/* * random.c -- A strong random number generator * * Copyright Matt Mackall <mpm@selenic.com>, 2003, 2004, 2005 * * Copyright Theodore Ts'o, 1994, 1995, 1996, 1997, 1998, 1999. All * rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, and the entire permission notice in its entirety, * including the disclaimer of warranties. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * 3. The name of the author may not be used to endorse or promote * products derived from this software without specific prior * written permission. * * ALTERNATIVELY, this product may be distributed under the terms of * the GNU General Public License, in which case the provisions of the GPL are * required INSTEAD OF the above restrictions. (This clause is * necessary due to a potential bad interaction between the GPL and * the restrictions contained in a BSD-style copyright.) * * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, ALL OF * WHICH ARE HEREBY DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR * BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE * USE OF THIS SOFTWARE, EVEN IF NOT ADVISED OF THE POSSIBILITY OF SUCH * DAMAGE. */ /* * (now, with legal B.S. out of the way.....) * * This routine gathers environmental noise from device drivers, etc., * and returns good random numbers, suitable for cryptographic use. * Besides the obvious cryptographic uses, these numbers are also good * for seeding TCP sequence numbers, and other places where it is * desirable to have numbers which are not only random, but hard to * predict by an attacker. * * Theory of operation * =================== * * Computers are very predictable devices. Hence it is extremely hard * to produce truly random numbers on a computer --- as opposed to * pseudo-random numbers, which can easily generated by using a * algorithm. Unfortunately, it is very easy for attackers to guess * the sequence of pseudo-random number generators, and for some * applications this is not acceptable. So instead, we must try to * gather "environmental noise" from the computer's environment, which * must be hard for outside attackers to observe, and use that to * generate random numbers. In a Unix environment, this is best done * from inside the kernel. * * Sources of randomness from the environment include inter-keyboard * timings, inter-interrupt timings from some interrupts, and other * events which are both (a) non-deterministic and (b) hard for an * outside observer to measure. Randomness from these sources are * added to an "entropy pool", which is mixed using a CRC-like function. * This is not cryptographically strong, but it is adequate assuming * the randomness is not chosen maliciously, and it is fast enough that * the overhead of doing it on every interrupt is very reasonable. * As random bytes are mixed into the entropy pool, the routines keep * an *estimate* of how many bits of randomness have been stored into * the random number generator's internal state. * * When random bytes are desired, they are obtained by taking the SHA * hash of the contents of the "entropy pool". The SHA hash avoids * exposing the internal state of the entropy pool. It is believed to * be computationally infeasible to derive any useful information * about the input of SHA from its output. Even if it is possible to * analyze SHA in some clever way, as long as the amount of data * returned from the generator is less than the inherent entropy in * the pool, the output data is totally unpredictable. For this * reason, the routine decreases its internal estimate of how many * bits of "true randomness" are contained in the entropy pool as it * outputs random numbers. * * If this estimate goes to zero, the routine can still generate * random numbers; however, an attacker may (at least in theory) be * able to infer the future output of the generator from prior * outputs. This requires successful cryptanalysis of SHA, which is * not believed to be feasible, but there is a remote possibility. * Nonetheless, these numbers should be useful for the vast majority * of purposes. * * Exported interfaces ---- output * =============================== * * There are three exported interfaces; the first is one designed to * be used from within the kernel: * * void get_random_bytes(void *buf, int nbytes); * * This interface will return the requested number of random bytes, * and place it in the requested buffer. * * The two other interfaces are two character devices /dev/random and * /dev/urandom. /dev/random is suitable for use when very high * quality randomness is desired (for example, for key generation or * one-time pads), as it will only return a maximum of the number of * bits of randomness (as estimated by the random number generator) * contained in the entropy pool. * * The /dev/urandom device does not have this limit, and will return * as many bytes as are requested. As more and more random bytes are * requested without giving time for the entropy pool to recharge, * this will result in random numbers that are merely cryptographically * strong. For many applications, however, this is acceptable. * * Exported interfaces ---- input * ============================== * * The current exported interfaces for gathering environmental noise * from the devices are: * * void add_input_randomness(unsigned int type, unsigned int code, * unsigned int value); * void add_interrupt_randomness(int irq); * * add_input_randomness() uses the input layer interrupt timing, as well as * the event type information from the hardware. * * add_interrupt_randomness() uses the inter-interrupt timing as random * inputs to the entropy pool. Note that not all interrupts are good * sources of randomness! For example, the timer interrupts is not a * good choice, because the periodicity of the interrupts is too * regular, and hence predictable to an attacker. Disk interrupts are * a better measure, since the timing of the disk interrupts are more * unpredictable. * * All of these routines try to estimate how many bits of randomness a * particular randomness source. They do this by keeping track of the * first and second order deltas of the event timings. * * Ensuring unpredictability at system startup * ============================================ * * When any operating system starts up, it will go through a sequence * of actions that are fairly predictable by an adversary, especially * if the start-up does not involve interaction with a human operator. * This reduces the actual number of bits of unpredictability in the * entropy pool below the value in entropy_count. In order to * counteract this effect, it helps to carry information in the * entropy pool across shut-downs and start-ups. To do this, put the * following lines an appropriate script which is run during the boot * sequence: * * echo "Initializing random number generator..." * random_seed=/var/run/random-seed * # Carry a random seed from start-up to start-up * # Load and then save the whole entropy pool * if [ -f $random_seed ]; then * cat $random_seed >/dev/urandom * else * touch $random_seed * fi * chmod 600 $random_seed * dd if=/dev/urandom of=$random_seed count=1 bs=512 * * and the following lines in an appropriate script which is run as * the system is shutdown: * * # Carry a random seed from shut-down to start-up * # Save the whole entropy pool * echo "Saving random seed..." * random_seed=/var/run/random-seed * touch $random_seed * chmod 600 $random_seed * dd if=/dev/urandom of=$random_seed count=1 bs=512 * * For example, on most modern systems using the System V init * scripts, such code fragments would be found in * /etc/rc.d/init.d/random. On older Linux systems, the correct script * location might be in /etc/rcb.d/rc.local or /etc/rc.d/rc.0. * * Effectively, these commands cause the contents of the entropy pool * to be saved at shut-down time and reloaded into the entropy pool at * start-up. (The 'dd' in the addition to the bootup script is to * make sure that /etc/random-seed is different for every start-up, * even if the system crashes without executing rc.0.) Even with * complete knowledge of the start-up activities, predicting the state * of the entropy pool requires knowledge of the previous history of * the system. * * Configuring the /dev/random driver under Linux * ============================================== * * The /dev/random driver under Linux uses minor numbers 8 and 9 of * the /dev/mem major number (#1). So if your system does not have * /dev/random and /dev/urandom created already, they can be created * by using the commands: * * mknod /dev/random c 1 8 * mknod /dev/urandom c 1 9 * * Acknowledgements: * ================= * * Ideas for constructing this random number generator were derived * from Pretty Good Privacy's random number generator, and from private * discussions with Phil Karn. Colin Plumb provided a faster random * number generator, which speed up the mixing function of the entropy * pool, taken from PGPfone. Dale Worley has also contributed many * useful ideas and suggestions to improve this driver. * * Any flaws in the design are solely my responsibility, and should * not be attributed to the Phil, Colin, or any of authors of PGP. * * Further background information on this topic may be obtained from * RFC 1750, "Randomness Recommendations for Security", by Donald * Eastlake, Steve Crocker, and Jeff Schiller. */ #include <linux/utsname.h> #include <linux/module.h> #include <linux/kernel.h> #include <linux/major.h> #include <linux/string.h> #include <linux/fcntl.h> #include <linux/slab.h> #include <linux/random.h> #include <linux/poll.h> #include <linux/init.h> #include <linux/fs.h> #include <linux/genhd.h> #include <linux/interrupt.h> #include <linux/mm.h> #include <linux/spinlock.h> #include <linux/percpu.h> #include <linux/cryptohash.h> #include <linux/fips.h> #ifdef CONFIG_GENERIC_HARDIRQS # include <linux/irq.h> #endif #include <asm/processor.h> #include <asm/uaccess.h> #include <asm/irq.h> #include <asm/io.h> /* * Configuration information */ #define INPUT_POOL_WORDS 128 #define OUTPUT_POOL_WORDS 32 #define SEC_XFER_SIZE 512 /* * The minimum number of bits of entropy before we wake up a read on * /dev/random. Should be enough to do a significant reseed. */ static int random_read_wakeup_thresh = 64; /* * If the entropy count falls under this number of bits, then we * should wake up processes which are selecting or polling on write * access to /dev/random. */ static int random_write_wakeup_thresh = 128; /* * When the input pool goes over trickle_thresh, start dropping most * samples to avoid wasting CPU time and reduce lock contention. */ static int trickle_thresh __read_mostly = INPUT_POOL_WORDS * 28; static DEFINE_PER_CPU(int, trickle_count); /* * A pool of size .poolwords is stirred with a primitive polynomial * of degree .poolwords over GF(2). The taps for various sizes are * defined below. They are chosen to be evenly spaced (minimum RMS * distance from evenly spaced; the numbers in the comments are a * scaled squared error sum) except for the last tap, which is 1 to * get the twisting happening as fast as possible. */ static struct poolinfo { int poolwords; int tap1, tap2, tap3, tap4, tap5; } poolinfo_table[] = { /* x^128 + x^103 + x^76 + x^51 +x^25 + x + 1 -- 105 */ { 128, 103, 76, 51, 25, 1 }, /* x^32 + x^26 + x^20 + x^14 + x^7 + x + 1 -- 15 */ { 32, 26, 20, 14, 7, 1 }, #if 0 /* x^2048 + x^1638 + x^1231 + x^819 + x^411 + x + 1 -- 115 */ { 2048, 1638, 1231, 819, 411, 1 }, /* x^1024 + x^817 + x^615 + x^412 + x^204 + x + 1 -- 290 */ { 1024, 817, 615, 412, 204, 1 }, /* x^1024 + x^819 + x^616 + x^410 + x^207 + x^2 + 1 -- 115 */ { 1024, 819, 616, 410, 207, 2 }, /* x^512 + x^411 + x^308 + x^208 + x^104 + x + 1 -- 225 */ { 512, 411, 308, 208, 104, 1 }, /* x^512 + x^409 + x^307 + x^206 + x^102 + x^2 + 1 -- 95 */ { 512, 409, 307, 206, 102, 2 }, /* x^512 + x^409 + x^309 + x^205 + x^103 + x^2 + 1 -- 95 */ { 512, 409, 309, 205, 103, 2 }, /* x^256 + x^205 + x^155 + x^101 + x^52 + x + 1 -- 125 */ { 256, 205, 155, 101, 52, 1 }, /* x^128 + x^103 + x^78 + x^51 + x^27 + x^2 + 1 -- 70 */ { 128, 103, 78, 51, 27, 2 }, /* x^64 + x^52 + x^39 + x^26 + x^14 + x + 1 -- 15 */ { 64, 52, 39, 26, 14, 1 }, #endif }; #define POOLBITS poolwords*32 #define POOLBYTES poolwords*4 /* * For the purposes of better mixing, we use the CRC-32 polynomial as * well to make a twisted Generalized Feedback Shift Reigster * * (See M. Matsumoto & Y. Kurita, 1992. Twisted GFSR generators. ACM * Transactions on Modeling and Computer Simulation 2(3):179-194. * Also see M. Matsumoto & Y. Kurita, 1994. Twisted GFSR generators * II. ACM Transactions on Mdeling and Computer Simulation 4:254-266) * * Thanks to Colin Plumb for suggesting this. * * We have not analyzed the resultant polynomial to prove it primitive; * in fact it almost certainly isn't. Nonetheless, the irreducible factors * of a random large-degree polynomial over GF(2) are more than large enough * that periodicity is not a concern. * * The input hash is much less sensitive than the output hash. All * that we want of it is that it be a good non-cryptographic hash; * i.e. it not produce collisions when fed "random" data of the sort * we expect to see. As long as the pool state differs for different * inputs, we have preserved the input entropy and done a good job. * The fact that an intelligent attacker can construct inputs that * will produce controlled alterations to the pool's state is not * important because we don't consider such inputs to contribute any * randomness. The only property we need with respect to them is that * the attacker can't increase his/her knowledge of the pool's state. * Since all additions are reversible (knowing the final state and the * input, you can reconstruct the initial state), if an attacker has * any uncertainty about the initial state, he/she can only shuffle * that uncertainty about, but never cause any collisions (which would * decrease the uncertainty). * * The chosen system lets the state of the pool be (essentially) the input * modulo the generator polymnomial. Now, for random primitive polynomials, * this is a universal class of hash functions, meaning that the chance * of a collision is limited by the attacker's knowledge of the generator * polynomail, so if it is chosen at random, an attacker can never force * a collision. Here, we use a fixed polynomial, but we *can* assume that * ###--> it is unknown to the processes generating the input entropy. <-### * Because of this important property, this is a good, collision-resistant * hash; hash collisions will occur no more often than chance. */ /* * Static global variables */ static DECLARE_WAIT_QUEUE_HEAD(random_read_wait); static DECLARE_WAIT_QUEUE_HEAD(random_write_wait); static struct fasync_struct *fasync; #if 0 static int debug; module_param(debug, bool, 0644); #define DEBUG_ENT(fmt, arg...) do { \ if (debug) \ printk(KERN_DEBUG "random %04d %04d %04d: " \ fmt,\ input_pool.entropy_count,\ blocking_pool.entropy_count,\ nonblocking_pool.entropy_count,\ ## arg); } while (0) #else #define DEBUG_ENT(fmt, arg...) do {} while (0) #endif /********************************************************************** * * OS independent entropy store. Here are the functions which handle * storing entropy in an entropy pool. * **********************************************************************/ struct entropy_store; struct entropy_store { /* read-only data: */ struct poolinfo *poolinfo; __u32 *pool; const char *name; int limit; struct entropy_store *pull; /* read-write data: */ spinlock_t lock; unsigned add_ptr; int entropy_count; int input_rotate; __u8 *last_data; }; static __u32 input_pool_data[INPUT_POOL_WORDS]; static __u32 blocking_pool_data[OUTPUT_POOL_WORDS]; static __u32 nonblocking_pool_data[OUTPUT_POOL_WORDS]; static struct entropy_store input_pool = { .poolinfo = &poolinfo_table[0], .name = "input", .limit = 1, .lock = __SPIN_LOCK_UNLOCKED(&input_pool.lock), .pool = input_pool_data }; static struct entropy_store blocking_pool = { .poolinfo = &poolinfo_table[1], .name = "blocking", .limit = 1, .pull = &input_pool, .lock = __SPIN_LOCK_UNLOCKED(&blocking_pool.lock), .pool = blocking_pool_data }; static struct entropy_store nonblocking_pool = { .poolinfo = &poolinfo_table[1], .name = "nonblocking", .pull = &input_pool, .lock = __SPIN_LOCK_UNLOCKED(&nonblocking_pool.lock), .pool = nonblocking_pool_data }; /* * This function adds bytes into the entropy "pool". It does not * update the entropy estimate. The caller should call * credit_entropy_bits if this is appropriate. * * The pool is stirred with a primitive polynomial of the appropriate * degree, and then twisted. We twist by three bits at a time because * it's cheap to do so and helps slightly in the expected case where * the entropy is concentrated in the low-order bits. */ static void mix_pool_bytes_extract(struct entropy_store *r, const void *in, int nbytes, __u8 out[64]) { static __u32 const twist_table[8] = { 0x00000000, 0x3b6e20c8, 0x76dc4190, 0x4db26158, 0xedb88320, 0xd6d6a3e8, 0x9b64c2b0, 0xa00ae278 }; unsigned long i, j, tap1, tap2, tap3, tap4, tap5; int input_rotate; int wordmask = r->poolinfo->poolwords - 1; const char *bytes = in; __u32 w; unsigned long flags; /* Taps are constant, so we can load them without holding r->lock. */ tap1 = r->poolinfo->tap1; tap2 = r->poolinfo->tap2; tap3 = r->poolinfo->tap3; tap4 = r->poolinfo->tap4; tap5 = r->poolinfo->tap5; spin_lock_irqsave(&r->lock, flags); input_rotate = r->input_rotate; i = r->add_ptr; /* mix one byte at a time to simplify size handling and churn faster */ while (nbytes--) { w = rol32(*bytes++, input_rotate & 31); i = (i - 1) & wordmask; /* XOR in the various taps */ w ^= r->pool[i]; w ^= r->pool[(i + tap1) & wordmask]; w ^= r->pool[(i + tap2) & wordmask]; w ^= r->pool[(i + tap3) & wordmask]; w ^= r->pool[(i + tap4) & wordmask]; w ^= r->pool[(i + tap5) & wordmask]; /* Mix the result back in with a twist */ r->pool[i] = (w >> 3) ^ twist_table[w & 7]; /* * Normally, we add 7 bits of rotation to the pool. * At the beginning of the pool, add an extra 7 bits * rotation, so that successive passes spread the * input bits across the pool evenly. */ input_rotate += i ? 7 : 14; } r->input_rotate = input_rotate; r->add_ptr = i; if (out) for (j = 0; j < 16; j++) ((__u32 *)out)[j] = r->pool[(i - j) & wordmask]; spin_unlock_irqrestore(&r->lock, flags); } static void mix_pool_bytes(struct entropy_store *r, const void *in, int bytes) { mix_pool_bytes_extract(r, in, bytes, NULL); } /* * Credit (or debit) the entropy store with n bits of entropy */ static void credit_entropy_bits(struct entropy_store *r, int nbits) { unsigned long flags; int entropy_count; if (!nbits) return; spin_lock_irqsave(&r->lock, flags); DEBUG_ENT("added %d entropy credits to %s\n", nbits, r->name); entropy_count = r->entropy_count; entropy_count += nbits; if (entropy_count < 0) { DEBUG_ENT("negative entropy/overflow\n"); entropy_count = 0; } else if (entropy_count > r->poolinfo->POOLBITS) entropy_count = r->poolinfo->POOLBITS; r->entropy_count = entropy_count; /* should we wake readers? */ if (r == &input_pool && entropy_count >= random_read_wakeup_thresh) { wake_up_interruptible(&random_read_wait); kill_fasync(&fasync, SIGIO, POLL_IN); } spin_unlock_irqrestore(&r->lock, flags); } /********************************************************************* * * Entropy input management * *********************************************************************/ /* There is one of these per entropy source */ struct timer_rand_state { cycles_t last_time; long last_delta, last_delta2; unsigned dont_count_entropy:1; }; #ifndef CONFIG_GENERIC_HARDIRQS static struct timer_rand_state *irq_timer_state[NR_IRQS]; static struct timer_rand_state *get_timer_rand_state(unsigned int irq) { return irq_timer_state[irq]; } static void set_timer_rand_state(unsigned int irq, struct timer_rand_state *state) { irq_timer_state[irq] = state; } #else static struct timer_rand_state *get_timer_rand_state(unsigned int irq) { struct irq_desc *desc; desc = irq_to_desc(irq); return desc->timer_rand_state; } static void set_timer_rand_state(unsigned int irq, struct timer_rand_state *state) { struct irq_desc *desc; desc = irq_to_desc(irq); desc->timer_rand_state = state; } #endif static struct timer_rand_state input_timer_state; /* * This function adds entropy to the entropy "pool" by using timing * delays. It uses the timer_rand_state structure to make an estimate * of how many bits of entropy this call has added to the pool. * * The number "num" is also added to the pool - it should somehow describe * the type of event which just happened. This is currently 0-255 for * keyboard scan codes, and 256 upwards for interrupts. * */ static void add_timer_randomness(struct timer_rand_state *state, unsigned num) { struct { cycles_t cycles; long jiffies; unsigned num; } sample; long delta, delta2, delta3; preempt_disable(); /* if over the trickle threshold, use only 1 in 4096 samples */ if (input_pool.entropy_count > trickle_thresh && (__get_cpu_var(trickle_count)++ & 0xfff)) goto out; sample.jiffies = jiffies; sample.cycles = get_cycles(); sample.num = num; mix_pool_bytes(&input_pool, &sample, sizeof(sample)); /* * Calculate number of bits of randomness we probably added. * We take into account the first, second and third-order deltas * in order to make our estimate. */ if (!state->dont_count_entropy) { delta = sample.jiffies - state->last_time; state->last_time = sample.jiffies; delta2 = delta - state->last_delta; state->last_delta = delta; delta3 = delta2 - state->last_delta2; state->last_delta2 = delta2; if (delta < 0) delta = -delta; if (delta2 < 0) delta2 = -delta2; if (delta3 < 0) delta3 = -delta3; if (delta > delta2) delta = delta2; if (delta > delta3) delta = delta3; /* * delta is now minimum absolute delta. * Round down by 1 bit on general principles, * and limit entropy entimate to 12 bits. */ credit_entropy_bits(&input_pool, min_t(int, fls(delta>>1), 11)); } out: preempt_enable(); } void add_input_randomness(unsigned int type, unsigned int code, unsigned int value) { static unsigned char last_value; /* ignore autorepeat and the like */ if (value == last_value) return; DEBUG_ENT("input event\n"); last_value = value; add_timer_randomness(&input_timer_state, (type << 4) ^ code ^ (code >> 4) ^ value); } EXPORT_SYMBOL_GPL(add_input_randomness); void add_interrupt_randomness(int irq) { struct timer_rand_state *state; state = get_timer_rand_state(irq); if (state == NULL) return; DEBUG_ENT("irq event %d\n", irq); add_timer_randomness(state, 0x100 + irq); } #ifdef CONFIG_BLOCK void add_disk_randomness(struct gendisk *disk) { if (!disk || !disk->random) return; /* first major is 1, so we get >= 0x200 here */ DEBUG_ENT("disk event %d:%d\n", MAJOR(disk_devt(disk)), MINOR(disk_devt(disk))); add_timer_randomness(disk->random, 0x100 + disk_devt(disk)); } #endif #define EXTRACT_SIZE 10 /********************************************************************* * * Entropy extraction routines * *********************************************************************/ static ssize_t extract_entropy(struct entropy_store *r, void *buf, size_t nbytes, int min, int rsvd); /* * This utility inline function is responsible for transfering entropy * from the primary pool to the secondary extraction pool. We make * sure we pull enough for a 'catastrophic reseed'. */ static void xfer_secondary_pool(struct entropy_store *r, size_t nbytes) { __u32 tmp[OUTPUT_POOL_WORDS]; if (r->pull && r->entropy_count < nbytes * 8 && r->entropy_count < r->poolinfo->POOLBITS) { /* If we're limited, always leave two wakeup worth's BITS */ int rsvd = r->limit ? 0 : random_read_wakeup_thresh/4; int bytes = nbytes; /* pull at least as many as BYTES as wakeup BITS */ bytes = max_t(int, bytes, random_read_wakeup_thresh / 8); /* but never more than the buffer size */ bytes = min_t(int, bytes, sizeof(tmp)); DEBUG_ENT("going to reseed %s with %d bits " "(%d of %d requested)\n", r->name, bytes * 8, nbytes * 8, r->entropy_count); bytes = extract_entropy(r->pull, tmp, bytes, random_read_wakeup_thresh / 8, rsvd); mix_pool_bytes(r, tmp, bytes); credit_entropy_bits(r, bytes*8); } } /* * These functions extracts randomness from the "entropy pool", and * returns it in a buffer. * * The min parameter specifies the minimum amount we can pull before * failing to avoid races that defeat catastrophic reseeding while the * reserved parameter indicates how much entropy we must leave in the * pool after each pull to avoid starving other readers. * * Note: extract_entropy() assumes that .poolwords is a multiple of 16 words. */ static size_t account(struct entropy_store *r, size_t nbytes, int min, int reserved) { unsigned long flags; /* Hold lock while accounting */ spin_lock_irqsave(&r->lock, flags); BUG_ON(r->entropy_count > r->poolinfo->POOLBITS); DEBUG_ENT("trying to extract %d bits from %s\n", nbytes * 8, r->name); /* Can we pull enough? */ if (r->entropy_count / 8 < min + reserved) { nbytes = 0; } else { /* If limited, never pull more than available */ if (r->limit && nbytes + reserved >= r->entropy_count / 8) nbytes = r->entropy_count/8 - reserved; if (r->entropy_count / 8 >= nbytes + reserved) r->entropy_count -= nbytes*8; else r->entropy_count = reserved; if (r->entropy_count < random_write_wakeup_thresh) { wake_up_interruptible(&random_write_wait); kill_fasync(&fasync, SIGIO, POLL_OUT); } } DEBUG_ENT("debiting %d entropy credits from %s%s\n", nbytes * 8, r->name, r->limit ? "" : " (unlimited)"); spin_unlock_irqrestore(&r->lock, flags); return nbytes; } static void extract_buf(struct entropy_store *r, __u8 *out) { int i; __u32 hash[5], workspace[SHA_WORKSPACE_WORDS]; __u8 extract[64]; /* Generate a hash across the pool, 16 words (512 bits) at a time */ sha_init(hash); for (i = 0; i < r->poolinfo->poolwords; i += 16) sha_transform(hash, (__u8 *)(r->pool + i), workspace); /* * We mix the hash back into the pool to prevent backtracking * attacks (where the attacker knows the state of the pool * plus the current outputs, and attempts to find previous * ouputs), unless the hash function can be inverted. By * mixing at least a SHA1 worth of hash data back, we make * brute-forcing the feedback as hard as brute-forcing the * hash. */ mix_pool_bytes_extract(r, hash, sizeof(hash), extract); /* * To avoid duplicates, we atomically extract a portion of the * pool while mixing, and hash one final time. */ sha_transform(hash, extract, workspace); memset(extract, 0, sizeof(extract)); memset(workspace, 0, sizeof(workspace)); /* * In case the hash function has some recognizable output * pattern, we fold it in half. Thus, we always feed back * twice as much data as we output. */ hash[0] ^= hash[3]; hash[1] ^= hash[4]; hash[2] ^= rol32(hash[2], 16); memcpy(out, hash, EXTRACT_SIZE); memset(hash, 0, sizeof(hash)); } static ssize_t extract_entropy(struct entropy_store *r, void *buf, size_t nbytes, int min, int reserved) { ssize_t ret = 0, i; __u8 tmp[EXTRACT_SIZE]; unsigned long flags; xfer_secondary_pool(r, nbytes); nbytes = account(r, nbytes, min, reserved); while (nbytes) { extract_buf(r, tmp); if (r->last_data) { spin_lock_irqsave(&r->lock, flags); if (!memcmp(tmp, r->last_data, EXTRACT_SIZE)) panic("Hardware RNG duplicated output!\n"); memcpy(r->last_data, tmp, EXTRACT_SIZE); spin_unlock_irqrestore(&r->lock, flags); } i = min_t(int, nbytes, EXTRACT_SIZE); memcpy(buf, tmp, i); nbytes -= i; buf += i; ret += i; } /* Wipe data just returned from memory */ memset(tmp, 0, sizeof(tmp)); return ret; } static ssize_t extract_entropy_user(struct entropy_store *r, void __user *buf, size_t nbytes) { ssize_t ret = 0, i; __u8 tmp[EXTRACT_SIZE]; xfer_secondary_pool(r, nbytes); nbytes = account(r, nbytes, 0, 0); while (nbytes) { if (need_resched()) { if (signal_pending(current)) { if (ret == 0) ret = -ERESTARTSYS; break; } schedule(); } extract_buf(r, tmp); i = min_t(int, nbytes, EXTRACT_SIZE); if (copy_to_user(buf, tmp, i)) { ret = -EFAULT; break; } nbytes -= i; buf += i; ret += i; } /* Wipe data just returned from memory */ memset(tmp, 0, sizeof(tmp)); return ret; } /* * This function is the exported kernel interface. It returns some * number of good random numbers, suitable for seeding TCP sequence * numbers, etc. */ void get_random_bytes(void *buf, int nbytes) { extract_entropy(&nonblocking_pool, buf, nbytes, 0, 0); } EXPORT_SYMBOL(get_random_bytes); /* * init_std_data - initialize pool with system data * * @r: pool to initialize * * This function clears the pool's entropy count and mixes some system * data into the pool to prepare it for use. The pool is not cleared * as that can only decrease the entropy in the pool. */ static void init_std_data(struct entropy_store *r) { ktime_t now; unsigned long flags; spin_lock_irqsave(&r->lock, flags); r->entropy_count = 0; spin_unlock_irqrestore(&r->lock, flags); now = ktime_get_real(); mix_pool_bytes(r, &now, sizeof(now)); mix_pool_bytes(r, utsname(), sizeof(*(utsname()))); /* Enable continuous test in fips mode */ if (fips_enabled) r->last_data = kmalloc(EXTRACT_SIZE, GFP_KERNEL); } static int rand_initialize(void) { init_std_data(&input_pool); init_std_data(&blocking_pool); init_std_data(&nonblocking_pool); return 0; } module_init(rand_initialize); void rand_initialize_irq(int irq) { struct timer_rand_state *state; state = get_timer_rand_state(irq); if (state) return; /* * If kzalloc returns null, we just won't use that entropy * source. */ state = kzalloc(sizeof(struct timer_rand_state), GFP_KERNEL); if (state) set_timer_rand_state(irq, state); } #ifdef CONFIG_BLOCK void rand_initialize_disk(struct gendisk *disk) { struct timer_rand_state *state; /* * If kzalloc returns null, we just won't use that entropy * source. */ state = kzalloc(sizeof(struct timer_rand_state), GFP_KERNEL); if (state) disk->random = state; } #endif static ssize_t random_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos) { ssize_t n, retval = 0, count = 0; if (nbytes == 0) return 0; while (nbytes > 0) { n = nbytes; if (n > SEC_XFER_SIZE) n = SEC_XFER_SIZE; DEBUG_ENT("reading %d bits\n", n*8); n = extract_entropy_user(&blocking_pool, buf, n); DEBUG_ENT("read got %d bits (%d still needed)\n", n*8, (nbytes-n)*8); if (n == 0) { if (file->f_flags & O_NONBLOCK) { retval = -EAGAIN; break; } DEBUG_ENT("sleeping?\n"); wait_event_interruptible(random_read_wait, input_pool.entropy_count >= random_read_wakeup_thresh); DEBUG_ENT("awake\n"); if (signal_pending(current)) { retval = -ERESTARTSYS; break; } continue; } if (n < 0) { retval = n; break; } count += n; buf += n; nbytes -= n; break; /* This break makes the device work */ /* like a named pipe */ } return (count ? count : retval); } static ssize_t urandom_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos) { return extract_entropy_user(&nonblocking_pool, buf, nbytes); } static unsigned int random_poll(struct file *file, poll_table * wait) { unsigned int mask; poll_wait(file, &random_read_wait, wait); poll_wait(file, &random_write_wait, wait); mask = 0; if (input_pool.entropy_count >= random_read_wakeup_thresh) mask |= POLLIN | POLLRDNORM; if (input_pool.entropy_count < random_write_wakeup_thresh) mask |= POLLOUT | POLLWRNORM; return mask; } static int write_pool(struct entropy_store *r, const char __user *buffer, size_t count) { size_t bytes; __u32 buf[16]; const char __user *p = buffer; while (count > 0) { bytes = min(count, sizeof(buf)); if (copy_from_user(&buf, p, bytes)) return -EFAULT; count -= bytes; p += bytes; mix_pool_bytes(r, buf, bytes); cond_resched(); } return 0; } static ssize_t random_write(struct file *file, const char __user *buffer, size_t count, loff_t *ppos) { size_t ret; ret = write_pool(&blocking_pool, buffer, count); if (ret) return ret; ret = write_pool(&nonblocking_pool, buffer, count); if (ret) return ret; return (ssize_t)count; } static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg) { int size, ent_count; int __user *p = (int __user *)arg; int retval; switch (cmd) { case RNDGETENTCNT: /* inherently racy, no point locking */ if (put_user(input_pool.entropy_count, p)) return -EFAULT; return 0; case RNDADDTOENTCNT: if (!capable(CAP_SYS_ADMIN)) return -EPERM; if (get_user(ent_count, p)) return -EFAULT; credit_entropy_bits(&input_pool, ent_count); return 0; case RNDADDENTROPY: if (!capable(CAP_SYS_ADMIN)) return -EPERM; if (get_user(ent_count, p++)) return -EFAULT; if (ent_count < 0) return -EINVAL; if (get_user(size, p++)) return -EFAULT; retval = write_pool(&input_pool, (const char __user *)p, size); if (retval < 0) return retval; credit_entropy_bits(&input_pool, ent_count); return 0; case RNDZAPENTCNT: case RNDCLEARPOOL: /* Clear the entropy pool counters. */ if (!capable(CAP_SYS_ADMIN)) return -EPERM; rand_initialize(); return 0; default: return -EINVAL; } } static int random_fasync(int fd, struct file *filp, int on) { return fasync_helper(fd, filp, on, &fasync); } const struct file_operations random_fops = { .read = random_read, .write = random_write, .poll = random_poll, .unlocked_ioctl = random_ioctl, .fasync = random_fasync, }; const struct file_operations urandom_fops = { .read = urandom_read, .write = random_write, .unlocked_ioctl = random_ioctl, .fasync = random_fasync, }; /*************************************************************** * Random UUID interface * * Used here for a Boot ID, but can be useful for other kernel * drivers. ***************************************************************/ /* * Generate random UUID */ void generate_random_uuid(unsigned char uuid_out[16]) { get_random_bytes(uuid_out, 16); /* Set UUID version to 4 --- truely random generation */ uuid_out[6] = (uuid_out[6] & 0x0F) | 0x40; /* Set the UUID variant to DCE */ uuid_out[8] = (uuid_out[8] & 0x3F) | 0x80; } EXPORT_SYMBOL(generate_random_uuid); /******************************************************************** * * Sysctl interface * ********************************************************************/ #ifdef CONFIG_SYSCTL #include <linux/sysctl.h> static int min_read_thresh = 8, min_write_thresh; static int max_read_thresh = INPUT_POOL_WORDS * 32; static int max_write_thresh = INPUT_POOL_WORDS * 32; static char sysctl_bootid[16]; /* * These functions is used to return both the bootid UUID, and random * UUID. The difference is in whether table->data is NULL; if it is, * then a new UUID is generated and returned to the user. * * If the user accesses this via the proc interface, it will be returned * as an ASCII string in the standard UUID format. If accesses via the * sysctl system call, it is returned as 16 bytes of binary data. */ static int proc_do_uuid(ctl_table *table, int write, void __user *buffer, size_t *lenp, loff_t *ppos) { ctl_table fake_table; unsigned char buf[64], tmp_uuid[16], *uuid; uuid = table->data; if (!uuid) { uuid = tmp_uuid; uuid[8] = 0; } if (uuid[8] == 0) generate_random_uuid(uuid); sprintf(buf, "%pU", uuid); fake_table.data = buf; fake_table.maxlen = sizeof(buf); return proc_dostring(&fake_table, write, buffer, lenp, ppos); } static int sysctl_poolsize = INPUT_POOL_WORDS * 32; ctl_table random_table[] = { { .procname = "poolsize", .data = &sysctl_poolsize, .maxlen = sizeof(int), .mode = 0444, .proc_handler = proc_dointvec, }, { .procname = "entropy_avail", .maxlen = sizeof(int), .mode = 0444, .proc_handler = proc_dointvec, .data = &input_pool.entropy_count, }, { .procname = "read_wakeup_threshold", .data = &random_read_wakeup_thresh, .maxlen = sizeof(int), .mode = 0644, .proc_handler = proc_dointvec_minmax, .extra1 = &min_read_thresh, .extra2 = &max_read_thresh, }, { .procname = "write_wakeup_threshold", .data = &random_write_wakeup_thresh, .maxlen = sizeof(int), .mode = 0644, .proc_handler = proc_dointvec_minmax, .extra1 = &min_write_thresh, .extra2 = &max_write_thresh, }, { .procname = "boot_id", .data = &sysctl_bootid, .maxlen = 16, .mode = 0444, .proc_handler = proc_do_uuid, }, { .procname = "uuid", .maxlen = 16, .mode = 0444, .proc_handler = proc_do_uuid, }, { } }; #endif /* CONFIG_SYSCTL */ /******************************************************************** * * Random functions for networking * ********************************************************************/ /* * TCP initial sequence number picking. This uses the random number * generator to pick an initial secret value. This value is hashed * along with the TCP endpoint information to provide a unique * starting point for each pair of TCP endpoints. This defeats * attacks which rely on guessing the initial TCP sequence number. * This algorithm was suggested by Steve Bellovin. * * Using a very strong hash was taking an appreciable amount of the total * TCP connection establishment time, so this is a weaker hash, * compensated for by changing the secret periodically. */ /* F, G and H are basic MD4 functions: selection, majority, parity */ #define F(x, y, z) ((z) ^ ((x) & ((y) ^ (z)))) #define G(x, y, z) (((x) & (y)) + (((x) ^ (y)) & (z))) #define H(x, y, z) ((x) ^ (y) ^ (z)) /* * The generic round function. The application is so specific that * we don't bother protecting all the arguments with parens, as is generally * good macro practice, in favor of extra legibility. * Rotation is separate from addition to prevent recomputation */ #define ROUND(f, a, b, c, d, x, s) \ (a += f(b, c, d) + x, a = (a << s) | (a >> (32 - s))) #define K1 0 #define K2 013240474631UL #define K3 015666365641UL #if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE) static __u32 twothirdsMD4Transform(__u32 const buf[4], __u32 const in[12]) { __u32 a = buf[0], b = buf[1], c = buf[2], d = buf[3]; /* Round 1 */ ROUND(F, a, b, c, d, in[ 0] + K1, 3); ROUND(F, d, a, b, c, in[ 1] + K1, 7); ROUND(F, c, d, a, b, in[ 2] + K1, 11); ROUND(F, b, c, d, a, in[ 3] + K1, 19); ROUND(F, a, b, c, d, in[ 4] + K1, 3); ROUND(F, d, a, b, c, in[ 5] + K1, 7); ROUND(F, c, d, a, b, in[ 6] + K1, 11); ROUND(F, b, c, d, a, in[ 7] + K1, 19); ROUND(F, a, b, c, d, in[ 8] + K1, 3); ROUND(F, d, a, b, c, in[ 9] + K1, 7); ROUND(F, c, d, a, b, in[10] + K1, 11); ROUND(F, b, c, d, a, in[11] + K1, 19); /* Round 2 */ ROUND(G, a, b, c, d, in[ 1] + K2, 3); ROUND(G, d, a, b, c, in[ 3] + K2, 5); ROUND(G, c, d, a, b, in[ 5] + K2, 9); ROUND(G, b, c, d, a, in[ 7] + K2, 13); ROUND(G, a, b, c, d, in[ 9] + K2, 3); ROUND(G, d, a, b, c, in[11] + K2, 5); ROUND(G, c, d, a, b, in[ 0] + K2, 9); ROUND(G, b, c, d, a, in[ 2] + K2, 13); ROUND(G, a, b, c, d, in[ 4] + K2, 3); ROUND(G, d, a, b, c, in[ 6] + K2, 5); ROUND(G, c, d, a, b, in[ 8] + K2, 9); ROUND(G, b, c, d, a, in[10] + K2, 13); /* Round 3 */ ROUND(H, a, b, c, d, in[ 3] + K3, 3); ROUND(H, d, a, b, c, in[ 7] + K3, 9); ROUND(H, c, d, a, b, in[11] + K3, 11); ROUND(H, b, c, d, a, in[ 2] + K3, 15); ROUND(H, a, b, c, d, in[ 6] + K3, 3); ROUND(H, d, a, b, c, in[10] + K3, 9); ROUND(H, c, d, a, b, in[ 1] + K3, 11); ROUND(H, b, c, d, a, in[ 5] + K3, 15); ROUND(H, a, b, c, d, in[ 9] + K3, 3); ROUND(H, d, a, b, c, in[ 0] + K3, 9); ROUND(H, c, d, a, b, in[ 4] + K3, 11); ROUND(H, b, c, d, a, in[ 8] + K3, 15); return buf[1] + b; /* "most hashed" word */ /* Alternative: return sum of all words? */ } #endif #undef ROUND #undef F #undef G #undef H #undef K1 #undef K2 #undef K3 /* This should not be decreased so low that ISNs wrap too fast. */ #define REKEY_INTERVAL (300 * HZ) /* * Bit layout of the tcp sequence numbers (before adding current time): * bit 24-31: increased after every key exchange * bit 0-23: hash(source,dest) * * The implementation is similar to the algorithm described * in the Appendix of RFC 1185, except that * - it uses a 1 MHz clock instead of a 250 kHz clock * - it performs a rekey every 5 minutes, which is equivalent * to a (source,dest) tulple dependent forward jump of the * clock by 0..2^(HASH_BITS+1) * * Thus the average ISN wraparound time is 68 minutes instead of * 4.55 hours. * * SMP cleanup and lock avoidance with poor man's RCU. * Manfred Spraul <manfred@colorfullife.com> * */ #define COUNT_BITS 8 #define COUNT_MASK ((1 << COUNT_BITS) - 1) #define HASH_BITS 24 #define HASH_MASK ((1 << HASH_BITS) - 1) static struct keydata { __u32 count; /* already shifted to the final position */ __u32 secret[12]; } ____cacheline_aligned ip_keydata[2]; static unsigned int ip_cnt; static void rekey_seq_generator(struct work_struct *work); static DECLARE_DELAYED_WORK(rekey_work, rekey_seq_generator); /* * Lock avoidance: * The ISN generation runs lockless - it's just a hash over random data. * State changes happen every 5 minutes when the random key is replaced. * Synchronization is performed by having two copies of the hash function * state and rekey_seq_generator always updates the inactive copy. * The copy is then activated by updating ip_cnt. * The implementation breaks down if someone blocks the thread * that processes SYN requests for more than 5 minutes. Should never * happen, and even if that happens only a not perfectly compliant * ISN is generated, nothing fatal. */ static void rekey_seq_generator(struct work_struct *work) { struct keydata *keyptr = &ip_keydata[1 ^ (ip_cnt & 1)]; get_random_bytes(keyptr->secret, sizeof(keyptr->secret)); keyptr->count = (ip_cnt & COUNT_MASK) << HASH_BITS; smp_wmb(); ip_cnt++; schedule_delayed_work(&rekey_work, round_jiffies_relative(REKEY_INTERVAL)); } static inline struct keydata *get_keyptr(void) { struct keydata *keyptr = &ip_keydata[ip_cnt & 1]; smp_rmb(); return keyptr; } static __init int seqgen_init(void) { rekey_seq_generator(NULL); return 0; } late_initcall(seqgen_init); #if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE) __u32 secure_tcpv6_sequence_number(__be32 *saddr, __be32 *daddr, __be16 sport, __be16 dport) { __u32 seq; __u32 hash[12]; struct keydata *keyptr = get_keyptr(); /* The procedure is the same as for IPv4, but addresses are longer. * Thus we must use twothirdsMD4Transform. */ memcpy(hash, saddr, 16); hash[4] = ((__force u16)sport << 16) + (__force u16)dport; memcpy(&hash[5], keyptr->secret, sizeof(__u32) * 7); seq = twothirdsMD4Transform((const __u32 *)daddr, hash) & HASH_MASK; seq += keyptr->count; seq += ktime_to_ns(ktime_get_real()); return seq; } EXPORT_SYMBOL(secure_tcpv6_sequence_number); #endif /* The code below is shamelessly stolen from secure_tcp_sequence_number(). * All blames to Andrey V. Savochkin <saw@msu.ru>. */ __u32 secure_ip_id(__be32 daddr) { struct keydata *keyptr; __u32 hash[4]; keyptr = get_keyptr(); /* * Pick a unique starting offset for each IP destination. * The dest ip address is placed in the starting vector, * which is then hashed with random data. */ hash[0] = (__force __u32)daddr; hash[1] = keyptr->secret[9]; hash[2] = keyptr->secret[10]; hash[3] = keyptr->secret[11]; return half_md4_transform(hash, keyptr->secret); } #ifdef CONFIG_INET __u32 secure_tcp_sequence_number(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport) { __u32 seq; __u32 hash[4]; struct keydata *keyptr = get_keyptr(); /* * Pick a unique starting offset for each TCP connection endpoints * (saddr, daddr, sport, dport). * Note that the words are placed into the starting vector, which is * then mixed with a partial MD4 over random data. */ hash[0] = (__force u32)saddr; hash[1] = (__force u32)daddr; hash[2] = ((__force u16)sport << 16) + (__force u16)dport; hash[3] = keyptr->secret[11]; seq = half_md4_transform(hash, keyptr->secret) & HASH_MASK; seq += keyptr->count; /* * As close as possible to RFC 793, which * suggests using a 250 kHz clock. * Further reading shows this assumes 2 Mb/s networks. * For 10 Mb/s Ethernet, a 1 MHz clock is appropriate. * For 10 Gb/s Ethernet, a 1 GHz clock should be ok, but * we also need to limit the resolution so that the u32 seq * overlaps less than one time per MSL (2 minutes). * Choosing a clock of 64 ns period is OK. (period of 274 s) */ seq += ktime_to_ns(ktime_get_real()) >> 6; return seq; } /* Generate secure starting point for ephemeral IPV4 transport port search */ u32 secure_ipv4_port_ephemeral(__be32 saddr, __be32 daddr, __be16 dport) { struct keydata *keyptr = get_keyptr(); u32 hash[4]; /* * Pick a unique starting offset for each ephemeral port search * (saddr, daddr, dport) and 48bits of random data. */ hash[0] = (__force u32)saddr; hash[1] = (__force u32)daddr; hash[2] = (__force u32)dport ^ keyptr->secret[10]; hash[3] = keyptr->secret[11]; return half_md4_transform(hash, keyptr->secret); } EXPORT_SYMBOL_GPL(secure_ipv4_port_ephemeral); #if defined(CONFIG_IPV6) || defined(CONFIG_IPV6_MODULE) u32 secure_ipv6_port_ephemeral(const __be32 *saddr, const __be32 *daddr, __be16 dport) { struct keydata *keyptr = get_keyptr(); u32 hash[12]; memcpy(hash, saddr, 16); hash[4] = (__force u32)dport; memcpy(&hash[5], keyptr->secret, sizeof(__u32) * 7); return twothirdsMD4Transform((const __u32 *)daddr, hash); } #endif #if defined(CONFIG_IP_DCCP) || defined(CONFIG_IP_DCCP_MODULE) /* Similar to secure_tcp_sequence_number but generate a 48 bit value * bit's 32-47 increase every key exchange * 0-31 hash(source, dest) */ u64 secure_dccp_sequence_number(__be32 saddr, __be32 daddr, __be16 sport, __be16 dport) { u64 seq; __u32 hash[4]; struct keydata *keyptr = get_keyptr(); hash[0] = (__force u32)saddr; hash[1] = (__force u32)daddr; hash[2] = ((__force u16)sport << 16) + (__force u16)dport; hash[3] = keyptr->secret[11]; seq = half_md4_transform(hash, keyptr->secret); seq |= ((u64)keyptr->count) << (32 - HASH_BITS); seq += ktime_to_ns(ktime_get_real()); seq &= (1ull << 48) - 1; return seq; } EXPORT_SYMBOL(secure_dccp_sequence_number); #endif #endif /* CONFIG_INET */ /* * Get a random word for internal kernel use only. Similar to urandom but * with the goal of minimal entropy pool depletion. As a result, the random * value is not cryptographically secure but for several uses the cost of * depleting entropy is too high */ DEFINE_PER_CPU(__u32 [4], get_random_int_hash); unsigned int get_random_int(void) { struct keydata *keyptr; __u32 *hash = get_cpu_var(get_random_int_hash); int ret; keyptr = get_keyptr(); hash[0] += current->pid + jiffies + get_cycles(); ret = half_md4_transform(hash, keyptr->secret); put_cpu_var(get_random_int_hash); return ret; } /* * randomize_range() returns a start address such that * * [...... <range> .....] * start end * * a <range> with size "len" starting at the return value is inside in the * area defined by [start, end], but is otherwise randomized. */ unsigned long randomize_range(unsigned long start, unsigned long end, unsigned long len) { unsigned long range = end - len - start; if (end <= start + len) return 0; return PAGE_ALIGN(get_random_int() % range + start); }