Stealthy IPC using offsets and file descriptors

dimensional-lock.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <sys/wait.h>

#define SIG_SYNC    0
#define SIG_END     1

int sendText(int fd, char *text) {
    for (int i = 0; i < strlen(text); i++) {
        // Use the character as the seek position!
        lseek(fd, text[i] + SIG_END, SEEK_SET);
        // Wait until the child process signals that it has read the character
        while (lseek(fd, 0, SEEK_CUR) != SIG_SYNC);
    }
    // Send the end character
    lseek(fd, SIG_END, SEEK_SET);
}

int receiveText(int fd, char *text) {
    int len = 0;
    while (1) {
        // Wait until the parent process has finished writing
        while (lseek(fd, 0, SEEK_CUR) == SIG_SYNC);
        // Read the character sent by the parent process
        int chr = lseek(fd, 0, SEEK_CUR);
        chr -= SIG_END;

        text[len++] = chr;
        if (chr == 0) {
            // Null byte received, we're done
            break;
        }

        // Signal the parent process that we have read the character, and it can take over
        lseek(fd, SIG_SYNC, SEEK_SET);
    }
}

int main(int argc, char **argv) {
    printf("[+] Parent process created by %d with pid %d\n", getppid(), getpid());

    srand(time(NULL));
    char text[50];
    int random = rand() % 100;
    sprintf(text, "Your random number is %d.", random);
    printf("[i] The text to send is: \"%s\"\n", text);

    int fd = open("random.txt", O_WRONLY | O_CREAT, 0644);
    if (fd < 0) {
        perror("open");
        return 1;
    }

    int pid = fork();
    if (pid < 0) {
        perror("fork");
        return 1;
    }
    if (pid == 0) {
        printf("[+] Child process created by %d with pid %d\n", getppid(), getpid());
        printf("[i] Child process receiving text\n");

        char text2[50];
        receiveText(fd, text2);
        printf("[i] Child process received text:\n");
        printf("\t\"%s\"\n", text2);

        printf("[-] Child process exited\n");
        return 0;
    } else {
        printf("[+] Parent process sending text\n");
        sendText(fd, text);
        printf("[i] Parent process sent text\n");

        wait(NULL);

        close(fd);
    }

    printf("[-] Parent process exited\n");
    return 0;
}