quixos-nix-helpers/restricted-execution/sandbox.c

#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <unistd.h>

/* Trusted, single-process admission gate. The supervisor charges this PID to
 * its engine cgroup before releasing stdin. All subsequently created namespace
 * helpers and threads inherit that charge. No authored code runs here. */
static void fail(const char *message) {
  fprintf(stderr, "RESTRICTED_SANDBOX: %s\n", message);
  exit(125);
}

int main(int argc, char **argv) {
  struct stat info;
  if (argc != 2 || argv[1][0] != '/' || strstr(argv[1], "/../") ||
      lstat(argv[1], &info) || !S_ISDIR(info.st_mode) ||
      info.st_uid != getuid() || (info.st_mode & 007))
    fail("invalid engine directory");
  char gate;
  if (read(STDIN_FILENO, &gate, 1) != 1 || gate != 1)
    fail("engine admission was not released");
  int input = open("/dev/null", O_RDONLY);
  if (input < 0 || dup2(input, STDIN_FILENO) < 0)
    fail("stdin");
  close(input);
  int filter = open(SANDBOX_FILTER, O_RDONLY);
  if (filter < 0)
    fail("seccomp filter");
  char filter_fd[32];
  snprintf(filter_fd, sizeof(filter_fd), "%d", filter);
  char *args[4096];
  size_t count = 0;
#define ARG(value) args[count++] = (value)
  ARG(SANDBOX_BWRAP);
  ARG("--unshare-all");
  ARG("--unshare-user");
  ARG("--die-with-parent");
  ARG("--disable-userns");
  ARG("--assert-userns-disabled");
  ARG("--cap-drop"); ARG("ALL");
  ARG("--hostname"); ARG("quixos-restricted");
  ARG("--clearenv");
  ARG("--setenv"); ARG("TZ"); ARG("UTC");
  ARG("--setenv"); ARG("LANG"); ARG("C.UTF-8");
  FILE *closure = fopen(SANDBOX_CLOSURE, "r");
  if (!closure)
    fail("runtime closure");
  char *line = NULL;
  size_t capacity = 0;
  while (getline(&line, &capacity, closure) >= 0) {
    line[strcspn(line, "\n")] = 0;
    if (strncmp(line, "/nix/store/", 11) || strchr(line, ' ') || count > 4000)
      fail("runtime closure entry");
    char *path = strdup(line);
    if (!path)
      fail("runtime closure allocation");
    ARG("--ro-bind"); ARG(path); ARG(path);
  }
  fclose(closure);
  free(line);
  ARG("--proc"); ARG("/proc");
  ARG("--dev"); ARG("/dev");
  ARG("--tmpfs"); ARG("/tmp");
  ARG("--bind"); ARG(argv[1]); ARG(argv[1]);
  ARG("--chdir"); ARG(argv[1]);
  ARG("--seccomp"); ARG(filter_fd);
  ARG("--"); ARG(SANDBOX_EXECUTABLE);
  ARG("serve"); ARG("--experimental"); ARG("config.capnp");
  ARG(NULL);
  execv(SANDBOX_BWRAP, args);
  fail("bubblewrap failed");
}