setup.c

/* The following program is an example of a setup program which generates a
 * secret key and writes it to a file which is only readably by the owner of
 * the file.
 *
 * In this example the program should be made owned by root and the wheel group
 * and the suid (set user ID) permission should be set so that when a regular
 * user runs the program, the secrets.txt file will be created and owned by the
 * root user.
 *
 * Your goal is to try to find a way without modifying the code of the program
 * to be able to read the secrets.txt file created without modifying the code
 * for this program or becoming root by using su, sudo or other means.
 *
 * This program has been tested on:
 * FreeBSD 10.2-RELEASE amd64 / FreeBSD clang version 3.4.1
 * OSX 10.10.5 / Apple LLVM version 7.0.2 (clang-700.1.81)
 * GNU/Linux 3.13.0-24 / gcc version 4.8.2 (Ubuntu 4.8.2-19ubuntu1)
 *
 * To compile and run on FreeBSD / OSX with clang:
 * $ cc -Wall -o setup setup.c
 * $ sudo chown root:wheel setup
 * $ sudo chmod u+s setup
 * $ ./setup
 * $ cat secrets.txt
 * cat: secrets.txt: Permission denied
 *
 * To compile on GNU/Linux with gcc:
 * $ sudo apt-get -y install libbsd-dev
 * $ cc -std=gnu99 -Wall -o setup setup.c -lbsd
 * $ sudo chown root:root setup
 * $ sudo chmod u+s setup
 * $ ./setup
 * $ cat secrets.txt
 * cat: secrets.txt: Permission denied
 *
 * Comments or Questions?
 * @MatthewClosson
 *
 */

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#ifdef __linux__
#include <bsd/stdlib.h>
#endif

void generate_secret(char *secret);
void write_file(char *secret);

int main(int argc, const char *argv[])
{
  char secret[22];

  generate_secret(secret);
  write_file(secret);

  exit(EXIT_SUCCESS);
}

void generate_secret (char *secret)
{
  int secret_size = 20;
  char hexchars[] = "0123456789ABCDEF";

  for (int i = 0; i < secret_size; i++) {
    secret[i] = hexchars[arc4random() % 16];
  }

  secret[secret_size]     = '\n';
  secret[secret_size + 1] = '\0';
}

void write_file(char *secret)
{
  int fd = open("secrets.txt", O_CREAT | O_WRONLY, 0700);

  if (fd == -1) {
    perror("secrets.txt");
    exit(EXIT_FAILURE);
  }

  printf("Writing to secrets.txt file...");
  write(fd, secret, strlen(secret));
  close(fd);
  printf("done.\n");
}