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[linux-2.6/linux-acpi-2.6/ibm-acpi-2.6.git] / drivers / char / random.c

/*
 * 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/spinlock.h>
#include <linux/percpu.h>
#include <linux/cryptohash.h>

#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) = 0;

/*
 * 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);

#if 0
static int debug = 0;
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 {
        /* mostly-read data: */
        struct poolinfo *poolinfo;
        __u32 *pool;
        const char *name;
        int limit;
        struct entropy_store *pull;

        /* read-write data: */
        spinlock_t lock ____cacheline_aligned_in_smp;
        unsigned add_ptr;
        int entropy_count;
        int input_rotate;
};

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 a byte into the entropy "pool".  It does not
 * update the entropy estimate.  The caller should call
 * credit_entropy_store 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 __add_entropy_words(struct entropy_store *r, const __u32 *in,
                                int nwords, __u32 out[16])
{
        static __u32 const twist_table[8] = {
                0x00000000, 0x3b6e20c8, 0x76dc4190, 0x4db26158,
                0xedb88320, 0xd6d6a3e8, 0x9b64c2b0, 0xa00ae278 };
        unsigned long i, add_ptr, tap1, tap2, tap3, tap4, tap5;
        int new_rotate, input_rotate;
        int wordmask = r->poolinfo->poolwords - 1;
        __u32 w, next_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;
        next_w = *in++;

        spin_lock_irqsave(&r->lock, flags);
        prefetch_range(r->pool, wordmask);
        input_rotate = r->input_rotate;
        add_ptr = r->add_ptr;

        while (nwords--) {
                w = rol32(next_w, input_rotate);
                if (nwords > 0)
                        next_w = *in++;
                i = add_ptr = (add_ptr - 1) & wordmask;
                /*
                 * 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.
                 */
                new_rotate = input_rotate + 14;
                if (i)
                        new_rotate = input_rotate + 7;
                input_rotate = new_rotate & 31;

                /* XOR in the various taps */
                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];
                w ^= r->pool[i];
                r->pool[i] = (w >> 3) ^ twist_table[w & 7];
        }

        r->input_rotate = input_rotate;
        r->add_ptr = add_ptr;

        if (out) {
                for (i = 0; i < 16; i++) {
                        out[i] = r->pool[add_ptr];
                        add_ptr = (add_ptr - 1) & wordmask;
                }
        }

        spin_unlock_irqrestore(&r->lock, flags);
}

static inline void add_entropy_words(struct entropy_store *r, const __u32 *in,
                                     int nwords)
{
        __add_entropy_words(r, in, nwords, NULL);
}

/*
 * Credit (or debit) the entropy store with n bits of entropy
 */
static void credit_entropy_store(struct entropy_store *r, int nbits)
{
        unsigned long flags;

        spin_lock_irqsave(&r->lock, flags);

        if (r->entropy_count + nbits < 0) {
                DEBUG_ENT("negative entropy/overflow (%d+%d)\n",
                          r->entropy_count, nbits);
                r->entropy_count = 0;
        } else if (r->entropy_count + nbits > r->poolinfo->POOLBITS) {
                r->entropy_count = r->poolinfo->POOLBITS;
        } else {
                r->entropy_count += nbits;
                if (nbits)
                        DEBUG_ENT("added %d entropy credits to %s\n",
                                  nbits, r->name);
        }

        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;
};

static struct timer_rand_state input_timer_state;
static struct timer_rand_state *irq_timer_state[NR_IRQS];

/*
 * 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;
        add_entropy_words(&input_pool, (u32 *)&sample, sizeof(sample)/4);

        /*
         * 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_store(&input_pool,
                                     min_t(int, fls(delta>>1), 11));
        }

        if(input_pool.entropy_count >= random_read_wakeup_thresh)
                wake_up_interruptible(&random_read_wait);

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)
{
        if (irq >= NR_IRQS || irq_timer_state[irq] == 0)
                return;

        DEBUG_ENT("irq event %d\n", irq);
        add_timer_randomness(irq_timer_state[irq], 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", disk->major, disk->first_minor);

        add_timer_randomness(disk->random,
                             0x100 + MKDEV(disk->major, disk->first_minor));
}

EXPORT_SYMBOL(add_disk_randomness);
#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);
                add_entropy_words(r, tmp, (bytes + 3) / 4);
                credit_entropy_store(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;

        BUG_ON(r->entropy_count > r->poolinfo->POOLBITS);

        /* Hold lock while accounting */
        spin_lock_irqsave(&r->lock, flags);

        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);
        }

        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 data[16], buf[5 + SHA_WORKSPACE_WORDS];

        sha_init(buf);
        /*
         * As we hash the pool, we mix intermediate values of
         * the hash back into the pool.  This eliminates
         * 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.
         */
        for (i = 0; i < r->poolinfo->poolwords; i += 16) {
                /* hash blocks of 16 words = 512 bits */
                sha_transform(buf, (__u8 *)(r->pool + i), buf + 5);
                /* feed back portion of the resulting hash */
                add_entropy_words(r, &buf[i % 5], 1);
        }

        /*
         * To avoid duplicates, we atomically extract a
         * portion of the pool while mixing, and hash one
         * final time.
         */
        __add_entropy_words(r, &buf[i % 5], 1, data);
        sha_transform(buf, (__u8 *)data, buf + 5);

        /*
         * In case the hash function has some recognizable
         * output pattern, we fold it in half.
         */

        buf[0] ^= buf[3];
        buf[1] ^= buf[4];
        buf[0] ^= rol32(buf[3], 16);
        memcpy(out, buf, EXTRACT_SIZE);
        memset(buf, 0, sizeof(buf));
}

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];

        xfer_secondary_pool(r, nbytes);
        nbytes = account(r, nbytes, min, reserved);

        while (nbytes) {
                extract_buf(r, tmp);
                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)
{
        struct timeval tv;
        unsigned long flags;

        spin_lock_irqsave(&r->lock, flags);
        r->entropy_count = 0;
        spin_unlock_irqrestore(&r->lock, flags);

        do_gettimeofday(&tv);
        add_entropy_words(r, (__u32 *)&tv, sizeof(tv)/4);
        add_entropy_words(r, (__u32 *)utsname(),
                          sizeof(*(utsname()))/4);
}

static int __init 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;

        if (irq >= NR_IRQS || irq_timer_state[irq])
                return;

        /*
         * If kmalloc returns null, we just won't use that entropy
         * source.
         */
        state = kmalloc(sizeof(struct timer_rand_state), GFP_KERNEL);
        if (state) {
                memset(state, 0, sizeof(struct timer_rand_state));
                irq_timer_state[irq] = state;
        }
}

#ifdef CONFIG_BLOCK
void rand_initialize_disk(struct gendisk *disk)
{
        struct timer_rand_state *state;

        /*
         * If kmalloc returns null, we just won't use that entropy
         * source.
         */
        state = kmalloc(sizeof(struct timer_rand_state), GFP_KERNEL);
        if (state) {
                memset(state, 0, sizeof(struct timer_rand_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 */
        }

        /*
         * If we gave the user some bytes, update the access time.
         */
        if (count)
                file_accessed(file);

        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;

                add_entropy_words(r, buf, (bytes + 3) / 4);
        }

        return 0;
}

static ssize_t
random_write(struct file * file, const char __user * buffer,
             size_t count, loff_t *ppos)
{
        size_t ret;
        struct inode *inode = file->f_path.dentry->d_inode;

        ret = write_pool(&blocking_pool, buffer, count);
        if (ret)
                return ret;
        ret = write_pool(&nonblocking_pool, buffer, count);
        if (ret)
                return ret;

        inode->i_mtime = current_fs_time(inode->i_sb);
        mark_inode_dirty(inode);
        return (ssize_t)count;
}

static int
random_ioctl(struct inode * inode, struct file * file,
             unsigned int cmd, unsigned long arg)
{
        int size, ent_count;
        int __user *p = (int __user *)arg;
        int retval;

        switch (cmd) {
        case RNDGETENTCNT:
                ent_count = input_pool.entropy_count;
                if (put_user(ent_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_store(&input_pool, ent_count);
                /*
                 * Wake up waiting processes if we have enough
                 * entropy.
                 */
                if (input_pool.entropy_count >= random_read_wakeup_thresh)
                        wake_up_interruptible(&random_read_wait);
                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_store(&input_pool, ent_count);
                /*
                 * Wake up waiting processes if we have enough
                 * entropy.
                 */
                if (input_pool.entropy_count >= random_read_wakeup_thresh)
                        wake_up_interruptible(&random_read_wait);
                return 0;
        case RNDZAPENTCNT:
        case RNDCLEARPOOL:
                /* Clear the entropy pool counters. */
                if (!capable(CAP_SYS_ADMIN))
                        return -EPERM;
                init_std_data(&input_pool);
                init_std_data(&blocking_pool);
                init_std_data(&nonblocking_pool);
                return 0;
        default:
                return -EINVAL;
        }
}

struct file_operations random_fops = {
        .read  = random_read,
        .write = random_write,
        .poll  = random_poll,
        .ioctl = random_ioctl,
};

struct file_operations urandom_fops = {
        .read  = urandom_read,
        .write = random_write,
        .ioctl = random_ioctl,
};

/***************************************************************
 * 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, struct file *filp,
                        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, "%02x%02x%02x%02x-%02x%02x-%02x%02x-%02x%02x-"
                "%02x%02x%02x%02x%02x%02x",
                uuid[0],  uuid[1],  uuid[2],  uuid[3],
                uuid[4],  uuid[5],  uuid[6],  uuid[7],
                uuid[8],  uuid[9],  uuid[10], uuid[11],
                uuid[12], uuid[13], uuid[14], uuid[15]);
        fake_table.data = buf;
        fake_table.maxlen = sizeof(buf);

        return proc_dostring(&fake_table, write, filp, buffer, lenp, ppos);
}

static int uuid_strategy(ctl_table *table, int __user *name, int nlen,
                         void __user *oldval, size_t __user *oldlenp,
                         void __user *newval, size_t newlen)
{
        unsigned char tmp_uuid[16], *uuid;
        unsigned int len;

        if (!oldval || !oldlenp)
                return 1;

        uuid = table->data;
        if (!uuid) {
                uuid = tmp_uuid;
                uuid[8] = 0;
        }
        if (uuid[8] == 0)
                generate_random_uuid(uuid);

        if (get_user(len, oldlenp))
                return -EFAULT;
        if (len) {
                if (len > 16)
                        len = 16;
                if (copy_to_user(oldval, uuid, len) ||
                    put_user(len, oldlenp))
                        return -EFAULT;
        }
        return 1;
}

static int sysctl_poolsize = INPUT_POOL_WORDS * 32;
ctl_table random_table[] = {
        {
                .ctl_name       = RANDOM_POOLSIZE,
                .procname       = "poolsize",
                .data           = &sysctl_poolsize,
                .maxlen         = sizeof(int),
                .mode           = 0444,
                .proc_handler   = &proc_dointvec,
        },
        {
                .ctl_name       = RANDOM_ENTROPY_COUNT,
                .procname       = "entropy_avail",
                .maxlen         = sizeof(int),
                .mode           = 0444,
                .proc_handler   = &proc_dointvec,
                .data           = &input_pool.entropy_count,
        },
        {
                .ctl_name       = RANDOM_READ_THRESH,
                .procname       = "read_wakeup_threshold",
                .data           = &random_read_wakeup_thresh,
                .maxlen         = sizeof(int),
                .mode           = 0644,
                .proc_handler   = &proc_dointvec_minmax,
                .strategy       = &sysctl_intvec,
                .extra1         = &min_read_thresh,
                .extra2         = &max_read_thresh,
        },
        {
                .ctl_name       = RANDOM_WRITE_THRESH,
                .procname       = "write_wakeup_threshold",
                .data           = &random_write_wakeup_thresh,
                .maxlen         = sizeof(int),
                .mode           = 0644,
                .proc_handler   = &proc_dointvec_minmax,
                .strategy       = &sysctl_intvec,
                .extra1         = &min_write_thresh,
                .extra2         = &max_write_thresh,
        },
        {
                .ctl_name       = RANDOM_BOOT_ID,
                .procname       = "boot_id",
                .data           = &sysctl_bootid,
                .maxlen         = 16,
                .mode           = 0444,
                .proc_handler   = &proc_do_uuid,
                .strategy       = &uuid_strategy,
        },
        {
                .ctl_name       = RANDOM_UUID,
                .procname       = "uuid",
                .maxlen         = 16,
                .mode           = 0444,
                .proc_handler   = &proc_do_uuid,
                .strategy       = &uuid_strategy,
        },
        { .ctl_name = 0 }
};
#endif  /* CONFIG_SYSCTL */

/********************************************************************
 *
 * Random funtions 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, 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)
{
        struct timeval tv;
        __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;

        do_gettimeofday(&tv);
        seq += tv.tv_usec + tv.tv_sec * 1000000;

        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)
{
        struct timeval tv;
        __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.
         *      That's funny, Linux has one built in!  Use it!
         *      (Networks are faster now - should this be increased?)
         */
        do_gettimeofday(&tv);
        seq += tv.tv_usec + tv.tv_sec * 1000000;
#if 0
        printk("init_seq(%lx, %lx, %d, %d) = %d\n",
               saddr, daddr, sport, dport, seq);
#endif
        return seq;
}

EXPORT_SYMBOL(secure_tcp_sequence_number);

/* 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);
}

#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)
{
        struct timeval tv;
        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);

        do_gettimeofday(&tv);
        seq += tv.tv_usec + tv.tv_sec * 1000000;
        seq &= (1ull << 48) - 1;
#if 0
        printk("dccp init_seq(%lx, %lx, %d, %d) = %d\n",
               saddr, daddr, sport, dport, seq);
#endif
        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
 */
unsigned int get_random_int(void)
{
        /*
         * Use IP's RNG. It suits our purpose perfectly: it re-keys itself
         * every second, from the entropy pool (and thus creates a limited
         * drain on it), and uses halfMD4Transform within the second. We
         * also mix it with jiffies and the PID:
         */
        return secure_ip_id((__force __be32)(current->pid + jiffies));
}

/*
 * 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);
}