Use of a Non-reentrant Function in a Concurrent Context

Demonstrations

The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

In this example, a signal handler uses syslog() to log a message:

char *message;
void sh(int dummy) {
  syslog(LOG_NOTICE,"%s\n",message);
  sleep(10);
  exit(0);
}
int main(int argc,char* argv[]) {
  ...
  signal(SIGHUP,sh);
  signal(SIGTERM,sh);
  sleep(10);
  exit(0);
}
If the execution of the first call to the signal handler is suspended after invoking syslog(), and the signal handler is called a second time, the memory allocated by syslog() enters an undefined, and possibly, exploitable state.

Example Two

The following code relies on getlogin() to determine whether or not a user is trusted. It is easily subverted.

pwd = getpwnam(getlogin());
if (isTrustedGroup(pwd->pw_gid)) {
  allow();
} else {
  deny();
}

See Also