } else if (errno
== ENOSYS
) {
158 /* We lack the syscall, continue with reading from /dev/urandom. */
159 have_syscall
= false;
162 } else if (errno
== EAGAIN
) {
163 /* The kernel has no entropy whatsoever. Let's remember to use the syscall the next
166 * If RANDOM_DONT_DRAIN is set, return an error so that random_bytes() can produce some
167 * pseudo-random bytes instead. Otherwise, fall back to /dev/urandom, which we know is empty,
168 * but the kernel will produce some bytes for us on a best-effort basis. */
171 if (got_some
&& FLAGS_SET(flags
, RANDOM_EXTEND_WITH_PSEUDO
)) {
172 /* Fill in the remaining bytes using pseudorandom values */
173 pseudo_random_bytes(p
, n
);
177 if (FLAGS_SET(flags
, RANDOM_DONT_DRAIN
))
180 /* Use /dev/urandom instead */
187 fd
= open("/dev/urandom", O_RDONLY
|O_CLOEXEC
|O_NOCTTY
);
189 return errno
== ENOENT
? -ENOSYS
: -errno
;
191 return loop_read_exact(fd
, p
, n
, true);
194 void initialize_srand(void) {
195 static bool srand_called
= false;
206 /* The kernel provides us with 16 bytes of entropy in auxv, so let's
207 * try to make use of that to seed the pseudo-random generator. It's
208 * better than nothing... */
210 auxv
= (const void*) getauxval(AT_RANDOM
);
212 assert_cc(sizeof(x
) <= 16);
213 memcpy(&x
, auxv
, sizeof(x
));
218 x
^= (unsigned) now(CLOCK_REALTIME
);
219 x
^= (unsigned) gettid();
228 /* INT_MAX gives us only 31 bits, so use 24 out of that. */
229 #if RAND_MAX >= INT_MAX
232 /* SHORT_INT_MAX or lower gives at most 15 bits, we just just 8 out of that. */
236 void pseudo_random_bytes(void *p
, size_t n
) {
241 for (q
= p
; q
< (uint8_t*) p
+ n
; q
+= RAND_STEP
) {
244 rr
= (unsigned) rand();
247 if ((size_t) (q
- (uint8_t*) p
+ 2) < n
)
251 if ((size_t) (q
- (uint8_t*) p
+ 1) < n
)
258 void random_bytes(void *p
, size_t n
) {
260 if (genuine_random_bytes(p
, n
, RANDOM_EXTEND_WITH_PSEUDO
|RANDOM_DONT_DRAIN
|RANDOM_ALLOW_RDRAND
) >= 0)
263 /* If for some reason some user made /dev/urandom unavailable to us, or the kernel has no entropy, use a PRNG instead. */
264 pseudo_random_bytes(p
, n
);