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 /*
  * Non-physical true random number generator based on timing jitter --
  * Jitter RNG standalone code.
  *
  * Copyright Stephan Mueller <smueller@chronox.de>, 2015 - 2020
  *
  * Design
  * ======
  *
  * See https://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 2.2.0 provided at https://www.chronox.de/jent.html
  */
 
 #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
 
 typedef unsigned long long  __u64;
 typedef long long       __s64;
 typedef unsigned int        __u32;
 #define NULL    ((void *) 0)
 
 /* 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 osr;   /* Oversample rate */
 #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 */
 
     /* Repetition Count Test */
     int rct_count;          /* Number of stuck values */
 
     /* Adaptive Proportion Test for a significance level of 2^-30 */
 #define JENT_APT_CUTOFF     325 /* Taken from SP800-90B sec 4.4.2 */
 #define JENT_APT_WINDOW_SIZE    512 /* Data window size */
     /* LSB of time stamp to process */
 #define JENT_APT_LSB        16
 #define JENT_APT_WORD_MASK  (JENT_APT_LSB - 1)
     unsigned int apt_observations;  /* Number of collected observations */
     unsigned int apt_count;     /* APT counter */
     unsigned int apt_base;      /* APT base reference */
     unsigned int apt_base_set:1;    /* APT base reference set? */
 
     unsigned int health_failure:1;  /* Permanent health failure */
 };
 
 /* Flags that can be used to initialize the RNG */
 #define JENT_DISABLE_MEMORY_ACCESS (1<<2) /* Disable memory access for more
                        * entropy, saves MEMORY_SIZE RAM for
                        * entropy collector */
 
 /* -- 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_EVARVAR        5 /* Timer does not produce variations of
                    * variations (2nd derivation of time is
                    * zero). */
 #define JENT_ESTUCK     8 /* Too many stuck results during init. */
 #define JENT_EHEALTH        9 /* Health test failed during initialization */
 #define JENT_ERCT       10 /* RCT failed during initialization */
 
 /*
  * The output n bits can receive more than n bits of min entropy, of course,
  * but the fixed output of the conditioning function can only asymptotically
  * approach the output size bits of min entropy, not attain that bound. Random
  * maps will tend to have output collisions, which reduces the creditable
  * output entropy (that is what SP 800-90B Section 3.1.5.1.2 attempts to bound).
  *
  * The value "64" is justified in Appendix A.4 of the current 90C draft,
  * and aligns with NIST's in "epsilon" definition in this document, which is
  * that a string can be considered "full entropy" if you can bound the min
  * entropy in each bit of output to at least 1-epsilon, where epsilon is
  * required to be <= 2^(-32).
  */
 #define JENT_ENTROPY_SAFETY_FACTOR  64
 
 #include <linux/fips.h>
 #include "jitterentropy.h"
 
 /***************************************************************************
  * Adaptive Proportion Test
  *
  * This test complies with SP800-90B section 4.4.2.
  ***************************************************************************/
 
 /*
  * Reset the APT counter
  *
  * @ec [in] Reference to entropy collector
  */
 static void jent_apt_reset(struct rand_data *ec, unsigned int delta_masked)
 {
     /* Reset APT counter */
     ec->apt_count = 0;
     ec->apt_base = delta_masked;
     ec->apt_observations = 0;
 }
 
 /*
  * Insert a new entropy event into APT
  *
  * @ec [in] Reference to entropy collector
  * @delta_masked [in] Masked time delta to process
  */
 static void jent_apt_insert(struct rand_data *ec, unsigned int delta_masked)
 {
     /* Initialize the base reference */
     if (!ec->apt_base_set) {
         ec->apt_base = delta_masked;
         ec->apt_base_set = 1;
         return;
     }
 
     if (delta_masked == ec->apt_base) {
         ec->apt_count++;
 
         if (ec->apt_count >= JENT_APT_CUTOFF)
             ec->health_failure = 1;
     }
 
     ec->apt_observations++;
 
     if (ec->apt_observations >= JENT_APT_WINDOW_SIZE)
         jent_apt_reset(ec, delta_masked);
 }
 
 /***************************************************************************
  * Stuck Test and its use as Repetition Count Test
  *
  * The Jitter RNG uses an enhanced version of the Repetition Count Test
  * (RCT) specified in SP800-90B section 4.4.1. Instead of counting identical
  * back-to-back values, the input to the RCT is the counting of the stuck
  * values during the generation of one Jitter RNG output block.
  *
  * The RCT is applied with an alpha of 2^{-30} compliant to FIPS 140-2 IG 9.8.
  *
  * During the counting operation, the Jitter RNG always calculates the RCT
  * cut-off value of C. If that value exceeds the allowed cut-off value,
  * the Jitter RNG output block will be calculated completely but discarded at
  * the end. The caller of the Jitter RNG is informed with an error code.
  ***************************************************************************/
 
 /*
  * Repetition Count Test as defined in SP800-90B section 4.4.1
  *
  * @ec [in] Reference to entropy collector
  * @stuck [in] Indicator whether the value is stuck
  */
 static void jent_rct_insert(struct rand_data *ec, int stuck)
 {
     /*
      * If we have a count less than zero, a previous RCT round identified
      * a failure. We will not overwrite it.
      */
     if (ec->rct_count < 0)
         return;
 
     if (stuck) {
         ec->rct_count++;
 
         /*
          * The cutoff value is based on the following consideration:
          * alpha = 2^-30 as recommended in FIPS 140-2 IG 9.8.
          * In addition, we require an entropy value H of 1/OSR as this
          * is the minimum entropy required to provide full entropy.
          * Note, we collect 64 * OSR deltas for inserting them into
          * the entropy pool which should then have (close to) 64 bits
          * of entropy.
          *
          * Note, ec->rct_count (which equals to value B in the pseudo
          * code of SP800-90B section 4.4.1) starts with zero. Hence
          * we need to subtract one from the cutoff value as calculated
          * following SP800-90B.
          */
         if ((unsigned int)ec->rct_count >= (31 * ec->osr)) {
             ec->rct_count = -1;
             ec->health_failure = 1;
         }
     } else {
         ec->rct_count = 0;
     }
 }
 
 /*
  * Is there an RCT health test failure?
  *
  * @ec [in] Reference to entropy collector
  *
  * @return
  *  0 No health test failure
  *  1 Permanent health test failure
  */
 static int jent_rct_failure(struct rand_data *ec)
 {
     if (ec->rct_count < 0)
         return 1;
     return 0;
 }
 
 static inline __u64 jent_delta(__u64 prev, __u64 next)
 {
 #define JENT_UINT64_MAX     (__u64)(~((__u64) 0))
     return (prev < next) ? (next - prev) :
                    (JENT_UINT64_MAX - prev + 1 + next);
 }
 
 /*
  * Stuck test by checking the:
  *  1st derivative of the jitter measurement (time delta)
  *  2nd derivative of the jitter measurement (delta of time deltas)
  *  3rd derivative of the jitter measurement (delta of delta of time deltas)
  *
  * All values must always be non-zero.
  *
  * @ec [in] Reference to entropy collector
  * @current_delta [in] Jitter time delta
  *
  * @return
  *  0 jitter measurement not stuck (good bit)
  *  1 jitter measurement stuck (reject bit)
  */
 static int jent_stuck(struct rand_data *ec, __u64 current_delta)
 {
     __u64 delta2 = jent_delta(ec->last_delta, current_delta);
     __u64 delta3 = jent_delta(ec->last_delta2, delta2);
 
     ec->last_delta = current_delta;
     ec->last_delta2 = delta2;
 
     /*
      * Insert the result of the comparison of two back-to-back time
      * deltas.
      */
     jent_apt_insert(ec, current_delta);
 
     if (!current_delta || !delta2 || !delta3) {
         /* RCT with a stuck bit */
         jent_rct_insert(ec, 1);
         return 1;
     }
 
     /* RCT with a non-stuck bit */
     jent_rct_insert(ec, 0);
 
     return 0;
 }
 
 /*
  * Report any health test failures
  *
  * @ec [in] Reference to entropy collector
  *
  * @return
  *  0 No health test failure
  *  1 Permanent health test failure
  */
 static int jent_health_failure(struct rand_data *ec)
 {
     return ec->health_failure;
 }
 
 /***************************************************************************
  * Noise sources
  ***************************************************************************/
 
 /*
  * 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 - 1) / 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));
 }
 
 /*
  * CPU Jitter noise source -- this is the noise source based on the CPU
  *                execution time jitter
  *
  * This function injects the individual bits of the time value into the
  * entropy pool using an LFSR.
  *
  * The code is deliberately inefficient with respect to the bit shifting
  * 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.
  *
  * @ec [in] entropy collector struct
  * @time [in] time stamp to be injected
  * @loop_cnt [in] if a value not equal to 0 is set, use the given value as
  *        number of loops to perform the folding
  * @stuck [in] Is the time stamp identified as stuck?
  *
  * Output:
  * updated ec->data
  *
  * @return Number of loops the folding operation is performed
  */
 static void jent_lfsr_time(struct rand_data *ec, __u64 time, __u64 loop_cnt,
                int stuck)
 {
     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 = ec->data;
         for (i = 1; (DATA_SIZE_BITS) >= i; i++) {
             __u64 tmp = time << (DATA_SIZE_BITS - i);
 
             tmp = tmp >> (DATA_SIZE_BITS - 1);
 
             /*
             * Fibonacci LSFR with polynomial 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 polynomial values minus one
             * due to counting bits from 0 to 63). As the current
             * position is always the LSB, the polynomial only needs
             * to shift data in from the left without wrap.
             */
             tmp ^= ((new >> 63) & 1);
             tmp ^= ((new >> 60) & 1);
             tmp ^= ((new >> 55) & 1);
             tmp ^= ((new >> 30) & 1);
             tmp ^= ((new >> 27) & 1);
             tmp ^= ((new >> 22) & 1);
             new <<= 1;
             new ^= tmp;
         }
     }
 
     /*
      * If the time stamp is stuck, do not finally insert the value into
      * the entropy pool. Although this operation should not do any harm
      * even when the time stamp has no entropy, SP800-90B requires that
      * any conditioning operation (SP800-90B considers the LFSR to be a
      * conditioning operation) to have an identical amount of input
      * data according to section 3.1.5.
      */
     if (!stuck)
         ec->data = new;
 }
 
 /*
  * 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.
  *
  * @ec [in] 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 [in] if a value not equal to 0 is set, use the given value
  *        number of loops to perform the LFSR
  */
 static void jent_memaccess(struct rand_data *ec, __u64 loop_cnt)
 {
     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;
     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++) {
         unsigned char *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;
     }
 }
 
 /***************************************************************************
  * Start of entropy processing logic
  ***************************************************************************/
 /*
  * This is the heart of the entropy generation: calculate time deltas and
  * use the CPU jitter in the time deltas. The jitter is injected into the
  * entropy pool.
  *
  * 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.
  *
  * @ec [in] Reference to entropy collector
  *
  * @return result of stuck test
  */
 static int jent_measure_jitter(struct rand_data *ec)
 {
     __u64 time = 0;
     __u64 current_delta = 0;
     int stuck;
 
     /* 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 = jent_delta(ec->prev_time, time);
     ec->prev_time = time;
 
     /* Check whether we have a stuck measurement. */
     stuck = jent_stuck(ec, current_delta);
 
     /* Now call the next noise sources which also injects the data */
     jent_lfsr_time(ec, current_delta, 0, stuck);
 
     return stuck;
 }
 
 /*
  * Generator of one 64 bit random number
  * Function fills rand_data->data
  *
  * @ec [in] Reference to entropy collector
  */
 static void jent_gen_entropy(struct rand_data *ec)
 {
     unsigned int k = 0, safety_factor = 0;
 
     if (fips_enabled)
         safety_factor = JENT_ENTROPY_SAFETY_FACTOR;
 
     /* priming of the ->prev_time value */
     jent_measure_jitter(ec);
 
     while (!jent_health_failure(ec)) {
         /* If a stuck measurement is received, repeat measurement */
         if (jent_measure_jitter(ec))
             continue;
 
         /*
          * We multiply the loop value with ->osr to obtain the
          * oversampling rate requested by the caller
          */
         if (++k >= ((DATA_SIZE_BITS + safety_factor) * ec->osr))
             break;
     }
 }
 
 /*
  * 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.
  *
  * @ec [in] Reference to entropy collector
  * @data [in] pointer to buffer for storing random data -- buffer must already
  *        exist
  * @len [in] 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
  *  -2  RCT failed
  *  -3  APT test failed
  */
 int jent_read_entropy(struct rand_data *ec, unsigned char *data,
               unsigned int len)
 {
     unsigned char *p = data;
 
     if (!ec)
         return -1;
 
     while (len > 0) {
         unsigned int tocopy;
 
         jent_gen_entropy(ec);
 
         if (jent_health_failure(ec)) {
             int ret;
 
             if (jent_rct_failure(ec))
                 ret = -2;
             else
                 ret = -3;
 
             /*
              * Re-initialize the noise source
              *
              * If the health test fails, the Jitter RNG remains
              * in failure state and will return a health failure
              * during next invocation.
              */
             if (jent_entropy_init())
                 return ret;
 
             /* Set APT to initial state */
             jent_apt_reset(ec, 0);
             ec->apt_base_set = 0;
 
             /* Set RCT to initial state */
             ec->rct_count = 0;
 
             /* Re-enable Jitter RNG */
             ec->health_failure = 0;
 
             /*
              * Return the health test failure status to the
              * caller as the generated value is not appropriate.
              */
             return ret;
         }
 
         if ((DATA_SIZE_BITS / 8) < len)
             tocopy = (DATA_SIZE_BITS / 8);
         else
             tocopy = len;
         jent_memcpy(p, &ec->data, tocopy);
 
         len -= tocopy;
         p += tocopy;
     }
 
     return 0;
 }
 
 /***************************************************************************
  * Initialization logic
  ***************************************************************************/
 
 struct rand_data *jent_entropy_collector_alloc(unsigned int osr,
                            unsigned int flags)
 {
     struct rand_data *entropy_collector;
 
     entropy_collector = jent_zalloc(sizeof(struct rand_data));
     if (!entropy_collector)
         return NULL;
 
     if (!(flags & JENT_DISABLE_MEMORY_ACCESS)) {
         /* Allocate memory for adding variations based on memory
          * access
          */
         entropy_collector->mem = jent_zalloc(JENT_MEMORY_SIZE);
         if (!entropy_collector->mem) {
             jent_zfree(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 (osr == 0)
         osr = 1; /* minimum sampling rate is 1 */
     entropy_collector->osr = osr;
 
     /* fill the data pad with non-zero values */
     jent_gen_entropy(entropy_collector);
 
     return entropy_collector;
 }
 
 void jent_entropy_collector_free(struct rand_data *entropy_collector)
 {
     jent_zfree(entropy_collector->mem);
     entropy_collector->mem = NULL;
     jent_zfree(entropy_collector);
 }
 
 int jent_entropy_init(void)
 {
     int i;
     __u64 delta_sum = 0;
     __u64 old_delta = 0;
     unsigned int nonstuck = 0;
     int time_backwards = 0;
     int count_mod = 0;
     int count_stuck = 0;
     struct rand_data ec = { 0 };
 
     /* Required for RCT */
     ec.osr = 1;
 
     /* 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.
      *
      * SP800-90B requires at least 1024 initial test cycles.
      */
 #define TESTLOOPCOUNT 1024
 #define CLEARCACHE 100
     for (i = 0; (TESTLOOPCOUNT + CLEARCACHE) > i; i++) {
         __u64 time = 0;
         __u64 time2 = 0;
         __u64 delta = 0;
         unsigned int lowdelta = 0;
         int stuck;
 
         /* Invoke core entropy collection logic */
         jent_get_nstime(&time);
         ec.prev_time = time;
         jent_lfsr_time(&ec, time, 0, 0);
         jent_get_nstime(&time2);
 
         /* test whether timer works */
         if (!time || !time2)
             return JENT_ENOTIME;
         delta = jent_delta(time, time2);
         /*
          * 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;
 
         stuck = jent_stuck(&ec, delta);
 
         /*
          * 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 (i < CLEARCACHE)
             continue;
 
         if (stuck)
             count_stuck++;
         else {
             nonstuck++;
 
             /*
              * Ensure that the APT succeeded.
              *
              * With the check below that count_stuck must be less
              * than 10% of the overall generated raw entropy values
              * it is guaranteed that the APT is invoked at
              * floor((TESTLOOPCOUNT * 0.9) / 64) == 14 times.
              */
             if ((nonstuck % JENT_APT_WINDOW_SIZE) == 0) {
                 jent_apt_reset(&ec,
                            delta & JENT_APT_WORD_MASK);
                 if (jent_health_failure(&ec))
                     return JENT_EHEALTH;
             }
         }
 
         /* Validate RCT */
         if (jent_rct_failure(&ec))
             return JENT_ERCT;
 
         /* test whether we have an increasing timer */
         if (!(time2 > time))
             time_backwards++;
 
         /* use 32 bit value to ensure compilation on 32 bit arches */
         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 (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 (time_backwards > 3)
         return JENT_ENOMONOTONIC;
 
     /*
      * 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_EVARVAR;
 
     /*
      * 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;
 
     /*
      * If we have more than 90% stuck results, then this Jitter RNG is
      * likely to not work well.
      */
     if ((TESTLOOPCOUNT/10 * 9) < count_stuck)
         return JENT_ESTUCK;
 
     return 0;
 }

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