1 /* 2 * random.c -- A strong random number generator 3 * 4 * Copyright Matt Mackall <mpm@selenic.com>, 2003, 2004, 2005 5 * 6 * Copyright Theodore Ts'o, 1994, 1995, 1996, 1997, 1998, 1999. All 7 * rights reserved. 8 * 9 * Redistribution and use in source and binary forms, with or without 10 * modification, are permitted provided that the following conditions 11 * are met: 12 * 1. Redistributions of source code must retain the above copyright 13 * notice, and the entire permission notice in its entirety, 14 * including the disclaimer of warranties. 15 * 2. Redistributions in binary form must reproduce the above copyright 16 * notice, this list of conditions and the following disclaimer in the 17 * documentation and/or other materials provided with the distribution. 18 * 3. The name of the author may not be used to endorse or promote 19 * products derived from this software without specific prior 20 * written permission. 21 * 22 * ALTERNATIVELY, this product may be distributed under the terms of 23 * the GNU General Public License, in which case the provisions of the GPL are 24 * required INSTEAD OF the above restrictions. (This clause is 25 * necessary due to a potential bad interaction between the GPL and 26 * the restrictions contained in a BSD-style copyright.) 27 * 28 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED 29 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 30 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, ALL OF 31 * WHICH ARE HEREBY DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE 32 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 33 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT 34 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR 35 * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF 36 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 37 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE 38 * USE OF THIS SOFTWARE, EVEN IF NOT ADVISED OF THE POSSIBILITY OF SUCH 39 * DAMAGE. 40 */ 41 42 /* 43 * (now, with legal B.S. out of the way.....) 44 * 45 * This routine gathers environmental noise from device drivers, etc., 46 * and returns good random numbers, suitable for cryptographic use. 47 * Besides the obvious cryptographic uses, these numbers are also good 48 * for seeding TCP sequence numbers, and other places where it is 49 * desirable to have numbers which are not only random, but hard to 50 * predict by an attacker. 51 * 52 * Theory of operation 53 * =================== 54 * 55 * Computers are very predictable devices. Hence it is extremely hard 56 * to produce truly random numbers on a computer --- as opposed to 57 * pseudo-random numbers, which can easily generated by using a 58 * algorithm. Unfortunately, it is very easy for attackers to guess 59 * the sequence of pseudo-random number generators, and for some 60 * applications this is not acceptable. So instead, we must try to 61 * gather "environmental noise" from the computer's environment, which 62 * must be hard for outside attackers to observe, and use that to 63 * generate random numbers. In a Unix environment, this is best done 64 * from inside the kernel. 65 * 66 * Sources of randomness from the environment include inter-keyboard 67 * timings, inter-interrupt timings from some interrupts, and other 68 * events which are both (a) non-deterministic and (b) hard for an 69 * outside observer to measure. Randomness from these sources are 70 * added to an "entropy pool", which is mixed using a CRC-like function. 71 * This is not cryptographically strong, but it is adequate assuming 72 * the randomness is not chosen maliciously, and it is fast enough that 73 * the overhead of doing it on every interrupt is very reasonable. 74 * As random bytes are mixed into the entropy pool, the routines keep 75 * an *estimate* of how many bits of randomness have been stored into 76 * the random number generator's internal state. 77 * 78 * When random bytes are desired, they are obtained by taking the SHA 79 * hash of the contents of the "entropy pool". The SHA hash avoids 80 * exposing the internal state of the entropy pool. It is believed to 81 * be computationally infeasible to derive any useful information 82 * about the input of SHA from its output. Even if it is possible to 83 * analyze SHA in some clever way, as long as the amount of data 84 * returned from the generator is less than the inherent entropy in 85 * the pool, the output data is totally unpredictable. For this 86 * reason, the routine decreases its internal estimate of how many 87 * bits of "true randomness" are contained in the entropy pool as it 88 * outputs random numbers. 89 * 90 * If this estimate goes to zero, the routine can still generate 91 * random numbers; however, an attacker may (at least in theory) be 92 * able to infer the future output of the generator from prior 93 * outputs. This requires successful cryptanalysis of SHA, which is 94 * not believed to be feasible, but there is a remote possibility. 95 * Nonetheless, these numbers should be useful for the vast majority 96 * of purposes. 97 * 98 * Exported interfaces ---- output 99 * =============================== 100 * 101 * There are three exported interfaces; the first is one designed to 102 * be used from within the kernel: 103 * 104 * void get_random_bytes(void *buf, int nbytes); 105 * 106 * This interface will return the requested number of random bytes, 107 * and place it in the requested buffer. 108 * 109 * The two other interfaces are two character devices /dev/random and 110 * /dev/urandom. /dev/random is suitable for use when very high 111 * quality randomness is desired (for example, for key generation or 112 * one-time pads), as it will only return a maximum of the number of 113 * bits of randomness (as estimated by the random number generator) 114 * contained in the entropy pool. 115 * 116 * The /dev/urandom device does not have this limit, and will return 117 * as many bytes as are requested. As more and more random bytes are 118 * requested without giving time for the entropy pool to recharge, 119 * this will result in random numbers that are merely cryptographically 120 * strong. For many applications, however, this is acceptable. 121 * 122 * Exported interfaces ---- input 123 * ============================== 124 * 125 * The current exported interfaces for gathering environmental noise 126 * from the devices are: 127 * 128 * void add_input_randomness(unsigned int type, unsigned int code, 129 * unsigned int value); 130 * void add_interrupt_randomness(int irq); 131 * void add_disk_randomness(struct gendisk *disk); 132 * 133 * add_input_randomness() uses the input layer interrupt timing, as well as 134 * the event type information from the hardware. 135 * 136 * add_interrupt_randomness() uses the inter-interrupt timing as random 137 * inputs to the entropy pool. Note that not all interrupts are good 138 * sources of randomness! For example, the timer interrupts is not a 139 * good choice, because the periodicity of the interrupts is too 140 * regular, and hence predictable to an attacker. Network Interface 141 * Controller interrupts are a better measure, since the timing of the 142 * NIC interrupts are more unpredictable. 143 * 144 * add_disk_randomness() uses what amounts to the seek time of block 145 * layer request events, on a per-disk_devt basis, as input to the 146 * entropy pool. Note that high-speed solid state drives with very low 147 * seek times do not make for good sources of entropy, as their seek 148 * times are usually fairly consistent. 149 * 150 * All of these routines try to estimate how many bits of randomness a 151 * particular randomness source. They do this by keeping track of the 152 * first and second order deltas of the event timings. 153 * 154 * Ensuring unpredictability at system startup 155 * ============================================ 156 * 157 * When any operating system starts up, it will go through a sequence 158 * of actions that are fairly predictable by an adversary, especially 159 * if the start-up does not involve interaction with a human operator. 160 * This reduces the actual number of bits of unpredictability in the 161 * entropy pool below the value in entropy_count. In order to 162 * counteract this effect, it helps to carry information in the 163 * entropy pool across shut-downs and start-ups. To do this, put the 164 * following lines an appropriate script which is run during the boot 165 * sequence: 166 * 167 * echo "Initializing random number generator..." 168 * random_seed=/var/run/random-seed 169 * # Carry a random seed from start-up to start-up 170 * # Load and then save the whole entropy pool 171 * if [ -f $random_seed ]; then 172 * cat $random_seed >/dev/urandom 173 * else 174 * touch $random_seed 175 * fi 176 * chmod 600 $random_seed 177 * dd if=/dev/urandom of=$random_seed count=1 bs=512 178 * 179 * and the following lines in an appropriate script which is run as 180 * the system is shutdown: 181 * 182 * # Carry a random seed from shut-down to start-up 183 * # Save the whole entropy pool 184 * echo "Saving random seed..." 185 * random_seed=/var/run/random-seed 186 * touch $random_seed 187 * chmod 600 $random_seed 188 * dd if=/dev/urandom of=$random_seed count=1 bs=512 189 * 190 * For example, on most modern systems using the System V init 191 * scripts, such code fragments would be found in 192 * /etc/rc.d/init.d/random. On older Linux systems, the correct script 193 * location might be in /etc/rcb.d/rc.local or /etc/rc.d/rc.0. 194 * 195 * Effectively, these commands cause the contents of the entropy pool 196 * to be saved at shut-down time and reloaded into the entropy pool at 197 * start-up. (The 'dd' in the addition to the bootup script is to 198 * make sure that /etc/random-seed is different for every start-up, 199 * even if the system crashes without executing rc.0.) Even with 200 * complete knowledge of the start-up activities, predicting the state 201 * of the entropy pool requires knowledge of the previous history of 202 * the system. 203 * 204 * Configuring the /dev/random driver under Linux 205 * ============================================== 206 * 207 * The /dev/random driver under Linux uses minor numbers 8 and 9 of 208 * the /dev/mem major number (#1). So if your system does not have 209 * /dev/random and /dev/urandom created already, they can be created 210 * by using the commands: 211 * 212 * mknod /dev/random c 1 8 213 * mknod /dev/urandom c 1 9 214 * 215 * Acknowledgements: 216 * ================= 217 * 218 * Ideas for constructing this random number generator were derived 219 * from Pretty Good Privacy's random number generator, and from private 220 * discussions with Phil Karn. Colin Plumb provided a faster random 221 * number generator, which speed up the mixing function of the entropy 222 * pool, taken from PGPfone. Dale Worley has also contributed many 223 * useful ideas and suggestions to improve this driver. 224 * 225 * Any flaws in the design are solely my responsibility, and should 226 * not be attributed to the Phil, Colin, or any of authors of PGP. 227 * 228 * Further background information on this topic may be obtained from 229 * RFC 1750, "Randomness Recommendations for Security", by Donald 230 * Eastlake, Steve Crocker, and Jeff Schiller. 231 */ 232 233 #include <linux/utsname.h> 234 #include <linux/module.h> 235 #include <linux/kernel.h> 236 #include <linux/major.h> 237 #include <linux/string.h> 238 #include <linux/fcntl.h> 239 #include <linux/slab.h> 240 #include <linux/random.h> 241 #include <linux/poll.h> 242 #include <linux/init.h> 243 #include <linux/fs.h> 244 #include <linux/genhd.h> 245 #include <linux/interrupt.h> 246 #include <linux/mm.h> 247 #include <linux/spinlock.h> 248 #include <linux/percpu.h> 249 #include <linux/cryptohash.h> 250 #include <linux/fips.h> 251 252 #ifdef CONFIG_GENERIC_HARDIRQS 253 # include <linux/irq.h> 254 #endif 255 256 #include <asm/processor.h> 257 #include <asm/uaccess.h> 258 #include <asm/irq.h> 259 #include <asm/io.h> 260 261 /* 262 * Configuration information 263 */ 264 #define INPUT_POOL_WORDS 128 265 #define OUTPUT_POOL_WORDS 32 266 #define SEC_XFER_SIZE 512 267 #define EXTRACT_SIZE 10 268 269 /* 270 * The minimum number of bits of entropy before we wake up a read on 271 * /dev/random. Should be enough to do a significant reseed. 272 */ 273 static int random_read_wakeup_thresh = 64; 274 275 /* 276 * If the entropy count falls under this number of bits, then we 277 * should wake up processes which are selecting or polling on write 278 * access to /dev/random. 279 */ 280 static int random_write_wakeup_thresh = 128; 281 282 /* 283 * When the input pool goes over trickle_thresh, start dropping most 284 * samples to avoid wasting CPU time and reduce lock contention. 285 */ 286 287 static int trickle_thresh __read_mostly = INPUT_POOL_WORDS * 28; 288 289 static DEFINE_PER_CPU(int, trickle_count); 290 291 /* 292 * A pool of size .poolwords is stirred with a primitive polynomial 293 * of degree .poolwords over GF(2). The taps for various sizes are 294 * defined below. They are chosen to be evenly spaced (minimum RMS 295 * distance from evenly spaced; the numbers in the comments are a 296 * scaled squared error sum) except for the last tap, which is 1 to 297 * get the twisting happening as fast as possible. 298 */ 299 static struct poolinfo { 300 int poolwords; 301 int tap1, tap2, tap3, tap4, tap5; 302 } poolinfo_table[] = { 303 /* x^128 + x^103 + x^76 + x^51 +x^25 + x + 1 -- 105 */ 304 { 128, 103, 76, 51, 25, 1 }, 305 /* x^32 + x^26 + x^20 + x^14 + x^7 + x + 1 -- 15 */ 306 { 32, 26, 20, 14, 7, 1 }, 307 #if 0 308 /* x^2048 + x^1638 + x^1231 + x^819 + x^411 + x + 1 -- 115 */ 309 { 2048, 1638, 1231, 819, 411, 1 }, 310 311 /* x^1024 + x^817 + x^615 + x^412 + x^204 + x + 1 -- 290 */ 312 { 1024, 817, 615, 412, 204, 1 }, 313 314 /* x^1024 + x^819 + x^616 + x^410 + x^207 + x^2 + 1 -- 115 */ 315 { 1024, 819, 616, 410, 207, 2 }, 316 317 /* x^512 + x^411 + x^308 + x^208 + x^104 + x + 1 -- 225 */ 318 { 512, 411, 308, 208, 104, 1 }, 319 320 /* x^512 + x^409 + x^307 + x^206 + x^102 + x^2 + 1 -- 95 */ 321 { 512, 409, 307, 206, 102, 2 }, 322 /* x^512 + x^409 + x^309 + x^205 + x^103 + x^2 + 1 -- 95 */ 323 { 512, 409, 309, 205, 103, 2 }, 324 325 /* x^256 + x^205 + x^155 + x^101 + x^52 + x + 1 -- 125 */ 326 { 256, 205, 155, 101, 52, 1 }, 327 328 /* x^128 + x^103 + x^78 + x^51 + x^27 + x^2 + 1 -- 70 */ 329 { 128, 103, 78, 51, 27, 2 }, 330 331 /* x^64 + x^52 + x^39 + x^26 + x^14 + x + 1 -- 15 */ 332 { 64, 52, 39, 26, 14, 1 }, 333 #endif 334 }; 335 336 #define POOLBITS poolwords*32 337 #define POOLBYTES poolwords*4 338 339 /* 340 * For the purposes of better mixing, we use the CRC-32 polynomial as 341 * well to make a twisted Generalized Feedback Shift Reigster 342 * 343 * (See M. Matsumoto & Y. Kurita, 1992. Twisted GFSR generators. ACM 344 * Transactions on Modeling and Computer Simulation 2(3):179-194. 345 * Also see M. Matsumoto & Y. Kurita, 1994. Twisted GFSR generators 346 * II. ACM Transactions on Mdeling and Computer Simulation 4:254-266) 347 * 348 * Thanks to Colin Plumb for suggesting this. 349 * 350 * We have not analyzed the resultant polynomial to prove it primitive; 351 * in fact it almost certainly isn't. Nonetheless, the irreducible factors 352 * of a random large-degree polynomial over GF(2) are more than large enough 353 * that periodicity is not a concern. 354 * 355 * The input hash is much less sensitive than the output hash. All 356 * that we want of it is that it be a good non-cryptographic hash; 357 * i.e. it not produce collisions when fed "random" data of the sort 358 * we expect to see. As long as the pool state differs for different 359 * inputs, we have preserved the input entropy and done a good job. 360 * The fact that an intelligent attacker can construct inputs that 361 * will produce controlled alterations to the pool's state is not 362 * important because we don't consider such inputs to contribute any 363 * randomness. The only property we need with respect to them is that 364 * the attacker can't increase his/her knowledge of the pool's state. 365 * Since all additions are reversible (knowing the final state and the 366 * input, you can reconstruct the initial state), if an attacker has 367 * any uncertainty about the initial state, he/she can only shuffle 368 * that uncertainty about, but never cause any collisions (which would 369 * decrease the uncertainty). 370 * 371 * The chosen system lets the state of the pool be (essentially) the input 372 * modulo the generator polymnomial. Now, for random primitive polynomials, 373 * this is a universal class of hash functions, meaning that the chance 374 * of a collision is limited by the attacker's knowledge of the generator 375 * polynomail, so if it is chosen at random, an attacker can never force 376 * a collision. Here, we use a fixed polynomial, but we *can* assume that 377 * ###--> it is unknown to the processes generating the input entropy. <-### 378 * Because of this important property, this is a good, collision-resistant 379 * hash; hash collisions will occur no more often than chance. 380 */ 381 382 /* 383 * Static global variables 384 */ 385 static DECLARE_WAIT_QUEUE_HEAD(random_read_wait); 386 static DECLARE_WAIT_QUEUE_HEAD(random_write_wait); 387 static struct fasync_struct *fasync; 388 389 #if 0 390 static bool debug; 391 module_param(debug, bool, 0644); 392 #define DEBUG_ENT(fmt, arg...) do { \ 393 if (debug) \ 394 printk(KERN_DEBUG "random %04d %04d %04d: " \ 395 fmt,\ 396 input_pool.entropy_count,\ 397 blocking_pool.entropy_count,\ 398 nonblocking_pool.entropy_count,\ 399 ## arg); } while (0) 400 #else 401 #define DEBUG_ENT(fmt, arg...) do {} while (0) 402 #endif 403 404 /********************************************************************** 405 * 406 * OS independent entropy store. Here are the functions which handle 407 * storing entropy in an entropy pool. 408 * 409 **********************************************************************/ 410 411 struct entropy_store; 412 struct entropy_store { 413 /* read-only data: */ 414 struct poolinfo *poolinfo; 415 __u32 *pool; 416 const char *name; 417 struct entropy_store *pull; 418 int limit; 419 420 /* read-write data: */ 421 spinlock_t lock; 422 unsigned add_ptr; 423 int entropy_count; 424 int input_rotate; 425 __u8 last_data[EXTRACT_SIZE]; 426 }; 427 428 static __u32 input_pool_data[INPUT_POOL_WORDS]; 429 static __u32 blocking_pool_data[OUTPUT_POOL_WORDS]; 430 static __u32 nonblocking_pool_data[OUTPUT_POOL_WORDS]; 431 432 static struct entropy_store input_pool = { 433 .poolinfo = &poolinfo_table[0], 434 .name = "input", 435 .limit = 1, 436 .lock = __SPIN_LOCK_UNLOCKED(&input_pool.lock), 437 .pool = input_pool_data 438 }; 439 440 static struct entropy_store blocking_pool = { 441 .poolinfo = &poolinfo_table[1], 442 .name = "blocking", 443 .limit = 1, 444 .pull = &input_pool, 445 .lock = __SPIN_LOCK_UNLOCKED(&blocking_pool.lock), 446 .pool = blocking_pool_data 447 }; 448 449 static struct entropy_store nonblocking_pool = { 450 .poolinfo = &poolinfo_table[1], 451 .name = "nonblocking", 452 .pull = &input_pool, 453 .lock = __SPIN_LOCK_UNLOCKED(&nonblocking_pool.lock), 454 .pool = nonblocking_pool_data 455 }; 456 457 /* 458 * This function adds bytes into the entropy "pool". It does not 459 * update the entropy estimate. The caller should call 460 * credit_entropy_bits if this is appropriate. 461 * 462 * The pool is stirred with a primitive polynomial of the appropriate 463 * degree, and then twisted. We twist by three bits at a time because 464 * it's cheap to do so and helps slightly in the expected case where 465 * the entropy is concentrated in the low-order bits. 466 */ 467 static void mix_pool_bytes_extract(struct entropy_store *r, const void *in, 468 int nbytes, __u8 out[64]) 469 { 470 static __u32 const twist_table[8] = { 471 0x00000000, 0x3b6e20c8, 0x76dc4190, 0x4db26158, 472 0xedb88320, 0xd6d6a3e8, 0x9b64c2b0, 0xa00ae278 }; 473 unsigned long i, j, tap1, tap2, tap3, tap4, tap5; 474 int input_rotate; 475 int wordmask = r->poolinfo->poolwords - 1; 476 const char *bytes = in; 477 __u32 w; 478 unsigned long flags; 479 480 /* Taps are constant, so we can load them without holding r->lock. */ 481 tap1 = r->poolinfo->tap1; 482 tap2 = r->poolinfo->tap2; 483 tap3 = r->poolinfo->tap3; 484 tap4 = r->poolinfo->tap4; 485 tap5 = r->poolinfo->tap5; 486 487 spin_lock_irqsave(&r->lock, flags); 488 input_rotate = r->input_rotate; 489 i = r->add_ptr; 490 491 /* mix one byte at a time to simplify size handling and churn faster */ 492 while (nbytes--) { 493 w = rol32(*bytes++, input_rotate & 31); 494 i = (i - 1) & wordmask; 495 496 /* XOR in the various taps */ 497 w ^= r->pool[i]; 498 w ^= r->pool[(i + tap1) & wordmask]; 499 w ^= r->pool[(i + tap2) & wordmask]; 500 w ^= r->pool[(i + tap3) & wordmask]; 501 w ^= r->pool[(i + tap4) & wordmask]; 502 w ^= r->pool[(i + tap5) & wordmask]; 503 504 /* Mix the result back in with a twist */ 505 r->pool[i] = (w >> 3) ^ twist_table[w & 7]; 506 507 /* 508 * Normally, we add 7 bits of rotation to the pool. 509 * At the beginning of the pool, add an extra 7 bits 510 * rotation, so that successive passes spread the 511 * input bits across the pool evenly. 512 */ 513 input_rotate += i ? 7 : 14; 514 } 515 516 r->input_rotate = input_rotate; 517 r->add_ptr = i; 518 519 if (out) 520 for (j = 0; j < 16; j++) 521 ((__u32 *)out)[j] = r->pool[(i - j) & wordmask]; 522 523 spin_unlock_irqrestore(&r->lock, flags); 524 } 525 526 static void mix_pool_bytes(struct entropy_store *r, const void *in, int bytes) 527 { 528 mix_pool_bytes_extract(r, in, bytes, NULL); 529 } 530 531 /* 532 * Credit (or debit) the entropy store with n bits of entropy 533 */ 534 static void credit_entropy_bits(struct entropy_store *r, int nbits) 535 { 536 unsigned long flags; 537 int entropy_count; 538 539 if (!nbits) 540 return; 541 542 spin_lock_irqsave(&r->lock, flags); 543 544 DEBUG_ENT("added %d entropy credits to %s\n", nbits, r->name); 545 entropy_count = r->entropy_count; 546 entropy_count += nbits; 547 if (entropy_count < 0) { 548 DEBUG_ENT("negative entropy/overflow\n"); 549 entropy_count = 0; 550 } else if (entropy_count > r->poolinfo->POOLBITS) 551 entropy_count = r->poolinfo->POOLBITS; 552 r->entropy_count = entropy_count; 553 554 /* should we wake readers? */ 555 if (r == &input_pool && entropy_count >= random_read_wakeup_thresh) { 556 wake_up_interruptible(&random_read_wait); 557 kill_fasync(&fasync, SIGIO, POLL_IN); 558 } 559 spin_unlock_irqrestore(&r->lock, flags); 560 } 561 562 /********************************************************************* 563 * 564 * Entropy input management 565 * 566 *********************************************************************/ 567 568 /* There is one of these per entropy source */ 569 struct timer_rand_state { 570 cycles_t last_time; 571 long last_delta, last_delta2; 572 unsigned dont_count_entropy:1; 573 }; 574 575 #ifndef CONFIG_GENERIC_HARDIRQS 576 577 static struct timer_rand_state *irq_timer_state[NR_IRQS]; 578 579 static struct timer_rand_state *get_timer_rand_state(unsigned int irq) 580 { 581 return irq_timer_state[irq]; 582 } 583 584 static void set_timer_rand_state(unsigned int irq, 585 struct timer_rand_state *state) 586 { 587 irq_timer_state[irq] = state; 588 } 589 590 #else 591 592 static struct timer_rand_state *get_timer_rand_state(unsigned int irq) 593 { 594 struct irq_desc *desc; 595 596 desc = irq_to_desc(irq); 597 598 return desc->timer_rand_state; 599 } 600 601 static void set_timer_rand_state(unsigned int irq, 602 struct timer_rand_state *state) 603 { 604 struct irq_desc *desc; 605 606 desc = irq_to_desc(irq); 607 608 desc->timer_rand_state = state; 609 } 610 #endif 611 612 static struct timer_rand_state input_timer_state; 613 614 /* 615 * This function adds entropy to the entropy "pool" by using timing 616 * delays. It uses the timer_rand_state structure to make an estimate 617 * of how many bits of entropy this call has added to the pool. 618 * 619 * The number "num" is also added to the pool - it should somehow describe 620 * the type of event which just happened. This is currently 0-255 for 621 * keyboard scan codes, and 256 upwards for interrupts. 622 * 623 */ 624 static void add_timer_randomness(struct timer_rand_state *state, unsigned num) 625 { 626 struct { 627 long jiffies; 628 unsigned cycles; 629 unsigned num; 630 } sample; 631 long delta, delta2, delta3; 632 633 preempt_disable(); 634 /* if over the trickle threshold, use only 1 in 4096 samples */ 635 if (input_pool.entropy_count > trickle_thresh && 636 ((__this_cpu_inc_return(trickle_count) - 1) & 0xfff)) 637 goto out; 638 639 sample.jiffies = jiffies; 640 641 /* Use arch random value, fall back to cycles */ 642 if (!arch_get_random_int(&sample.cycles)) 643 sample.cycles = get_cycles(); 644 645 sample.num = num; 646 mix_pool_bytes(&input_pool, &sample, sizeof(sample)); 647 648 /* 649 * Calculate number of bits of randomness we probably added. 650 * We take into account the first, second and third-order deltas 651 * in order to make our estimate. 652 */ 653 654 if (!state->dont_count_entropy) { 655 delta = sample.jiffies - state->last_time; 656 state->last_time = sample.jiffies; 657 658 delta2 = delta - state->last_delta; 659 state->last_delta = delta; 660 661 delta3 = delta2 - state->last_delta2; 662 state->last_delta2 = delta2; 663 664 if (delta < 0) 665 delta = -delta; 666 if (delta2 < 0) 667 delta2 = -delta2; 668 if (delta3 < 0) 669 delta3 = -delta3; 670 if (delta > delta2) 671 delta = delta2; 672 if (delta > delta3) 673 delta = delta3; 674 675 /* 676 * delta is now minimum absolute delta. 677 * Round down by 1 bit on general principles, 678 * and limit entropy entimate to 12 bits. 679 */ 680 credit_entropy_bits(&input_pool, 681 min_t(int, fls(delta>>1), 11)); 682 } 683 out: 684 preempt_enable(); 685 } 686 687 void add_input_randomness(unsigned int type, unsigned int code, 688 unsigned int value) 689 { 690 static unsigned char last_value; 691 692 /* ignore autorepeat and the like */ 693 if (value == last_value) 694 return; 695 696 DEBUG_ENT("input event\n"); 697 last_value = value; 698 add_timer_randomness(&input_timer_state, 699 (type << 4) ^ code ^ (code >> 4) ^ value); 700 } 701 EXPORT_SYMBOL_GPL(add_input_randomness); 702 703 void add_interrupt_randomness(int irq) 704 { 705 struct timer_rand_state *state; 706 707 state = get_timer_rand_state(irq); 708 709 if (state == NULL) 710 return; 711 712 DEBUG_ENT("irq event %d\n", irq); 713 add_timer_randomness(state, 0x100 + irq); 714 } 715 716 #ifdef CONFIG_BLOCK 717 void add_disk_randomness(struct gendisk *disk) 718 { 719 if (!disk || !disk->random) 720 return; 721 /* first major is 1, so we get >= 0x200 here */ 722 DEBUG_ENT("disk event %d:%d\n", 723 MAJOR(disk_devt(disk)), MINOR(disk_devt(disk))); 724 725 add_timer_randomness(disk->random, 0x100 + disk_devt(disk)); 726 } 727 #endif 728 729 /********************************************************************* 730 * 731 * Entropy extraction routines 732 * 733 *********************************************************************/ 734 735 static ssize_t extract_entropy(struct entropy_store *r, void *buf, 736 size_t nbytes, int min, int rsvd); 737 738 /* 739 * This utility inline function is responsible for transferring entropy 740 * from the primary pool to the secondary extraction pool. We make 741 * sure we pull enough for a 'catastrophic reseed'. 742 */ 743 static void xfer_secondary_pool(struct entropy_store *r, size_t nbytes) 744 { 745 __u32 tmp[OUTPUT_POOL_WORDS]; 746 747 if (r->pull && r->entropy_count < nbytes * 8 && 748 r->entropy_count < r->poolinfo->POOLBITS) { 749 /* If we're limited, always leave two wakeup worth's BITS */ 750 int rsvd = r->limit ? 0 : random_read_wakeup_thresh/4; 751 int bytes = nbytes; 752 753 /* pull at least as many as BYTES as wakeup BITS */ 754 bytes = max_t(int, bytes, random_read_wakeup_thresh / 8); 755 /* but never more than the buffer size */ 756 bytes = min_t(int, bytes, sizeof(tmp)); 757 758 DEBUG_ENT("going to reseed %s with %d bits " 759 "(%d of %d requested)\n", 760 r->name, bytes * 8, nbytes * 8, r->entropy_count); 761 762 bytes = extract_entropy(r->pull, tmp, bytes, 763 random_read_wakeup_thresh / 8, rsvd); 764 mix_pool_bytes(r, tmp, bytes); 765 credit_entropy_bits(r, bytes*8); 766 } 767 } 768 769 /* 770 * These functions extracts randomness from the "entropy pool", and 771 * returns it in a buffer. 772 * 773 * The min parameter specifies the minimum amount we can pull before 774 * failing to avoid races that defeat catastrophic reseeding while the 775 * reserved parameter indicates how much entropy we must leave in the 776 * pool after each pull to avoid starving other readers. 777 * 778 * Note: extract_entropy() assumes that .poolwords is a multiple of 16 words. 779 */ 780 781 static size_t account(struct entropy_store *r, size_t nbytes, int min, 782 int reserved) 783 { 784 unsigned long flags; 785 786 /* Hold lock while accounting */ 787 spin_lock_irqsave(&r->lock, flags); 788 789 BUG_ON(r->entropy_count > r->poolinfo->POOLBITS); 790 DEBUG_ENT("trying to extract %d bits from %s\n", 791 nbytes * 8, r->name); 792 793 /* Can we pull enough? */ 794 if (r->entropy_count / 8 < min + reserved) { 795 nbytes = 0; 796 } else { 797 /* If limited, never pull more than available */ 798 if (r->limit && nbytes + reserved >= r->entropy_count / 8) 799 nbytes = r->entropy_count/8 - reserved; 800 801 if (r->entropy_count / 8 >= nbytes + reserved) 802 r->entropy_count -= nbytes*8; 803 else 804 r->entropy_count = reserved; 805 806 if (r->entropy_count < random_write_wakeup_thresh) { 807 wake_up_interruptible(&random_write_wait); 808 kill_fasync(&fasync, SIGIO, POLL_OUT); 809 } 810 } 811 812 DEBUG_ENT("debiting %d entropy credits from %s%s\n", 813 nbytes * 8, r->name, r->limit ? "" : " (unlimited)"); 814 815 spin_unlock_irqrestore(&r->lock, flags); 816 817 return nbytes; 818 } 819 820 static void extract_buf(struct entropy_store *r, __u8 *out) 821 { 822 int i; 823 __u32 hash[5], workspace[SHA_WORKSPACE_WORDS]; 824 __u8 extract[64]; 825 826 /* Generate a hash across the pool, 16 words (512 bits) at a time */ 827 sha_init(hash); 828 for (i = 0; i < r->poolinfo->poolwords; i += 16) 829 sha_transform(hash, (__u8 *)(r->pool + i), workspace); 830 831 /* 832 * We mix the hash back into the pool to prevent backtracking 833 * attacks (where the attacker knows the state of the pool 834 * plus the current outputs, and attempts to find previous 835 * ouputs), unless the hash function can be inverted. By 836 * mixing at least a SHA1 worth of hash data back, we make 837 * brute-forcing the feedback as hard as brute-forcing the 838 * hash. 839 */ 840 mix_pool_bytes_extract(r, hash, sizeof(hash), extract); 841 842 /* 843 * To avoid duplicates, we atomically extract a portion of the 844 * pool while mixing, and hash one final time. 845 */ 846 sha_transform(hash, extract, workspace); 847 memset(extract, 0, sizeof(extract)); 848 memset(workspace, 0, sizeof(workspace)); 849 850 /* 851 * In case the hash function has some recognizable output 852 * pattern, we fold it in half. Thus, we always feed back 853 * twice as much data as we output. 854 */ 855 hash[0] ^= hash[3]; 856 hash[1] ^= hash[4]; 857 hash[2] ^= rol32(hash[2], 16); 858 memcpy(out, hash, EXTRACT_SIZE); 859 memset(hash, 0, sizeof(hash)); 860 } 861 862 static ssize_t extract_entropy(struct entropy_store *r, void *buf, 863 size_t nbytes, int min, int reserved) 864 { 865 ssize_t ret = 0, i; 866 __u8 tmp[EXTRACT_SIZE]; 867 unsigned long flags; 868 869 xfer_secondary_pool(r, nbytes); 870 nbytes = account(r, nbytes, min, reserved); 871 872 while (nbytes) { 873 extract_buf(r, tmp); 874 875 if (fips_enabled) { 876 spin_lock_irqsave(&r->lock, flags); 877 if (!memcmp(tmp, r->last_data, EXTRACT_SIZE)) 878 panic("Hardware RNG duplicated output!\n"); 879 memcpy(r->last_data, tmp, EXTRACT_SIZE); 880 spin_unlock_irqrestore(&r->lock, flags); 881 } 882 i = min_t(int, nbytes, EXTRACT_SIZE); 883 memcpy(buf, tmp, i); 884 nbytes -= i; 885 buf += i; 886 ret += i; 887 } 888 889 /* Wipe data just returned from memory */ 890 memset(tmp, 0, sizeof(tmp)); 891 892 return ret; 893 } 894 895 static ssize_t extract_entropy_user(struct entropy_store *r, void __user *buf, 896 size_t nbytes) 897 { 898 ssize_t ret = 0, i; 899 __u8 tmp[EXTRACT_SIZE]; 900 901 xfer_secondary_pool(r, nbytes); 902 nbytes = account(r, nbytes, 0, 0); 903 904 while (nbytes) { 905 if (need_resched()) { 906 if (signal_pending(current)) { 907 if (ret == 0) 908 ret = -ERESTARTSYS; 909 break; 910 } 911 schedule(); 912 } 913 914 extract_buf(r, tmp); 915 i = min_t(int, nbytes, EXTRACT_SIZE); 916 if (copy_to_user(buf, tmp, i)) { 917 ret = -EFAULT; 918 break; 919 } 920 921 nbytes -= i; 922 buf += i; 923 ret += i; 924 } 925 926 /* Wipe data just returned from memory */ 927 memset(tmp, 0, sizeof(tmp)); 928 929 return ret; 930 } 931 932 /* 933 * This function is the exported kernel interface. It returns some 934 * number of good random numbers, suitable for seeding TCP sequence 935 * numbers, etc. 936 */ 937 void get_random_bytes(void *buf, int nbytes) 938 { 939 char *p = buf; 940 941 while (nbytes) { 942 unsigned long v; 943 int chunk = min(nbytes, (int)sizeof(unsigned long)); 944 945 if (!arch_get_random_long(&v)) 946 break; 947 948 memcpy(p, &v, chunk); 949 p += chunk; 950 nbytes -= chunk; 951 } 952 953 extract_entropy(&nonblocking_pool, p, nbytes, 0, 0); 954 } 955 EXPORT_SYMBOL(get_random_bytes); 956 957 /* 958 * init_std_data - initialize pool with system data 959 * 960 * @r: pool to initialize 961 * 962 * This function clears the pool's entropy count and mixes some system 963 * data into the pool to prepare it for use. The pool is not cleared 964 * as that can only decrease the entropy in the pool. 965 */ 966 static void init_std_data(struct entropy_store *r) 967 { 968 int i; 969 ktime_t now; 970 unsigned long flags; 971 972 spin_lock_irqsave(&r->lock, flags); 973 r->entropy_count = 0; 974 spin_unlock_irqrestore(&r->lock, flags); 975 976 now = ktime_get_real(); 977 mix_pool_bytes(r, &now, sizeof(now)); 978 for (i = r->poolinfo->POOLBYTES; i > 0; i -= sizeof flags) { 979 if (!arch_get_random_long(&flags)) 980 break; 981 mix_pool_bytes(r, &flags, sizeof(flags)); 982 } 983 mix_pool_bytes(r, utsname(), sizeof(*(utsname()))); 984 } 985 986 static int rand_initialize(void) 987 { 988 init_std_data(&input_pool); 989 init_std_data(&blocking_pool); 990 init_std_data(&nonblocking_pool); 991 return 0; 992 } 993 module_init(rand_initialize); 994 995 void rand_initialize_irq(int irq) 996 { 997 struct timer_rand_state *state; 998 999 state = get_timer_rand_state(irq); 1000 1001 if (state) 1002 return; 1003 1004 /* 1005 * If kzalloc returns null, we just won't use that entropy 1006 * source. 1007 */ 1008 state = kzalloc(sizeof(struct timer_rand_state), GFP_KERNEL); 1009 if (state) 1010 set_timer_rand_state(irq, state); 1011 } 1012 1013 #ifdef CONFIG_BLOCK 1014 void rand_initialize_disk(struct gendisk *disk) 1015 { 1016 struct timer_rand_state *state; 1017 1018 /* 1019 * If kzalloc returns null, we just won't use that entropy 1020 * source. 1021 */ 1022 state = kzalloc(sizeof(struct timer_rand_state), GFP_KERNEL); 1023 if (state) 1024 disk->random = state; 1025 } 1026 #endif 1027 1028 static ssize_t 1029 random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos) 1030 { 1031 ssize_t n, retval = 0, count = 0; 1032 1033 if (nbytes == 0) 1034 return 0; 1035 1036 while (nbytes > 0) { 1037 n = nbytes; 1038 if (n > SEC_XFER_SIZE) 1039 n = SEC_XFER_SIZE; 1040 1041 DEBUG_ENT("reading %d bits\n", n*8); 1042 1043 n = extract_entropy_user(&blocking_pool, buf, n); 1044 1045 DEBUG_ENT("read got %d bits (%d still needed)\n", 1046 n*8, (nbytes-n)*8); 1047 1048 if (n == 0) { 1049 if (file->f_flags & O_NONBLOCK) { 1050 retval = -EAGAIN; 1051 break; 1052 } 1053 1054 DEBUG_ENT("sleeping?\n"); 1055 1056 wait_event_interruptible(random_read_wait, 1057 input_pool.entropy_count >= 1058 random_read_wakeup_thresh); 1059 1060 DEBUG_ENT("awake\n"); 1061 1062 if (signal_pending(current)) { 1063 retval = -ERESTARTSYS; 1064 break; 1065 } 1066 1067 continue; 1068 } 1069 1070 if (n < 0) { 1071 retval = n; 1072 break; 1073 } 1074 count += n; 1075 buf += n; 1076 nbytes -= n; 1077 break; /* This break makes the device work */ 1078 /* like a named pipe */ 1079 } 1080 1081 return (count ? count : retval); 1082 } 1083 1084 static ssize_t 1085 urandom_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos) 1086 { 1087 return extract_entropy_user(&nonblocking_pool, buf, nbytes); 1088 } 1089 1090 static unsigned int 1091 random_poll(struct file *file, poll_table * wait) 1092 { 1093 unsigned int mask; 1094 1095 poll_wait(file, &random_read_wait, wait); 1096 poll_wait(file, &random_write_wait, wait); 1097 mask = 0; 1098 if (input_pool.entropy_count >= random_read_wakeup_thresh) 1099 mask |= POLLIN | POLLRDNORM; 1100 if (input_pool.entropy_count < random_write_wakeup_thresh) 1101 mask |= POLLOUT | POLLWRNORM; 1102 return mask; 1103 } 1104 1105 static int 1106 write_pool(struct entropy_store *r, const char __user *buffer, size_t count) 1107 { 1108 size_t bytes; 1109 __u32 buf[16]; 1110 const char __user *p = buffer; 1111 1112 while (count > 0) { 1113 bytes = min(count, sizeof(buf)); 1114 if (copy_from_user(&buf, p, bytes)) 1115 return -EFAULT; 1116 1117 count -= bytes; 1118 p += bytes; 1119 1120 mix_pool_bytes(r, buf, bytes); 1121 cond_resched(); 1122 } 1123 1124 return 0; 1125 } 1126 1127 static ssize_t random_write(struct file *file, const char __user *buffer, 1128 size_t count, loff_t *ppos) 1129 { 1130 size_t ret; 1131 1132 ret = write_pool(&blocking_pool, buffer, count); 1133 if (ret) 1134 return ret; 1135 ret = write_pool(&nonblocking_pool, buffer, count); 1136 if (ret) 1137 return ret; 1138 1139 return (ssize_t)count; 1140 } 1141 1142 static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg) 1143 { 1144 int size, ent_count; 1145 int __user *p = (int __user *)arg; 1146 int retval; 1147 1148 switch (cmd) { 1149 case RNDGETENTCNT: 1150 /* inherently racy, no point locking */ 1151 if (put_user(input_pool.entropy_count, p)) 1152 return -EFAULT; 1153 return 0; 1154 case RNDADDTOENTCNT: 1155 if (!capable(CAP_SYS_ADMIN)) 1156 return -EPERM; 1157 if (get_user(ent_count, p)) 1158 return -EFAULT; 1159 credit_entropy_bits(&input_pool, ent_count); 1160 return 0; 1161 case RNDADDENTROPY: 1162 if (!capable(CAP_SYS_ADMIN)) 1163 return -EPERM; 1164 if (get_user(ent_count, p++)) 1165 return -EFAULT; 1166 if (ent_count < 0) 1167 return -EINVAL; 1168 if (get_user(size, p++)) 1169 return -EFAULT; 1170 retval = write_pool(&input_pool, (const char __user *)p, 1171 size); 1172 if (retval < 0) 1173 return retval; 1174 credit_entropy_bits(&input_pool, ent_count); 1175 return 0; 1176 case RNDZAPENTCNT: 1177 case RNDCLEARPOOL: 1178 /* Clear the entropy pool counters. */ 1179 if (!capable(CAP_SYS_ADMIN)) 1180 return -EPERM; 1181 rand_initialize(); 1182 return 0; 1183 default: 1184 return -EINVAL; 1185 } 1186 } 1187 1188 static int random_fasync(int fd, struct file *filp, int on) 1189 { 1190 return fasync_helper(fd, filp, on, &fasync); 1191 } 1192 1193 const struct file_operations random_fops = { 1194 .read = random_read, 1195 .write = random_write, 1196 .poll = random_poll, 1197 .unlocked_ioctl = random_ioctl, 1198 .fasync = random_fasync, 1199 .llseek = noop_llseek, 1200 }; 1201 1202 const struct file_operations urandom_fops = { 1203 .read = urandom_read, 1204 .write = random_write, 1205 .unlocked_ioctl = random_ioctl, 1206 .fasync = random_fasync, 1207 .llseek = noop_llseek, 1208 }; 1209 1210 /*************************************************************** 1211 * Random UUID interface 1212 * 1213 * Used here for a Boot ID, but can be useful for other kernel 1214 * drivers. 1215 ***************************************************************/ 1216 1217 /* 1218 * Generate random UUID 1219 */ 1220 void generate_random_uuid(unsigned char uuid_out[16]) 1221 { 1222 get_random_bytes(uuid_out, 16); 1223 /* Set UUID version to 4 --- truly random generation */ 1224 uuid_out[6] = (uuid_out[6] & 0x0F) | 0x40; 1225 /* Set the UUID variant to DCE */ 1226 uuid_out[8] = (uuid_out[8] & 0x3F) | 0x80; 1227 } 1228 EXPORT_SYMBOL(generate_random_uuid); 1229 1230 /******************************************************************** 1231 * 1232 * Sysctl interface 1233 * 1234 ********************************************************************/ 1235 1236 #ifdef CONFIG_SYSCTL 1237 1238 #include <linux/sysctl.h> 1239 1240 static int min_read_thresh = 8, min_write_thresh; 1241 static int max_read_thresh = INPUT_POOL_WORDS * 32; 1242 static int max_write_thresh = INPUT_POOL_WORDS * 32; 1243 static char sysctl_bootid[16]; 1244 1245 /* 1246 * These functions is used to return both the bootid UUID, and random 1247 * UUID. The difference is in whether table->data is NULL; if it is, 1248 * then a new UUID is generated and returned to the user. 1249 * 1250 * If the user accesses this via the proc interface, it will be returned 1251 * as an ASCII string in the standard UUID format. If accesses via the 1252 * sysctl system call, it is returned as 16 bytes of binary data. 1253 */ 1254 static int proc_do_uuid(ctl_table *table, int write, 1255 void __user *buffer, size_t *lenp, loff_t *ppos) 1256 { 1257 ctl_table fake_table; 1258 unsigned char buf[64], tmp_uuid[16], *uuid; 1259 1260 uuid = table->data; 1261 if (!uuid) { 1262 uuid = tmp_uuid; 1263 uuid[8] = 0; 1264 } 1265 if (uuid[8] == 0) 1266 generate_random_uuid(uuid); 1267 1268 sprintf(buf, "%pU", uuid); 1269 1270 fake_table.data = buf; 1271 fake_table.maxlen = sizeof(buf); 1272 1273 return proc_dostring(&fake_table, write, buffer, lenp, ppos); 1274 } 1275 1276 static int sysctl_poolsize = INPUT_POOL_WORDS * 32; 1277 ctl_table random_table[] = { 1278 { 1279 .procname = "poolsize", 1280 .data = &sysctl_poolsize, 1281 .maxlen = sizeof(int), 1282 .mode = 0444, 1283 .proc_handler = proc_dointvec, 1284 }, 1285 { 1286 .procname = "entropy_avail", 1287 .maxlen = sizeof(int), 1288 .mode = 0444, 1289 .proc_handler = proc_dointvec, 1290 .data = &input_pool.entropy_count, 1291 }, 1292 { 1293 .procname = "read_wakeup_threshold", 1294 .data = &random_read_wakeup_thresh, 1295 .maxlen = sizeof(int), 1296 .mode = 0644, 1297 .proc_handler = proc_dointvec_minmax, 1298 .extra1 = &min_read_thresh, 1299 .extra2 = &max_read_thresh, 1300 }, 1301 { 1302 .procname = "write_wakeup_threshold", 1303 .data = &random_write_wakeup_thresh, 1304 .maxlen = sizeof(int), 1305 .mode = 0644, 1306 .proc_handler = proc_dointvec_minmax, 1307 .extra1 = &min_write_thresh, 1308 .extra2 = &max_write_thresh, 1309 }, 1310 { 1311 .procname = "boot_id", 1312 .data = &sysctl_bootid, 1313 .maxlen = 16, 1314 .mode = 0444, 1315 .proc_handler = proc_do_uuid, 1316 }, 1317 { 1318 .procname = "uuid", 1319 .maxlen = 16, 1320 .mode = 0444, 1321 .proc_handler = proc_do_uuid, 1322 }, 1323 { } 1324 }; 1325 #endif /* CONFIG_SYSCTL */ 1326 1327 static u32 random_int_secret[MD5_MESSAGE_BYTES / 4] ____cacheline_aligned; 1328 1329 static int __init random_int_secret_init(void) 1330 { 1331 get_random_bytes(random_int_secret, sizeof(random_int_secret)); 1332 return 0; 1333 } 1334 late_initcall(random_int_secret_init); 1335 1336 /* 1337 * Get a random word for internal kernel use only. Similar to urandom but 1338 * with the goal of minimal entropy pool depletion. As a result, the random 1339 * value is not cryptographically secure but for several uses the cost of 1340 * depleting entropy is too high 1341 */ 1342 DEFINE_PER_CPU(__u32 [MD5_DIGEST_WORDS], get_random_int_hash); 1343 unsigned int get_random_int(void) 1344 { 1345 __u32 *hash; 1346 unsigned int ret; 1347 1348 if (arch_get_random_int(&ret)) 1349 return ret; 1350 1351 hash = get_cpu_var(get_random_int_hash); 1352 1353 hash[0] += current->pid + jiffies + get_cycles(); 1354 md5_transform(hash, random_int_secret); 1355 ret = hash[0]; 1356 put_cpu_var(get_random_int_hash); 1357 1358 return ret; 1359 } 1360 1361 /* 1362 * randomize_range() returns a start address such that 1363 * 1364 * [...... <range> .....] 1365 * start end 1366 * 1367 * a <range> with size "len" starting at the return value is inside in the 1368 * area defined by [start, end], but is otherwise randomized. 1369 */ 1370 unsigned long 1371 randomize_range(unsigned long start, unsigned long end, unsigned long len) 1372 { 1373 unsigned long range = end - len - start; 1374 1375 if (end <= start + len) 1376 return 0; 1377 return PAGE_ALIGN(get_random_int() % range + start); 1378 } 1379