1 /*
2 * Non-physical true random number generator based on timing jitter --
3 * Jitter RNG standalone code.
4 *
5 * Copyright Stephan Mueller <smueller@chronox.de>, 2015 - 2023
6 *
7 * Design
8 * ======
9 *
10 * See https://www.chronox.de/jent.html
11 *
12 * License
13 * =======
14 *
15 * Redistribution and use in source and binary forms, with or without
16 * modification, are permitted provided that the following conditions
17 * are met:
18 * 1. Redistributions of source code must retain the above copyright
19 * notice, and the entire permission notice in its entirety,
20 * including the disclaimer of warranties.
21 * 2. Redistributions in binary form must reproduce the above copyright
22 * notice, this list of conditions and the following disclaimer in the
23 * documentation and/or other materials provided with the distribution.
24 * 3. The name of the author may not be used to endorse or promote
25 * products derived from this software without specific prior
26 * written permission.
27 *
28 * ALTERNATIVELY, this product may be distributed under the terms of
29 * the GNU General Public License, in which case the provisions of the GPL2 are
30 * required INSTEAD OF the above restrictions. (This clause is
31 * necessary due to a potential bad interaction between the GPL and
32 * the restrictions contained in a BSD-style copyright.)
33 *
34 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
35 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
36 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, ALL OF
37 * WHICH ARE HEREBY DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE
38 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
39 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
40 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
41 * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
42 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
43 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
44 * USE OF THIS SOFTWARE, EVEN IF NOT ADVISED OF THE POSSIBILITY OF SUCH
45 * DAMAGE.
46 */
47
48 /*
49 * This Jitterentropy RNG is based on the jitterentropy library
50 * version 3.4.0 provided at https://www.chronox.de/jent.html
51 */
52
53 #ifdef __OPTIMIZE__
54 #error "The CPU Jitter random number generator must not be compiled with optimizations. See documentation. Use the compiler switch -O0 for compiling jitterentropy.c."
55 #endif
56
57 typedef unsigned long long __u64;
58 typedef long long __s64;
59 typedef unsigned int __u32;
60 typedef unsigned char u8;
61 #define NULL ((void *) 0)
62
63 /* The entropy pool */
64 struct rand_data {
65 /* SHA3-256 is used as conditioner */
66 #define DATA_SIZE_BITS 256
67 /* all data values that are vital to maintain the security
68 * of the RNG are marked as SENSITIVE. A user must not
69 * access that information while the RNG executes its loops to
70 * calculate the next random value. */
71 void *hash_state; /* SENSITIVE hash state entropy pool */
72 __u64 prev_time; /* SENSITIVE Previous time stamp */
73 __u64 last_delta; /* SENSITIVE stuck test */
74 __s64 last_delta2; /* SENSITIVE stuck test */
75 unsigned int osr; /* Oversample rate */
76 #define JENT_MEMORY_BLOCKS 64
77 #define JENT_MEMORY_BLOCKSIZE 32
78 #define JENT_MEMORY_ACCESSLOOPS 128
79 #define JENT_MEMORY_SIZE (JENT_MEMORY_BLOCKS*JENT_MEMORY_BLOCKSIZE)
80 unsigned char *mem; /* Memory access location with size of
81 * memblocks * memblocksize */
82 unsigned int memlocation; /* Pointer to byte in *mem */
83 unsigned int memblocks; /* Number of memory blocks in *mem */
84 unsigned int memblocksize; /* Size of one memory block in bytes */
85 unsigned int memaccessloops; /* Number of memory accesses per random
86 * bit generation */
87
88 /* Repetition Count Test */
89 unsigned int rct_count; /* Number of stuck values */
90
91 /* Intermittent health test failure threshold of 2^-30 */
92 /* From an SP800-90B perspective, this RCT cutoff value is equal to 31. */
93 /* However, our RCT implementation starts at 1, so we subtract 1 here. */
94 #define JENT_RCT_CUTOFF (31 - 1) /* Taken from SP800-90B sec 4.4.1 */
95 #define JENT_APT_CUTOFF 325 /* Taken from SP800-90B sec 4.4.2 */
96 /* Permanent health test failure threshold of 2^-60 */
97 /* From an SP800-90B perspective, this RCT cutoff value is equal to 61. */
98 /* However, our RCT implementation starts at 1, so we subtract 1 here. */
99 #define JENT_RCT_CUTOFF_PERMANENT (61 - 1)
100 #define JENT_APT_CUTOFF_PERMANENT 355
101 #define JENT_APT_WINDOW_SIZE 512 /* Data window size */
102 /* LSB of time stamp to process */
103 #define JENT_APT_LSB 16
104 #define JENT_APT_WORD_MASK (JENT_APT_LSB - 1)
105 unsigned int apt_observations; /* Number of collected observations */
106 unsigned int apt_count; /* APT counter */
107 unsigned int apt_base; /* APT base reference */
108 unsigned int apt_base_set:1; /* APT base reference set? */
109 };
110
111 /* Flags that can be used to initialize the RNG */
112 #define JENT_DISABLE_MEMORY_ACCESS (1<<2) /* Disable memory access for more
113 * entropy, saves MEMORY_SIZE RAM for
114 * entropy collector */
115
116 /* -- error codes for init function -- */
117 #define JENT_ENOTIME 1 /* Timer service not available */
118 #define JENT_ECOARSETIME 2 /* Timer too coarse for RNG */
119 #define JENT_ENOMONOTONIC 3 /* Timer is not monotonic increasing */
120 #define JENT_EVARVAR 5 /* Timer does not produce variations of
121 * variations (2nd derivation of time is
122 * zero). */
123 #define JENT_ESTUCK 8 /* Too many stuck results during init. */
124 #define JENT_EHEALTH 9 /* Health test failed during initialization */
125
126 /*
127 * The output n bits can receive more than n bits of min entropy, of course,
128 * but the fixed output of the conditioning function can only asymptotically
129 * approach the output size bits of min entropy, not attain that bound. Random
130 * maps will tend to have output collisions, which reduces the creditable
131 * output entropy (that is what SP 800-90B Section 3.1.5.1.2 attempts to bound).
132 *
133 * The value "64" is justified in Appendix A.4 of the current 90C draft,
134 * and aligns with NIST's in "epsilon" definition in this document, which is
135 * that a string can be considered "full entropy" if you can bound the min
136 * entropy in each bit of output to at least 1-epsilon, where epsilon is
137 * required to be <= 2^(-32).
138 */
139 #define JENT_ENTROPY_SAFETY_FACTOR 64
140
141 #include <linux/fips.h>
142 #include "jitterentropy.h"
143
144 /***************************************************************************
145 * Adaptive Proportion Test
146 *
147 * This test complies with SP800-90B section 4.4.2.
148 ***************************************************************************/
149
150 /*
151 * Reset the APT counter
152 *
153 * @ec [in] Reference to entropy collector
154 */
jent_apt_reset(struct rand_data * ec,unsigned int delta_masked)155 static void jent_apt_reset(struct rand_data *ec, unsigned int delta_masked)
156 {
157 /* Reset APT counter */
158 ec->apt_count = 0;
159 ec->apt_base = delta_masked;
160 ec->apt_observations = 0;
161 }
162
163 /*
164 * Insert a new entropy event into APT
165 *
166 * @ec [in] Reference to entropy collector
167 * @delta_masked [in] Masked time delta to process
168 */
jent_apt_insert(struct rand_data * ec,unsigned int delta_masked)169 static void jent_apt_insert(struct rand_data *ec, unsigned int delta_masked)
170 {
171 /* Initialize the base reference */
172 if (!ec->apt_base_set) {
173 ec->apt_base = delta_masked;
174 ec->apt_base_set = 1;
175 return;
176 }
177
178 if (delta_masked == ec->apt_base)
179 ec->apt_count++;
180
181 ec->apt_observations++;
182
183 if (ec->apt_observations >= JENT_APT_WINDOW_SIZE)
184 jent_apt_reset(ec, delta_masked);
185 }
186
187 /* APT health test failure detection */
jent_apt_permanent_failure(struct rand_data * ec)188 static int jent_apt_permanent_failure(struct rand_data *ec)
189 {
190 return (ec->apt_count >= JENT_APT_CUTOFF_PERMANENT) ? 1 : 0;
191 }
192
jent_apt_failure(struct rand_data * ec)193 static int jent_apt_failure(struct rand_data *ec)
194 {
195 return (ec->apt_count >= JENT_APT_CUTOFF) ? 1 : 0;
196 }
197
198 /***************************************************************************
199 * Stuck Test and its use as Repetition Count Test
200 *
201 * The Jitter RNG uses an enhanced version of the Repetition Count Test
202 * (RCT) specified in SP800-90B section 4.4.1. Instead of counting identical
203 * back-to-back values, the input to the RCT is the counting of the stuck
204 * values during the generation of one Jitter RNG output block.
205 *
206 * The RCT is applied with an alpha of 2^{-30} compliant to FIPS 140-2 IG 9.8.
207 *
208 * During the counting operation, the Jitter RNG always calculates the RCT
209 * cut-off value of C. If that value exceeds the allowed cut-off value,
210 * the Jitter RNG output block will be calculated completely but discarded at
211 * the end. The caller of the Jitter RNG is informed with an error code.
212 ***************************************************************************/
213
214 /*
215 * Repetition Count Test as defined in SP800-90B section 4.4.1
216 *
217 * @ec [in] Reference to entropy collector
218 * @stuck [in] Indicator whether the value is stuck
219 */
jent_rct_insert(struct rand_data * ec,int stuck)220 static void jent_rct_insert(struct rand_data *ec, int stuck)
221 {
222 if (stuck) {
223 ec->rct_count++;
224 } else {
225 /* Reset RCT */
226 ec->rct_count = 0;
227 }
228 }
229
jent_delta(__u64 prev,__u64 next)230 static inline __u64 jent_delta(__u64 prev, __u64 next)
231 {
232 #define JENT_UINT64_MAX (__u64)(~((__u64) 0))
233 return (prev < next) ? (next - prev) :
234 (JENT_UINT64_MAX - prev + 1 + next);
235 }
236
237 /*
238 * Stuck test by checking the:
239 * 1st derivative of the jitter measurement (time delta)
240 * 2nd derivative of the jitter measurement (delta of time deltas)
241 * 3rd derivative of the jitter measurement (delta of delta of time deltas)
242 *
243 * All values must always be non-zero.
244 *
245 * @ec [in] Reference to entropy collector
246 * @current_delta [in] Jitter time delta
247 *
248 * @return
249 * 0 jitter measurement not stuck (good bit)
250 * 1 jitter measurement stuck (reject bit)
251 */
jent_stuck(struct rand_data * ec,__u64 current_delta)252 static int jent_stuck(struct rand_data *ec, __u64 current_delta)
253 {
254 __u64 delta2 = jent_delta(ec->last_delta, current_delta);
255 __u64 delta3 = jent_delta(ec->last_delta2, delta2);
256
257 ec->last_delta = current_delta;
258 ec->last_delta2 = delta2;
259
260 /*
261 * Insert the result of the comparison of two back-to-back time
262 * deltas.
263 */
264 jent_apt_insert(ec, current_delta);
265
266 if (!current_delta || !delta2 || !delta3) {
267 /* RCT with a stuck bit */
268 jent_rct_insert(ec, 1);
269 return 1;
270 }
271
272 /* RCT with a non-stuck bit */
273 jent_rct_insert(ec, 0);
274
275 return 0;
276 }
277
278 /* RCT health test failure detection */
jent_rct_permanent_failure(struct rand_data * ec)279 static int jent_rct_permanent_failure(struct rand_data *ec)
280 {
281 return (ec->rct_count >= JENT_RCT_CUTOFF_PERMANENT) ? 1 : 0;
282 }
283
jent_rct_failure(struct rand_data * ec)284 static int jent_rct_failure(struct rand_data *ec)
285 {
286 return (ec->rct_count >= JENT_RCT_CUTOFF) ? 1 : 0;
287 }
288
289 /* Report of health test failures */
jent_health_failure(struct rand_data * ec)290 static int jent_health_failure(struct rand_data *ec)
291 {
292 return jent_rct_failure(ec) | jent_apt_failure(ec);
293 }
294
jent_permanent_health_failure(struct rand_data * ec)295 static int jent_permanent_health_failure(struct rand_data *ec)
296 {
297 return jent_rct_permanent_failure(ec) | jent_apt_permanent_failure(ec);
298 }
299
300 /***************************************************************************
301 * Noise sources
302 ***************************************************************************/
303
304 /*
305 * Update of the loop count used for the next round of
306 * an entropy collection.
307 *
308 * Input:
309 * @bits is the number of low bits of the timer to consider
310 * @min is the number of bits we shift the timer value to the right at
311 * the end to make sure we have a guaranteed minimum value
312 *
313 * @return Newly calculated loop counter
314 */
jent_loop_shuffle(unsigned int bits,unsigned int min)315 static __u64 jent_loop_shuffle(unsigned int bits, unsigned int min)
316 {
317 __u64 time = 0;
318 __u64 shuffle = 0;
319 unsigned int i = 0;
320 unsigned int mask = (1<<bits) - 1;
321
322 jent_get_nstime(&time);
323
324 /*
325 * We fold the time value as much as possible to ensure that as many
326 * bits of the time stamp are included as possible.
327 */
328 for (i = 0; ((DATA_SIZE_BITS + bits - 1) / bits) > i; i++) {
329 shuffle ^= time & mask;
330 time = time >> bits;
331 }
332
333 /*
334 * We add a lower boundary value to ensure we have a minimum
335 * RNG loop count.
336 */
337 return (shuffle + (1<<min));
338 }
339
340 /*
341 * CPU Jitter noise source -- this is the noise source based on the CPU
342 * execution time jitter
343 *
344 * This function injects the individual bits of the time value into the
345 * entropy pool using a hash.
346 *
347 * ec [in] entropy collector
348 * time [in] time stamp to be injected
349 * stuck [in] Is the time stamp identified as stuck?
350 *
351 * Output:
352 * updated hash context in the entropy collector or error code
353 */
jent_condition_data(struct rand_data * ec,__u64 time,int stuck)354 static int jent_condition_data(struct rand_data *ec, __u64 time, int stuck)
355 {
356 #define SHA3_HASH_LOOP (1<<3)
357 struct {
358 int rct_count;
359 unsigned int apt_observations;
360 unsigned int apt_count;
361 unsigned int apt_base;
362 } addtl = {
363 ec->rct_count,
364 ec->apt_observations,
365 ec->apt_count,
366 ec->apt_base
367 };
368
369 return jent_hash_time(ec->hash_state, time, (u8 *)&addtl, sizeof(addtl),
370 SHA3_HASH_LOOP, stuck);
371 }
372
373 /*
374 * Memory Access noise source -- this is a noise source based on variations in
375 * memory access times
376 *
377 * This function performs memory accesses which will add to the timing
378 * variations due to an unknown amount of CPU wait states that need to be
379 * added when accessing memory. The memory size should be larger than the L1
380 * caches as outlined in the documentation and the associated testing.
381 *
382 * The L1 cache has a very high bandwidth, albeit its access rate is usually
383 * slower than accessing CPU registers. Therefore, L1 accesses only add minimal
384 * variations as the CPU has hardly to wait. Starting with L2, significant
385 * variations are added because L2 typically does not belong to the CPU any more
386 * and therefore a wider range of CPU wait states is necessary for accesses.
387 * L3 and real memory accesses have even a wider range of wait states. However,
388 * to reliably access either L3 or memory, the ec->mem memory must be quite
389 * large which is usually not desirable.
390 *
391 * @ec [in] Reference to the entropy collector with the memory access data -- if
392 * the reference to the memory block to be accessed is NULL, this noise
393 * source is disabled
394 * @loop_cnt [in] if a value not equal to 0 is set, use the given value
395 * number of loops to perform the LFSR
396 */
jent_memaccess(struct rand_data * ec,__u64 loop_cnt)397 static void jent_memaccess(struct rand_data *ec, __u64 loop_cnt)
398 {
399 unsigned int wrap = 0;
400 __u64 i = 0;
401 #define MAX_ACC_LOOP_BIT 7
402 #define MIN_ACC_LOOP_BIT 0
403 __u64 acc_loop_cnt =
404 jent_loop_shuffle(MAX_ACC_LOOP_BIT, MIN_ACC_LOOP_BIT);
405
406 if (NULL == ec || NULL == ec->mem)
407 return;
408 wrap = ec->memblocksize * ec->memblocks;
409
410 /*
411 * testing purposes -- allow test app to set the counter, not
412 * needed during runtime
413 */
414 if (loop_cnt)
415 acc_loop_cnt = loop_cnt;
416
417 for (i = 0; i < (ec->memaccessloops + acc_loop_cnt); i++) {
418 unsigned char *tmpval = ec->mem + ec->memlocation;
419 /*
420 * memory access: just add 1 to one byte,
421 * wrap at 255 -- memory access implies read
422 * from and write to memory location
423 */
424 *tmpval = (*tmpval + 1) & 0xff;
425 /*
426 * Addition of memblocksize - 1 to pointer
427 * with wrap around logic to ensure that every
428 * memory location is hit evenly
429 */
430 ec->memlocation = ec->memlocation + ec->memblocksize - 1;
431 ec->memlocation = ec->memlocation % wrap;
432 }
433 }
434
435 /***************************************************************************
436 * Start of entropy processing logic
437 ***************************************************************************/
438 /*
439 * This is the heart of the entropy generation: calculate time deltas and
440 * use the CPU jitter in the time deltas. The jitter is injected into the
441 * entropy pool.
442 *
443 * WARNING: ensure that ->prev_time is primed before using the output
444 * of this function! This can be done by calling this function
445 * and not using its result.
446 *
447 * @ec [in] Reference to entropy collector
448 *
449 * @return result of stuck test
450 */
jent_measure_jitter(struct rand_data * ec)451 static int jent_measure_jitter(struct rand_data *ec)
452 {
453 __u64 time = 0;
454 __u64 current_delta = 0;
455 int stuck;
456
457 /* Invoke one noise source before time measurement to add variations */
458 jent_memaccess(ec, 0);
459
460 /*
461 * Get time stamp and calculate time delta to previous
462 * invocation to measure the timing variations
463 */
464 jent_get_nstime(&time);
465 current_delta = jent_delta(ec->prev_time, time);
466 ec->prev_time = time;
467
468 /* Check whether we have a stuck measurement. */
469 stuck = jent_stuck(ec, current_delta);
470
471 /* Now call the next noise sources which also injects the data */
472 if (jent_condition_data(ec, current_delta, stuck))
473 stuck = 1;
474
475 return stuck;
476 }
477
478 /*
479 * Generator of one 64 bit random number
480 * Function fills rand_data->hash_state
481 *
482 * @ec [in] Reference to entropy collector
483 */
jent_gen_entropy(struct rand_data * ec)484 static void jent_gen_entropy(struct rand_data *ec)
485 {
486 unsigned int k = 0, safety_factor = 0;
487
488 if (fips_enabled)
489 safety_factor = JENT_ENTROPY_SAFETY_FACTOR;
490
491 /* priming of the ->prev_time value */
492 jent_measure_jitter(ec);
493
494 while (!jent_health_failure(ec)) {
495 /* If a stuck measurement is received, repeat measurement */
496 if (jent_measure_jitter(ec))
497 continue;
498
499 /*
500 * We multiply the loop value with ->osr to obtain the
501 * oversampling rate requested by the caller
502 */
503 if (++k >= ((DATA_SIZE_BITS + safety_factor) * ec->osr))
504 break;
505 }
506 }
507
508 /*
509 * Entry function: Obtain entropy for the caller.
510 *
511 * This function invokes the entropy gathering logic as often to generate
512 * as many bytes as requested by the caller. The entropy gathering logic
513 * creates 64 bit per invocation.
514 *
515 * This function truncates the last 64 bit entropy value output to the exact
516 * size specified by the caller.
517 *
518 * @ec [in] Reference to entropy collector
519 * @data [in] pointer to buffer for storing random data -- buffer must already
520 * exist
521 * @len [in] size of the buffer, specifying also the requested number of random
522 * in bytes
523 *
524 * @return 0 when request is fulfilled or an error
525 *
526 * The following error codes can occur:
527 * -1 entropy_collector is NULL or the generation failed
528 * -2 Intermittent health failure
529 * -3 Permanent health failure
530 */
jent_read_entropy(struct rand_data * ec,unsigned char * data,unsigned int len)531 int jent_read_entropy(struct rand_data *ec, unsigned char *data,
532 unsigned int len)
533 {
534 unsigned char *p = data;
535
536 if (!ec)
537 return -1;
538
539 while (len > 0) {
540 unsigned int tocopy;
541
542 jent_gen_entropy(ec);
543
544 if (jent_permanent_health_failure(ec)) {
545 /*
546 * At this point, the Jitter RNG instance is considered
547 * as a failed instance. There is no rerun of the
548 * startup test any more, because the caller
549 * is assumed to not further use this instance.
550 */
551 return -3;
552 } else if (jent_health_failure(ec)) {
553 /*
554 * Perform startup health tests and return permanent
555 * error if it fails.
556 */
557 if (jent_entropy_init(ec->hash_state))
558 return -3;
559
560 return -2;
561 }
562
563 if ((DATA_SIZE_BITS / 8) < len)
564 tocopy = (DATA_SIZE_BITS / 8);
565 else
566 tocopy = len;
567 if (jent_read_random_block(ec->hash_state, p, tocopy))
568 return -1;
569
570 len -= tocopy;
571 p += tocopy;
572 }
573
574 return 0;
575 }
576
577 /***************************************************************************
578 * Initialization logic
579 ***************************************************************************/
580
jent_entropy_collector_alloc(unsigned int osr,unsigned int flags,void * hash_state)581 struct rand_data *jent_entropy_collector_alloc(unsigned int osr,
582 unsigned int flags,
583 void *hash_state)
584 {
585 struct rand_data *entropy_collector;
586
587 entropy_collector = jent_zalloc(sizeof(struct rand_data));
588 if (!entropy_collector)
589 return NULL;
590
591 if (!(flags & JENT_DISABLE_MEMORY_ACCESS)) {
592 /* Allocate memory for adding variations based on memory
593 * access
594 */
595 entropy_collector->mem = jent_zalloc(JENT_MEMORY_SIZE);
596 if (!entropy_collector->mem) {
597 jent_zfree(entropy_collector);
598 return NULL;
599 }
600 entropy_collector->memblocksize = JENT_MEMORY_BLOCKSIZE;
601 entropy_collector->memblocks = JENT_MEMORY_BLOCKS;
602 entropy_collector->memaccessloops = JENT_MEMORY_ACCESSLOOPS;
603 }
604
605 /* verify and set the oversampling rate */
606 if (osr == 0)
607 osr = 1; /* minimum sampling rate is 1 */
608 entropy_collector->osr = osr;
609
610 entropy_collector->hash_state = hash_state;
611
612 /* fill the data pad with non-zero values */
613 jent_gen_entropy(entropy_collector);
614
615 return entropy_collector;
616 }
617
jent_entropy_collector_free(struct rand_data * entropy_collector)618 void jent_entropy_collector_free(struct rand_data *entropy_collector)
619 {
620 jent_zfree(entropy_collector->mem);
621 entropy_collector->mem = NULL;
622 jent_zfree(entropy_collector);
623 }
624
jent_entropy_init(void * hash_state)625 int jent_entropy_init(void *hash_state)
626 {
627 int i;
628 __u64 delta_sum = 0;
629 __u64 old_delta = 0;
630 unsigned int nonstuck = 0;
631 int time_backwards = 0;
632 int count_mod = 0;
633 int count_stuck = 0;
634 struct rand_data ec = { 0 };
635
636 /* Required for RCT */
637 ec.osr = 1;
638 ec.hash_state = hash_state;
639
640 /* We could perform statistical tests here, but the problem is
641 * that we only have a few loop counts to do testing. These
642 * loop counts may show some slight skew and we produce
643 * false positives.
644 *
645 * Moreover, only old systems show potentially problematic
646 * jitter entropy that could potentially be caught here. But
647 * the RNG is intended for hardware that is available or widely
648 * used, but not old systems that are long out of favor. Thus,
649 * no statistical tests.
650 */
651
652 /*
653 * We could add a check for system capabilities such as clock_getres or
654 * check for CONFIG_X86_TSC, but it does not make much sense as the
655 * following sanity checks verify that we have a high-resolution
656 * timer.
657 */
658 /*
659 * TESTLOOPCOUNT needs some loops to identify edge systems. 100 is
660 * definitely too little.
661 *
662 * SP800-90B requires at least 1024 initial test cycles.
663 */
664 #define TESTLOOPCOUNT 1024
665 #define CLEARCACHE 100
666 for (i = 0; (TESTLOOPCOUNT + CLEARCACHE) > i; i++) {
667 __u64 time = 0;
668 __u64 time2 = 0;
669 __u64 delta = 0;
670 unsigned int lowdelta = 0;
671 int stuck;
672
673 /* Invoke core entropy collection logic */
674 jent_get_nstime(&time);
675 ec.prev_time = time;
676 jent_condition_data(&ec, time, 0);
677 jent_get_nstime(&time2);
678
679 /* test whether timer works */
680 if (!time || !time2)
681 return JENT_ENOTIME;
682 delta = jent_delta(time, time2);
683 /*
684 * test whether timer is fine grained enough to provide
685 * delta even when called shortly after each other -- this
686 * implies that we also have a high resolution timer
687 */
688 if (!delta)
689 return JENT_ECOARSETIME;
690
691 stuck = jent_stuck(&ec, delta);
692
693 /*
694 * up to here we did not modify any variable that will be
695 * evaluated later, but we already performed some work. Thus we
696 * already have had an impact on the caches, branch prediction,
697 * etc. with the goal to clear it to get the worst case
698 * measurements.
699 */
700 if (i < CLEARCACHE)
701 continue;
702
703 if (stuck)
704 count_stuck++;
705 else {
706 nonstuck++;
707
708 /*
709 * Ensure that the APT succeeded.
710 *
711 * With the check below that count_stuck must be less
712 * than 10% of the overall generated raw entropy values
713 * it is guaranteed that the APT is invoked at
714 * floor((TESTLOOPCOUNT * 0.9) / 64) == 14 times.
715 */
716 if ((nonstuck % JENT_APT_WINDOW_SIZE) == 0) {
717 jent_apt_reset(&ec,
718 delta & JENT_APT_WORD_MASK);
719 }
720 }
721
722 /* Validate health test result */
723 if (jent_health_failure(&ec))
724 return JENT_EHEALTH;
725
726 /* test whether we have an increasing timer */
727 if (!(time2 > time))
728 time_backwards++;
729
730 /* use 32 bit value to ensure compilation on 32 bit arches */
731 lowdelta = time2 - time;
732 if (!(lowdelta % 100))
733 count_mod++;
734
735 /*
736 * ensure that we have a varying delta timer which is necessary
737 * for the calculation of entropy -- perform this check
738 * only after the first loop is executed as we need to prime
739 * the old_data value
740 */
741 if (delta > old_delta)
742 delta_sum += (delta - old_delta);
743 else
744 delta_sum += (old_delta - delta);
745 old_delta = delta;
746 }
747
748 /*
749 * we allow up to three times the time running backwards.
750 * CLOCK_REALTIME is affected by adjtime and NTP operations. Thus,
751 * if such an operation just happens to interfere with our test, it
752 * should not fail. The value of 3 should cover the NTP case being
753 * performed during our test run.
754 */
755 if (time_backwards > 3)
756 return JENT_ENOMONOTONIC;
757
758 /*
759 * Variations of deltas of time must on average be larger
760 * than 1 to ensure the entropy estimation
761 * implied with 1 is preserved
762 */
763 if ((delta_sum) <= 1)
764 return JENT_EVARVAR;
765
766 /*
767 * Ensure that we have variations in the time stamp below 10 for at
768 * least 10% of all checks -- on some platforms, the counter increments
769 * in multiples of 100, but not always
770 */
771 if ((TESTLOOPCOUNT/10 * 9) < count_mod)
772 return JENT_ECOARSETIME;
773
774 /*
775 * If we have more than 90% stuck results, then this Jitter RNG is
776 * likely to not work well.
777 */
778 if ((TESTLOOPCOUNT/10 * 9) < count_stuck)
779 return JENT_ESTUCK;
780
781 return 0;
782 }
783