Line data Source code
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 "fileio.h"
29 : #include "io-util.h"
30 : #include "missing.h"
31 : #include "parse-util.h"
32 : #include "random-util.h"
33 : #include "siphash24.h"
34 : #include "time-util.h"
35 :
36 376132 : int rdrand(unsigned long *ret) {
37 :
38 : /* So, you are a "security researcher", and you wonder why we bother with using raw RDRAND here,
39 : * instead of sticking to /dev/urandom or getrandom()?
40 : *
41 : * Here's why: early boot. On Linux, during early boot the random pool that backs /dev/urandom and
42 : * getrandom() is generally not initialized yet. It is very common that initialization of the random
43 : * pool takes a longer time (up to many minutes), in particular on embedded devices that have no
44 : * explicit hardware random generator, as well as in virtualized environments such as major cloud
45 : * installations that do not provide virtio-rng or a similar mechanism.
46 : *
47 : * In such an environment using getrandom() synchronously means we'd block the entire system boot-up
48 : * until the pool is initialized, i.e. *very* long. Using getrandom() asynchronously (GRND_NONBLOCK)
49 : * would mean acquiring randomness during early boot would simply fail. Using /dev/urandom would mean
50 : * generating many kmsg log messages about our use of it before the random pool is properly
51 : * initialized. Neither of these outcomes is desirable.
52 : *
53 : * Thus, for very specific purposes we use RDRAND instead of either of these three options. RDRAND
54 : * provides us quickly and relatively reliably with random values, without having to delay boot,
55 : * without triggering warning messages in kmsg.
56 : *
57 : * Note that we use RDRAND only under very specific circumstances, when the requirements on the
58 : * quality of the returned entropy permit it. Specifically, here are some cases where we *do* use
59 : * RDRAND:
60 : *
61 : * • UUID generation: UUIDs are supposed to be universally unique but are not cryptographic
62 : * key material. The quality and trust level of RDRAND should hence be OK: UUIDs should be
63 : * generated in a way that is reliably unique, but they do not require ultimate trust into
64 : * the entropy generator. systemd generates a number of UUIDs during early boot, including
65 : * 'invocation IDs' for every unit spawned that identify the specific invocation of the
66 : * service globally, and a number of others. Other alternatives for generating these UUIDs
67 : * have been considered, but don't really work: for example, hashing uuids from a local
68 : * system identifier combined with a counter falls flat because during early boot disk
69 : * storage is not yet available (think: initrd) and thus a system-specific ID cannot be
70 : * stored or retrieved yet.
71 : *
72 : * • Hash table seed generation: systemd uses many hash tables internally. Hash tables are
73 : * generally assumed to have O(1) access complexity, but can deteriorate to prohibitive
74 : * O(n) access complexity if an attacker manages to trigger a large number of hash
75 : * collisions. Thus, systemd (as any software employing hash tables should) uses seeded
76 : * hash functions for its hash tables, with a seed generated randomly. The hash tables
77 : * systemd employs watch the fill level closely and reseed if necessary. This allows use of
78 : * a low quality RNG initially, as long as it improves should a hash table be under attack:
79 : * the attacker after all needs to to trigger many collisions to exploit it for the purpose
80 : * of DoS, but if doing so improves the seed the attack surface is reduced as the attack
81 : * takes place.
82 : *
83 : * Some cases where we do NOT use RDRAND are:
84 : *
85 : * • Generation of cryptographic key material 🔑
86 : *
87 : * • Generation of cryptographic salt values 🧂
88 : *
89 : * This function returns:
90 : *
91 : * -EOPNOTSUPP → RDRAND is not available on this system 😔
92 : * -EAGAIN → The operation failed this time, but is likely to work if you try again a few
93 : * times ♻
94 : * -EUCLEAN → We got some random value, but it looked strange, so we refused using it.
95 : * This failure might or might not be temporary. 😕
96 : */
97 :
98 : #if defined(__i386__) || defined(__x86_64__)
99 : static int have_rdrand = -1;
100 : unsigned long v;
101 : uint8_t success;
102 :
103 376132 : if (have_rdrand < 0) {
104 : uint32_t eax, ebx, ecx, edx;
105 :
106 : /* Check if RDRAND is supported by the CPU */
107 192 : if (__get_cpuid(1, &eax, &ebx, &ecx, &edx) == 0) {
108 0 : have_rdrand = false;
109 0 : return -EOPNOTSUPP;
110 : }
111 :
112 : /* Compat with old gcc where bit_RDRND didn't exist yet */
113 : #ifndef bit_RDRND
114 : #define bit_RDRND (1U << 30)
115 : #endif
116 :
117 192 : have_rdrand = !!(ecx & bit_RDRND);
118 : }
119 :
120 376132 : if (have_rdrand == 0)
121 0 : return -EOPNOTSUPP;
122 :
123 376132 : asm volatile("rdrand %0;"
124 : "setc %1"
125 : : "=r" (v),
126 : "=qm" (success));
127 : msan_unpoison(&success, sizeof(success));
128 376132 : if (!success)
129 0 : return -EAGAIN;
130 :
131 : /* Apparently on some AMD CPUs RDRAND will sometimes (after a suspend/resume cycle?) report success
132 : * via the carry flag but nonetheless return the same fixed value -1 in all cases. This appears to be
133 : * a bad bug in the CPU or firmware. Let's deal with that and work-around this by explicitly checking
134 : * for this special value (and also 0, just to be sure) and filtering it out. This is a work-around
135 : * only however and something AMD really should fix properly. The Linux kernel should probably work
136 : * around this issue by turning off RDRAND altogether on those CPUs. See:
137 : * https://github.com/systemd/systemd/issues/11810 */
138 376132 : if (v == 0 || v == ULONG_MAX)
139 0 : return log_debug_errno(SYNTHETIC_ERRNO(EUCLEAN),
140 : "RDRAND returned suspicious value %lx, assuming bad hardware RNG, not using value.", v);
141 :
142 376132 : *ret = v;
143 376132 : return 0;
144 : #else
145 : return -EOPNOTSUPP;
146 : #endif
147 : }
148 :
149 9746 : int genuine_random_bytes(void *p, size_t n, RandomFlags flags) {
150 : static int have_syscall = -1;
151 9746 : _cleanup_close_ int fd = -1;
152 9746 : bool got_some = false;
153 : int r;
154 :
155 : /* Gathers some high-quality randomness from the kernel (or potentially mid-quality randomness from
156 : * the CPU if the RANDOM_ALLOW_RDRAND flag is set). This call won't block, unless the RANDOM_BLOCK
157 : * flag is set. If RANDOM_MAY_FAIL is set, an error is returned if the random pool is not
158 : * initialized. Otherwise it will always return some data from the kernel, regardless of whether the
159 : * random pool is fully initialized or not. If RANDOM_EXTEND_WITH_PSEUDO is set, and some but not
160 : * enough better quality randomness could be acquired, the rest is filled up with low quality
161 : * randomness.
162 : *
163 : * Of course, when creating cryptographic key material you really shouldn't use RANDOM_ALLOW_DRDRAND
164 : * or even RANDOM_EXTEND_WITH_PSEUDO.
165 : *
166 : * When generating UUIDs it's fine to use RANDOM_ALLOW_RDRAND but not OK to use
167 : * RANDOM_EXTEND_WITH_PSEUDO. In fact RANDOM_EXTEND_WITH_PSEUDO is only really fine when invoked via
168 : * an "all bets are off" wrapper, such as random_bytes(), see below. */
169 :
170 9746 : if (n == 0)
171 0 : return 0;
172 :
173 9746 : if (FLAGS_SET(flags, RANDOM_ALLOW_RDRAND))
174 : /* Try x86-64' RDRAND intrinsic if we have it. We only use it if high quality randomness is
175 : * not required, as we don't trust it (who does?). Note that we only do a single iteration of
176 : * RDRAND here, even though the Intel docs suggest calling this in a tight loop of 10
177 : * invocations or so. That's because we don't really care about the quality here. We
178 : * generally prefer using RDRAND if the caller allows us to, since this way we won't upset
179 : * the kernel's random subsystem by accessing it before the pool is initialized (after all it
180 : * will kmsg log about every attempt to do so)..*/
181 366422 : for (;;) {
182 : unsigned long u;
183 : size_t m;
184 :
185 376120 : if (rdrand(&u) < 0) {
186 0 : if (got_some && FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
187 : /* Fill in the remaining bytes using pseudo-random values */
188 0 : pseudo_random_bytes(p, n);
189 9698 : return 0;
190 : }
191 :
192 : /* OK, this didn't work, let's go to getrandom() + /dev/urandom instead */
193 0 : break;
194 : }
195 :
196 376120 : m = MIN(sizeof(u), n);
197 376120 : memcpy(p, &u, m);
198 :
199 376120 : p = (uint8_t*) p + m;
200 376120 : n -= m;
201 :
202 376120 : if (n == 0)
203 9698 : return 0; /* Yay, success! */
204 :
205 366422 : got_some = true;
206 : }
207 :
208 : /* Use the getrandom() syscall unless we know we don't have it. */
209 48 : if (have_syscall != 0 && !HAS_FEATURE_MEMORY_SANITIZER) {
210 :
211 : for (;;) {
212 48 : r = getrandom(p, n, FLAGS_SET(flags, RANDOM_BLOCK) ? 0 : GRND_NONBLOCK);
213 48 : if (r > 0) {
214 48 : have_syscall = true;
215 :
216 48 : if ((size_t) r == n)
217 48 : return 0; /* Yay, success! */
218 :
219 0 : assert((size_t) r < n);
220 0 : p = (uint8_t*) p + r;
221 0 : n -= r;
222 :
223 0 : if (FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
224 : /* Fill in the remaining bytes using pseudo-random values */
225 0 : pseudo_random_bytes(p, n);
226 0 : return 0;
227 : }
228 :
229 0 : got_some = true;
230 :
231 : /* Hmm, we didn't get enough good data but the caller insists on good data? Then try again */
232 0 : if (FLAGS_SET(flags, RANDOM_BLOCK))
233 0 : continue;
234 :
235 : /* Fill in the rest with /dev/urandom */
236 0 : break;
237 :
238 0 : } else if (r == 0) {
239 0 : have_syscall = true;
240 0 : return -EIO;
241 :
242 0 : } else if (errno == ENOSYS) {
243 : /* We lack the syscall, continue with reading from /dev/urandom. */
244 0 : have_syscall = false;
245 0 : break;
246 :
247 0 : } else if (errno == EAGAIN) {
248 : /* The kernel has no entropy whatsoever. Let's remember to use the syscall
249 : * the next time again though.
250 : *
251 : * If RANDOM_MAY_FAIL is set, return an error so that random_bytes() can
252 : * produce some pseudo-random bytes instead. Otherwise, fall back to
253 : * /dev/urandom, which we know is empty, but the kernel will produce some
254 : * bytes for us on a best-effort basis. */
255 0 : have_syscall = true;
256 :
257 0 : if (got_some && FLAGS_SET(flags, RANDOM_EXTEND_WITH_PSEUDO)) {
258 : /* Fill in the remaining bytes using pseudorandom values */
259 0 : pseudo_random_bytes(p, n);
260 0 : return 0;
261 : }
262 :
263 0 : if (FLAGS_SET(flags, RANDOM_MAY_FAIL))
264 0 : return -ENODATA;
265 :
266 : /* Use /dev/urandom instead */
267 0 : break;
268 : } else
269 0 : return -errno;
270 : }
271 : }
272 :
273 0 : fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
274 0 : if (fd < 0)
275 0 : return errno == ENOENT ? -ENOSYS : -errno;
276 :
277 0 : return loop_read_exact(fd, p, n, true);
278 : }
279 :
280 17 : void initialize_srand(void) {
281 : static bool srand_called = false;
282 : unsigned x;
283 : #if HAVE_SYS_AUXV_H
284 : const void *auxv;
285 : #endif
286 : unsigned long k;
287 :
288 17 : if (srand_called)
289 15 : return;
290 :
291 : #if HAVE_SYS_AUXV_H
292 : /* The kernel provides us with 16 bytes of entropy in auxv, so let's try to make use of that to seed
293 : * the pseudo-random generator. It's better than nothing... But let's first hash it to make it harder
294 : * to recover the original value by watching any pseudo-random bits we generate. After all the
295 : * AT_RANDOM data might be used by other stuff too (in particular: ASLR), and we probably shouldn't
296 : * leak the seed for that. */
297 :
298 2 : auxv = ULONG_TO_PTR(getauxval(AT_RANDOM));
299 2 : if (auxv) {
300 : static const uint8_t auxval_hash_key[16] = {
301 : 0x92, 0x6e, 0xfe, 0x1b, 0xcf, 0x00, 0x52, 0x9c, 0xcc, 0x42, 0xcf, 0xdc, 0x94, 0x1f, 0x81, 0x0f
302 : };
303 :
304 2 : x = (unsigned) siphash24(auxv, 16, auxval_hash_key);
305 : } else
306 : #endif
307 0 : x = 0;
308 :
309 2 : x ^= (unsigned) now(CLOCK_REALTIME);
310 2 : x ^= (unsigned) gettid();
311 :
312 2 : if (rdrand(&k) >= 0)
313 2 : x ^= (unsigned) k;
314 :
315 2 : srand(x);
316 2 : srand_called = true;
317 : }
318 :
319 : /* INT_MAX gives us only 31 bits, so use 24 out of that. */
320 : #if RAND_MAX >= INT_MAX
321 : # define RAND_STEP 3
322 : #else
323 : /* SHORT_INT_MAX or lower gives at most 15 bits, we just just 8 out of that. */
324 : # define RAND_STEP 1
325 : #endif
326 :
327 17 : void pseudo_random_bytes(void *p, size_t n) {
328 : uint8_t *q;
329 :
330 : /* This returns pseudo-random data using libc's rand() function. You probably never want to call this
331 : * directly, because why would you use this if you can get better stuff cheaply? Use random_bytes()
332 : * instead, see below: it will fall back to this function if there's nothing better to get, but only
333 : * then. */
334 :
335 17 : initialize_srand();
336 :
337 66 : for (q = p; q < (uint8_t*) p + n; q += RAND_STEP) {
338 : unsigned rr;
339 :
340 49 : rr = (unsigned) rand();
341 :
342 : #if RAND_STEP >= 3
343 49 : if ((size_t) (q - (uint8_t*) p + 2) < n)
344 37 : q[2] = rr >> 16;
345 : #endif
346 : #if RAND_STEP >= 2
347 49 : if ((size_t) (q - (uint8_t*) p + 1) < n)
348 42 : q[1] = rr >> 8;
349 : #endif
350 49 : q[0] = rr;
351 : }
352 17 : }
353 :
354 8158 : void random_bytes(void *p, size_t n) {
355 :
356 : /* This returns high quality randomness if we can get it cheaply. If we can't because for some reason
357 : * it is not available we'll try some crappy fallbacks.
358 : *
359 : * What this function will do:
360 : *
361 : * • This function will preferably use the CPU's RDRAND operation, if it is available, in
362 : * order to return "mid-quality" random values cheaply.
363 : *
364 : * • Use getrandom() with GRND_NONBLOCK, to return high-quality random values if they are
365 : * cheaply available.
366 : *
367 : * • This function will return pseudo-random data, generated via libc rand() if nothing
368 : * better is available.
369 : *
370 : * • This function will work fine in early boot
371 : *
372 : * • This function will always succeed
373 : *
374 : * What this function won't do:
375 : *
376 : * • This function will never fail: it will give you randomness no matter what. It might not
377 : * be high quality, but it will return some, possibly generated via libc's rand() call.
378 : *
379 : * • This function will never block: if the only way to get good randomness is a blocking,
380 : * synchronous getrandom() we'll instead provide you with pseudo-random data.
381 : *
382 : * This function is hence great for things like seeding hash tables, generating random numeric UNIX
383 : * user IDs (that are checked for collisions before use) and such.
384 : *
385 : * This function is hence not useful for generating UUIDs or cryptographic key material.
386 : */
387 :
388 8158 : if (genuine_random_bytes(p, n, RANDOM_EXTEND_WITH_PSEUDO|RANDOM_MAY_FAIL|RANDOM_ALLOW_RDRAND) >= 0)
389 8158 : return;
390 :
391 : /* If for some reason some user made /dev/urandom unavailable to us, or the kernel has no entropy, use a PRNG instead. */
392 0 : pseudo_random_bytes(p, n);
393 : }
394 :
395 0 : size_t random_pool_size(void) {
396 0 : _cleanup_free_ char *s = NULL;
397 : int r;
398 :
399 : /* Read pool size, if possible */
400 0 : r = read_one_line_file("/proc/sys/kernel/random/poolsize", &s);
401 0 : if (r < 0)
402 0 : log_debug_errno(r, "Failed to read pool size from kernel: %m");
403 : else {
404 : unsigned sz;
405 :
406 0 : r = safe_atou(s, &sz);
407 0 : if (r < 0)
408 0 : log_debug_errno(r, "Failed to parse pool size: %s", s);
409 : else
410 : /* poolsize is in bits on 2.6, but we want bytes */
411 0 : return CLAMP(sz / 8, RANDOM_POOL_SIZE_MIN, RANDOM_POOL_SIZE_MAX);
412 : }
413 :
414 : /* Use the minimum as default, if we can't retrieve the correct value */
415 0 : return RANDOM_POOL_SIZE_MIN;
416 : }
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