crypto: jitterentropy - add jitterentropy RNG

The CPU Jitter RNG provides a source of good entropy by
collecting CPU executing time jitter. The entropy in the CPU
execution time jitter is magnified by the CPU Jitter Random
Number Generator. The CPU Jitter Random Number Generator uses
the CPU execution timing jitter to generate a bit stream
which complies with different statistical measurements that
determine the bit stream is random.

The CPU Jitter Random Number Generator delivers entropy which
follows information theoretical requirements. Based on these
studies and the implementation, the caller can assume that
one bit of data extracted from the CPU Jitter Random Number
Generator holds one bit of entropy.

The CPU Jitter Random Number Generator provides a decentralized
source of entropy, i.e. every caller can operate on a private
state of the entropy pool.

The RNG does not have any dependencies on any other service
in the kernel. The RNG only needs a high-resolution time
stamp.

Further design details, the cryptographic assessment and
large array of test results are documented at
http://www.chronox.de/jent.html.

CC: Andreas Steffen <andreas.steffen@strongswan.org>
CC: Theodore Ts'o <tytso@mit.edu>
CC: Sandy Harris <sandyinchina@gmail.com>
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 909 insertions, 0 deletions

diff --git a/crypto/jitterentropy.c b/crypto/jitterentropy.c
new file mode 100644
index 000000000000..1ebe58a26619
--- /dev/null
+++ b/crypto/jitterentropy.c
@@ -0,0 +1,909 @@
+/*
+ * Non-physical true random number generator based on timing jitter.
+ *
+ * Copyright Stephan Mueller <smueller@chronox.de>, 2014
+ *
+ * Design
+ * ======
+ *
+ * See http://www.chronox.de/jent.html
+ *
+ * License
+ * =======
+ *
+ * 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 GPL2 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.
+ */
+
+/*
+ * This Jitterentropy RNG is based on the jitterentropy library
+ * version 1.1.0 provided at http://www.chronox.de/jent.html
+ */
+
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/module.h>
+#include <linux/fips.h>
+#include <linux/time.h>
+#include <linux/crypto.h>
+#include <crypto/internal/rng.h>
+
+#ifdef __OPTIMIZE__
+ #error "The CPU Jitter random number generator must not be compiled with optimizations. See documentation. Use the compiler switch -O0 for compiling jitterentropy.c."
+#endif
+
+/* The entropy pool */
+struct rand_data {
+        /* all data values that are vital to maintain the security
+         * of the RNG are marked as SENSITIVE. A user must not
+         * access that information while the RNG executes its loops to
+         * calculate the next random value. */
+        __u64 data;             /* SENSITIVE Actual random number */
+        __u64 old_data;         /* SENSITIVE Previous random number */
+        __u64 prev_time;        /* SENSITIVE Previous time stamp */
+#define DATA_SIZE_BITS ((sizeof(__u64)) * 8)
+        __u64 last_delta;       /* SENSITIVE stuck test */
+        __s64 last_delta2;      /* SENSITIVE stuck test */
+        unsigned int stuck:1;   /* Time measurement stuck */
+        unsigned int osr;       /* Oversample rate */
+        unsigned int stir:1;            /* Post-processing stirring */
+        unsigned int disable_unbias:1;  /* Deactivate Von-Neuman unbias */
+#define JENT_MEMORY_BLOCKS 64
+#define JENT_MEMORY_BLOCKSIZE 32
+#define JENT_MEMORY_ACCESSLOOPS 128
+#define JENT_MEMORY_SIZE (JENT_MEMORY_BLOCKS*JENT_MEMORY_BLOCKSIZE)
+        unsigned char *mem;     /* Memory access location with size of
+                                 * memblocks * memblocksize */
+        unsigned int memlocation; /* Pointer to byte in *mem */
+        unsigned int memblocks; /* Number of memory blocks in *mem */
+        unsigned int memblocksize; /* Size of one memory block in bytes */
+        unsigned int memaccessloops; /* Number of memory accesses per random
+                                      * bit generation */
+};
+
+/* Flags that can be used to initialize the RNG */
+#define JENT_DISABLE_STIR (1<<0) /* Disable stirring the entropy pool */
+#define JENT_DISABLE_UNBIAS (1<<1) /* Disable the Von-Neuman Unbiaser */
+#define JENT_DISABLE_MEMORY_ACCESS (1<<2) /* Disable memory access for more
+                                           * entropy, saves MEMORY_SIZE RAM for
+                                           * entropy collector */
+
+#define DRIVER_NAME     "jitterentropy"
+
+/* -- error codes for init function -- */
+#define JENT_ENOTIME            1 /* Timer service not available */
+#define JENT_ECOARSETIME        2 /* Timer too coarse for RNG */
+#define JENT_ENOMONOTONIC       3 /* Timer is not monotonic increasing */
+#define JENT_EMINVARIATION      4 /* Timer variations too small for RNG */
+#define JENT_EVARVAR            5 /* Timer does not produce variations of
+                                   * variations (2nd derivation of time is
+                                   * zero). */
+#define JENT_EMINVARVAR         6 /* Timer variations of variations is tooi
+                                   * small. */
+
+/***************************************************************************
+ * Helper functions
+ ***************************************************************************/
+
+static inline void jent_get_nstime(__u64 *out)
+{
+        struct timespec ts;
+        __u64 tmp = 0;
+
+        tmp = random_get_entropy();
+
+        /*
+         * If random_get_entropy does not return a value (which is possible on,
+         * for example, MIPS), invoke __getnstimeofday
+         * hoping that there are timers we can work with.
+         *
+         * The list of available timers can be obtained from
+         * /sys/devices/system/clocksource/clocksource0/available_clocksource
+         * and are registered with clocksource_register()
+         */
+        if ((0 == tmp) &&
+#ifndef MODULE
+           (0 == timekeeping_valid_for_hres()) &&
+#endif
+           (0 == __getnstimeofday(&ts))) {
+                tmp = ts.tv_sec;
+                tmp = tmp << 32;
+                tmp = tmp | ts.tv_nsec;
+        }
+
+        *out = tmp;
+}
+
+
+/**
+ * Update of the loop count used for the next round of
+ * an entropy collection.
+ *
+ * Input:
+ * @ec entropy collector struct -- may be NULL
+ * @bits is the number of low bits of the timer to consider
+ * @min is the number of bits we shift the timer value to the right at
+ *      the end to make sure we have a guaranteed minimum value
+ *
+ * @return Newly calculated loop counter
+ */
+static __u64 jent_loop_shuffle(struct rand_data *ec,
+                               unsigned int bits, unsigned int min)
+{
+        __u64 time = 0;
+        __u64 shuffle = 0;
+        unsigned int i = 0;
+        unsigned int mask = (1<<bits) - 1;
+
+        jent_get_nstime(&time);
+        /*
+         * mix the current state of the random number into the shuffle
+         * calculation to balance that shuffle a bit more
+         */
+        if (ec)
+                time ^= ec->data;
+        /*
+         * we fold the time value as much as possible to ensure that as many
+         * bits of the time stamp are included as possible
+         */
+        for (i = 0; (DATA_SIZE_BITS / bits) > i; i++) {
+                shuffle ^= time & mask;
+                time = time >> bits;
+        }
+
+        /*
+         * We add a lower boundary value to ensure we have a minimum
+         * RNG loop count.
+         */
+        return (shuffle + (1<<min));
+}
+
+/***************************************************************************
+ * Noise sources
+ ***************************************************************************/
+
+/**
+ * CPU Jitter noise source -- this is the noise source based on the CPU
+ *                            execution time jitter
+ *
+ * This function folds the time into one bit units by iterating
+ * through the DATA_SIZE_BITS bit time value as follows: assume our time value
+ * is 0xabcd
+ * 1st loop, 1st shift generates 0xd000
+ * 1st loop, 2nd shift generates 0x000d
+ * 2nd loop, 1st shift generates 0xcd00
+ * 2nd loop, 2nd shift generates 0x000c
+ * 3rd loop, 1st shift generates 0xbcd0
+ * 3rd loop, 2nd shift generates 0x000b
+ * 4th loop, 1st shift generates 0xabcd
+ * 4th loop, 2nd shift generates 0x000a
+ * Now, the values at the end of the 2nd shifts are XORed together.
+ *
+ * The code is deliberately inefficient and shall stay that way. This function
+ * is the root cause why the code shall be compiled without optimization. This
+ * function not only acts as folding operation, but this function's execution
+ * is used to measure the CPU execution time jitter. Any change to the loop in
+ * this function implies that careful retesting must be done.
+ *
+ * Input:
+ * @ec entropy collector struct -- may be NULL
+ * @time time stamp to be folded
+ * @loop_cnt if a value not equal to 0 is set, use the given value as number of
+ *           loops to perform the folding
+ *
+ * Output:
+ * @folded result of folding operation
+ *
+ * @return Number of loops the folding operation is performed
+ */
+static __u64 jent_fold_time(struct rand_data *ec, __u64 time,
+                            __u64 *folded, __u64 loop_cnt)
+{
+        unsigned int i;
+        __u64 j = 0;
+        __u64 new = 0;
+#define MAX_FOLD_LOOP_BIT 4
+#define MIN_FOLD_LOOP_BIT 0
+        __u64 fold_loop_cnt =
+                jent_loop_shuffle(ec, MAX_FOLD_LOOP_BIT, MIN_FOLD_LOOP_BIT);
+
+        /*
+         * testing purposes -- allow test app to set the counter, not
+         * needed during runtime
+         */
+        if (loop_cnt)
+                fold_loop_cnt = loop_cnt;
+        for (j = 0; j < fold_loop_cnt; j++) {
+                new = 0;
+                for (i = 1; (DATA_SIZE_BITS) >= i; i++) {
+                        __u64 tmp = time << (DATA_SIZE_BITS - i);
+
+                        tmp = tmp >> (DATA_SIZE_BITS - 1);
+                        new ^= tmp;
+                }
+        }
+        *folded = new;
+        return fold_loop_cnt;
+}
+
+/**
+ * Memory Access noise source -- this is a noise source based on variations in
+ *                               memory access times
+ *
+ * This function performs memory accesses which will add to the timing
+ * variations due to an unknown amount of CPU wait states that need to be
+ * added when accessing memory. The memory size should be larger than the L1
+ * caches as outlined in the documentation and the associated testing.
+ *
+ * The L1 cache has a very high bandwidth, albeit its access rate is  usually
+ * slower than accessing CPU registers. Therefore, L1 accesses only add minimal
+ * variations as the CPU has hardly to wait. Starting with L2, significant
+ * variations are added because L2 typically does not belong to the CPU any more
+ * and therefore a wider range of CPU wait states is necessary for accesses.
+ * L3 and real memory accesses have even a wider range of wait states. However,
+ * to reliably access either L3 or memory, the ec->mem memory must be quite
+ * large which is usually not desirable.
+ *
+ * Input:
+ * @ec Reference to the entropy collector with the memory access data -- if
+ *     the reference to the memory block to be accessed is NULL, this noise
+ *     source is disabled
+ * @loop_cnt if a value not equal to 0 is set, use the given value as number of
+ *           loops to perform the folding
+ *
+ * @return Number of memory access operations
+ */
+static unsigned int jent_memaccess(struct rand_data *ec, __u64 loop_cnt)
+{
+        unsigned char *tmpval = NULL;
+        unsigned int wrap = 0;
+        __u64 i = 0;
+#define MAX_ACC_LOOP_BIT 7
+#define MIN_ACC_LOOP_BIT 0
+        __u64 acc_loop_cnt =
+                jent_loop_shuffle(ec, MAX_ACC_LOOP_BIT, MIN_ACC_LOOP_BIT);
+
+        if (NULL == ec || NULL == ec->mem)
+                return 0;
+        wrap = ec->memblocksize * ec->memblocks;
+
+        /*
+         * testing purposes -- allow test app to set the counter, not
+         * needed during runtime
+         */
+        if (loop_cnt)
+                acc_loop_cnt = loop_cnt;
+
+        for (i = 0; i < (ec->memaccessloops + acc_loop_cnt); i++) {
+                tmpval = ec->mem + ec->memlocation;
+                /*
+                 * memory access: just add 1 to one byte,
+                 * wrap at 255 -- memory access implies read
+                 * from and write to memory location
+                 */
+                *tmpval = (*tmpval + 1) & 0xff;
+                /*
+                 * Addition of memblocksize - 1 to pointer
+                 * with wrap around logic to ensure that every
+                 * memory location is hit evenly
+                 */
+                ec->memlocation = ec->memlocation + ec->memblocksize - 1;
+                ec->memlocation = ec->memlocation % wrap;
+        }
+        return i;
+}
+
+/***************************************************************************
+ * Start of entropy processing logic
+ ***************************************************************************/
+
+/**
+ * Stuck test by checking the:
+ *      1st derivation of the jitter measurement (time delta)
+ *      2nd derivation of the jitter measurement (delta of time deltas)
+ *      3rd derivation of the jitter measurement (delta of delta of time deltas)
+ *
+ * All values must always be non-zero.
+ *
+ * Input:
+ * @ec Reference to entropy collector
+ * @current_delta Jitter time delta
+ *
+ * @return
+ *      0 jitter measurement not stuck (good bit)
+ *      1 jitter measurement stuck (reject bit)
+ */
+static void jent_stuck(struct rand_data *ec, __u64 current_delta)
+{
+        __s64 delta2 = ec->last_delta - current_delta;
+        __s64 delta3 = delta2 - ec->last_delta2;
+
+        ec->last_delta = current_delta;
+        ec->last_delta2 = delta2;
+
+        if (!current_delta || !delta2 || !delta3)
+                ec->stuck = 1;
+}
+
+/**
+ * This is the heart of the entropy generation: calculate time deltas and
+ * use the CPU jitter in the time deltas. The jitter is folded into one
+ * bit. You can call this function the "random bit generator" as it
+ * produces one random bit per invocation.
+ *
+ * WARNING: ensure that ->prev_time is primed before using the output
+ *          of this function! This can be done by calling this function
+ *          and not using its result.
+ *
+ * Input:
+ * @entropy_collector Reference to entropy collector
+ *
+ * @return One random bit
+ */
+static __u64 jent_measure_jitter(struct rand_data *ec)
+{
+        __u64 time = 0;
+        __u64 data = 0;
+        __u64 current_delta = 0;
+
+        /* Invoke one noise source before time measurement to add variations */
+        jent_memaccess(ec, 0);
+
+        /*
+         * Get time stamp and calculate time delta to previous
+         * invocation to measure the timing variations
+         */
+        jent_get_nstime(&time);
+        current_delta = time - ec->prev_time;
+        ec->prev_time = time;
+
+        /* Now call the next noise sources which also folds the data */
+        jent_fold_time(ec, current_delta, &data, 0);
+
+        /*
+         * Check whether we have a stuck measurement. The enforcement
+         * is performed after the stuck value has been mixed into the
+         * entropy pool.
+         */
+        jent_stuck(ec, current_delta);
+
+        return data;
+}
+
+/**
+ * Von Neuman unbias as explained in RFC 4086 section 4.2. As shown in the
+ * documentation of that RNG, the bits from jent_measure_jitter are considered
+ * independent which implies that the Von Neuman unbias operation is applicable.
+ * A proof of the Von-Neumann unbias operation to remove skews is given in the
+ * document "A proposal for: Functionality classes for random number
+ * generators", version 2.0 by Werner Schindler, section 5.4.1.
+ *
+ * Input:
+ * @entropy_collector Reference to entropy collector
+ *
+ * @return One random bit
+ */
+static __u64 jent_unbiased_bit(struct rand_data *entropy_collector)
+{
+        do {
+                __u64 a = jent_measure_jitter(entropy_collector);
+                __u64 b = jent_measure_jitter(entropy_collector);
+
+                if (a == b)
+                        continue;
+                if (1 == a)
+                        return 1;
+                else
+                        return 0;
+        } while (1);
+}
+
+/**
+ * Shuffle the pool a bit by mixing some value with a bijective function (XOR)
+ * into the pool.
+ *
+ * The function generates a mixer value that depends on the bits set and the
+ * location of the set bits in the random number generated by the entropy
+ * source. Therefore, based on the generated random number, this mixer value
+ * can have 2**64 different values. That mixer value is initialized with the
+ * first two SHA-1 constants. After obtaining the mixer value, it is XORed into
+ * the random number.
+ *
+ * The mixer value is not assumed to contain any entropy. But due to the XOR
+ * operation, it can also not destroy any entropy present in the entropy pool.
+ *
+ * Input:
+ * @entropy_collector Reference to entropy collector
+ */
+static void jent_stir_pool(struct rand_data *entropy_collector)
+{
+        /*
+         * to shut up GCC on 32 bit, we have to initialize the 64 variable
+         * with two 32 bit variables
+         */
+        union c {
+                __u64 u64;
+                __u32 u32[2];
+        };
+        /*
+         * This constant is derived from the first two 32 bit initialization
+         * vectors of SHA-1 as defined in FIPS 180-4 section 5.3.1
+         */
+        union c constant;
+        /*
+         * The start value of the mixer variable is derived from the third
+         * and fourth 32 bit initialization vector of SHA-1 as defined in
+         * FIPS 180-4 section 5.3.1
+         */
+        union c mixer;
+        unsigned int i = 0;
+
+        /*
+         * Store the SHA-1 constants in reverse order to make up the 64 bit
+         * value -- this applies to a little endian system, on a big endian
+         * system, it reverses as expected. But this really does not matter
+         * as we do not rely on the specific numbers. We just pick the SHA-1
+         * constants as they have a good mix of bit set and unset.
+         */
+        constant.u32[1] = 0x67452301;
+        constant.u32[0] = 0xefcdab89;
+        mixer.u32[1] = 0x98badcfe;
+        mixer.u32[0] = 0x10325476;
+
+        for (i = 0; i < DATA_SIZE_BITS; i++) {
+                /*
+                 * get the i-th bit of the input random number and only XOR
+                 * the constant into the mixer value when that bit is set
+                 */
+                if ((entropy_collector->data >> i) & 1)
+                        mixer.u64 ^= constant.u64;
+                mixer.u64 = rol64(mixer.u64, 1);
+        }
+        entropy_collector->data ^= mixer.u64;
+}
+
+/**
+ * Generator of one 64 bit random number
+ * Function fills rand_data->data
+ *
+ * Input:
+ * @ec Reference to entropy collector
+ */
+static void jent_gen_entropy(struct rand_data *ec)
+{
+        unsigned int k = 0;
+
+        /* priming of the ->prev_time value */
+        jent_measure_jitter(ec);
+
+        while (1) {
+                __u64 data = 0;
+
+                if (ec->disable_unbias == 1)
+                        data = jent_measure_jitter(ec);
+                else
+                        data = jent_unbiased_bit(ec);
+
+                /* enforcement of the jent_stuck test */
+                if (ec->stuck) {
+                        /*
+                         * We only mix in the bit considered not appropriate
+                         * without the LSFR. The reason is that if we apply
+                         * the LSFR and we do not rotate, the 2nd bit with LSFR
+                         * will cancel out the first LSFR application on the
+                         * bad bit.
+                         *
+                         * And we do not rotate as we apply the next bit to the
+                         * current bit location again.
+                         */
+                        ec->data ^= data;
+                        ec->stuck = 0;
+                        continue;
+                }
+
+                /*
+                 * Fibonacci LSFR with polynom of
+                 *  x^64 + x^61 + x^56 + x^31 + x^28 + x^23 + 1 which is
+                 *  primitive according to
+                 *   http://poincare.matf.bg.ac.rs/~ezivkovm/publications/primpol1.pdf
+                 * (the shift values are the polynom values minus one
+                 * due to counting bits from 0 to 63). As the current
+                 * position is always the LSB, the polynom only needs
+                 * to shift data in from the left without wrap.
+                 */
+                ec->data ^= data;
+                ec->data ^= ((ec->data >> 63) & 1);
+                ec->data ^= ((ec->data >> 60) & 1);
+                ec->data ^= ((ec->data >> 55) & 1);
+                ec->data ^= ((ec->data >> 30) & 1);
+                ec->data ^= ((ec->data >> 27) & 1);
+                ec->data ^= ((ec->data >> 22) & 1);
+                ec->data = rol64(ec->data, 1);
+
+                /*
+                 * We multiply the loop value with ->osr to obtain the
+                 * oversampling rate requested by the caller
+                 */
+                if (++k >= (DATA_SIZE_BITS * ec->osr))
+                        break;
+        }
+        if (ec->stir)
+                jent_stir_pool(ec);
+}
+
+/**
+ * The continuous test required by FIPS 140-2 -- the function automatically
+ * primes the test if needed.
+ *
+ * Return:
+ * 0 if FIPS test passed
+ * < 0 if FIPS test failed
+ */
+static void jent_fips_test(struct rand_data *ec)
+{
+        if (!fips_enabled)
+                return;
+
+        /* prime the FIPS test */
+        if (!ec->old_data) {
+                ec->old_data = ec->data;
+                jent_gen_entropy(ec);
+        }
+
+        if (ec->data == ec->old_data)
+                panic(DRIVER_NAME ": Duplicate output detected\n");
+
+        ec->old_data = ec->data;
+}
+
+
+/**
+ * Entry function: Obtain entropy for the caller.
+ *
+ * This function invokes the entropy gathering logic as often to generate
+ * as many bytes as requested by the caller. The entropy gathering logic
+ * creates 64 bit per invocation.
+ *
+ * This function truncates the last 64 bit entropy value output to the exact
+ * size specified by the caller.
+ *
+ * Input:
+ * @ec Reference to entropy collector
+ * @data pointer to buffer for storing random data -- buffer must already
+ *       exist
+ * @len size of the buffer, specifying also the requested number of random
+ *      in bytes
+ *
+ * @return 0 when request is fulfilled or an error
+ *
+ * The following error codes can occur:
+ *      -1      entropy_collector is NULL
+ */
+static ssize_t jent_read_entropy(struct rand_data *ec, u8 *data, size_t len)
+{
+        u8 *p = data;
+
+        if (!ec)
+                return -EINVAL;
+
+        while (0 < len) {
+                size_t tocopy;
+
+                jent_gen_entropy(ec);
+                jent_fips_test(ec);
+                if ((DATA_SIZE_BITS / 8) < len)
+                        tocopy = (DATA_SIZE_BITS / 8);
+                else
+                        tocopy = len;
+                memcpy(p, &ec->data, tocopy);
+
+                len -= tocopy;
+                p += tocopy;
+        }
+
+        return 0;
+}
+
+/***************************************************************************
+ * Initialization logic
+ ***************************************************************************/
+
+static struct rand_data *jent_entropy_collector_alloc(unsigned int osr,
+                                                      unsigned int flags)
+{
+        struct rand_data *entropy_collector;
+
+        entropy_collector = kzalloc(sizeof(struct rand_data), GFP_KERNEL);
+        if (!entropy_collector)
+                return NULL;
+
+        if (!(flags & JENT_DISABLE_MEMORY_ACCESS)) {
+                /* Allocate memory for adding variations based on memory
+                 * access
+                 */
+                entropy_collector->mem = kzalloc(JENT_MEMORY_SIZE, GFP_KERNEL);
+                if (!entropy_collector->mem) {
+                        kfree(entropy_collector);
+                        return NULL;
+                }
+                entropy_collector->memblocksize = JENT_MEMORY_BLOCKSIZE;
+                entropy_collector->memblocks = JENT_MEMORY_BLOCKS;
+                entropy_collector->memaccessloops = JENT_MEMORY_ACCESSLOOPS;
+        }
+
+        /* verify and set the oversampling rate */
+        if (0 == osr)
+                osr = 1; /* minimum sampling rate is 1 */
+        entropy_collector->osr = osr;
+
+        entropy_collector->stir = 1;
+        if (flags & JENT_DISABLE_STIR)
+                entropy_collector->stir = 0;
+        if (flags & JENT_DISABLE_UNBIAS)
+                entropy_collector->disable_unbias = 1;
+
+        /* fill the data pad with non-zero values */
+        jent_gen_entropy(entropy_collector);
+
+        return entropy_collector;
+}
+
+static void jent_entropy_collector_free(struct rand_data *entropy_collector)
+{
+        if (entropy_collector->mem)
+                kzfree(entropy_collector->mem);
+        entropy_collector->mem = NULL;
+        if (entropy_collector)
+                kzfree(entropy_collector);
+        entropy_collector = NULL;
+}
+
+static int jent_entropy_init(void)
+{
+        int i;
+        __u64 delta_sum = 0;
+        __u64 old_delta = 0;
+        int time_backwards = 0;
+        int count_var = 0;
+        int count_mod = 0;
+
+        /* We could perform statistical tests here, but the problem is
+         * that we only have a few loop counts to do testing. These
+         * loop counts may show some slight skew and we produce
+         * false positives.
+         *
+         * Moreover, only old systems show potentially problematic
+         * jitter entropy that could potentially be caught here. But
+         * the RNG is intended for hardware that is available or widely
+         * used, but not old systems that are long out of favor. Thus,
+         * no statistical tests.
+         */
+
+        /*
+         * We could add a check for system capabilities such as clock_getres or
+         * check for CONFIG_X86_TSC, but it does not make much sense as the
+         * following sanity checks verify that we have a high-resolution
+         * timer.
+         */
+        /*
+         * TESTLOOPCOUNT needs some loops to identify edge systems. 100 is
+         * definitely too little.
+         */
+#define TESTLOOPCOUNT 300
+#define CLEARCACHE 100
+        for (i = 0; (TESTLOOPCOUNT + CLEARCACHE) > i; i++) {
+                __u64 time = 0;
+                __u64 time2 = 0;
+                __u64 folded = 0;
+                __u64 delta = 0;
+                unsigned int lowdelta = 0;
+
+                jent_get_nstime(&time);
+                jent_fold_time(NULL, time, &folded, 1<<MIN_FOLD_LOOP_BIT);
+                jent_get_nstime(&time2);
+
+                /* test whether timer works */
+                if (!time || !time2)
+                        return JENT_ENOTIME;
+                delta = time2 - time;
+                /*
+                 * test whether timer is fine grained enough to provide
+                 * delta even when called shortly after each other -- this
+                 * implies that we also have a high resolution timer
+                 */
+                if (!delta)
+                        return JENT_ECOARSETIME;
+
+                /*
+                 * up to here we did not modify any variable that will be
+                 * evaluated later, but we already performed some work. Thus we
+                 * already have had an impact on the caches, branch prediction,
+                 * etc. with the goal to clear it to get the worst case
+                 * measurements.
+                 */
+                if (CLEARCACHE > i)
+                        continue;
+
+                /* test whether we have an increasing timer */
+                if (!(time2 > time))
+                        time_backwards++;
+
+                /*
+                 * Avoid modulo of 64 bit integer to allow code to compile
+                 * on 32 bit architectures.
+                 */
+                lowdelta = time2 - time;
+                if (!(lowdelta % 100))
+                        count_mod++;
+
+                /*
+                 * ensure that we have a varying delta timer which is necessary
+                 * for the calculation of entropy -- perform this check
+                 * only after the first loop is executed as we need to prime
+                 * the old_data value
+                 */
+                if (i) {
+                        if (delta != old_delta)
+                                count_var++;
+                        if (delta > old_delta)
+                                delta_sum += (delta - old_delta);
+                        else
+                                delta_sum += (old_delta - delta);
+                }
+                old_delta = delta;
+        }
+
+        /*
+         * we allow up to three times the time running backwards.
+         * CLOCK_REALTIME is affected by adjtime and NTP operations. Thus,
+         * if such an operation just happens to interfere with our test, it
+         * should not fail. The value of 3 should cover the NTP case being
+         * performed during our test run.
+         */
+        if (3 < time_backwards)
+                return JENT_ENOMONOTONIC;
+        /* Error if the time variances are always identical */
+        if (!delta_sum)
+                return JENT_EVARVAR;
+
+        /*
+         * Variations of deltas of time must on average be larger
+         * than 1 to ensure the entropy estimation
+         * implied with 1 is preserved
+         */
+        if (delta_sum <= 1)
+                return JENT_EMINVARVAR;
+
+        /*
+         * Ensure that we have variations in the time stamp below 10 for at
+         * least 10% of all checks -- on some platforms, the counter
+         * increments in multiples of 100, but not always
+         */
+        if ((TESTLOOPCOUNT/10 * 9) < count_mod)
+                return JENT_ECOARSETIME;
+
+        return 0;
+}
+
+/***************************************************************************
+ * Kernel crypto API interface
+ ***************************************************************************/
+
+struct jitterentropy {
+        spinlock_t jent_lock;
+        struct rand_data *entropy_collector;
+};
+
+static int jent_kcapi_init(struct crypto_tfm *tfm)
+{
+        struct jitterentropy *rng = crypto_tfm_ctx(tfm);
+        int ret = 0;
+
+        rng->entropy_collector = jent_entropy_collector_alloc(1, 0);
+        if (!rng->entropy_collector)
+                ret = -ENOMEM;
+
+        spin_lock_init(&rng->jent_lock);
+        return ret;
+}
+
+static void jent_kcapi_cleanup(struct crypto_tfm *tfm)
+{
+        struct jitterentropy *rng = crypto_tfm_ctx(tfm);
+
+        spin_lock(&rng->jent_lock);
+        if (rng->entropy_collector)
+                jent_entropy_collector_free(rng->entropy_collector);
+        rng->entropy_collector = NULL;
+        spin_unlock(&rng->jent_lock);
+}
+
+static int jent_kcapi_random(struct crypto_rng *tfm,
+                             const u8 *src, unsigned int slen,
+                             u8 *rdata, unsigned int dlen)
+{
+        struct jitterentropy *rng = crypto_rng_ctx(tfm);
+        int ret = 0;
+
+        spin_lock(&rng->jent_lock);
+        ret = jent_read_entropy(rng->entropy_collector, rdata, dlen);
+        spin_unlock(&rng->jent_lock);
+
+        return ret;
+}
+
+static int jent_kcapi_reset(struct crypto_rng *tfm,
+                            const u8 *seed, unsigned int slen)
+{
+        return 0;
+}
+
+static struct rng_alg jent_alg = {
+        .generate               = jent_kcapi_random,
+        .seed                   = jent_kcapi_reset,
+        .seedsize               = 0,
+        .base                   = {
+                .cra_name               = "jitterentropy_rng",
+                .cra_driver_name        = "jitterentropy_rng",
+                .cra_priority           = 100,
+                .cra_ctxsize            = sizeof(struct jitterentropy),
+                .cra_module             = THIS_MODULE,
+                .cra_init               = jent_kcapi_init,
+                .cra_exit               = jent_kcapi_cleanup,
+
+        }
+};
+
+static int __init jent_mod_init(void)
+{
+        int ret = 0;
+
+        ret = jent_entropy_init();
+        if (ret) {
+                pr_info(DRIVER_NAME ": Initialization failed with host not compliant with requirements: %d\n", ret);
+                return -EFAULT;
+        }
+        return crypto_register_rng(&jent_alg);
+}
+
+static void __exit jent_mod_exit(void)
+{
+        crypto_unregister_rng(&jent_alg);
+}
+
+module_init(jent_mod_init);
+module_exit(jent_mod_exit);
+
+MODULE_LICENSE("Dual BSD/GPL");
+MODULE_AUTHOR("Stephan Mueller <smueller@chronox.de>");
+MODULE_DESCRIPTION("Non-physical True Random Number Generator based on CPU Jitter");
+MODULE_ALIAS_CRYPTO("jitterentropy_rng");