Signal Handler Race Condition

Description

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

This code registers the same signal handler function with two different signals (CWE-831). If those signals are sent to the process, the handler creates a log message (specified in the first argument to the program) and exits.

char *logMessage;

void handler (int sigNum) {

  syslog(LOG_NOTICE, "%s\n", logMessage);
  free(logMessage);
  /* artificially increase the size of the timing window to make demonstration of this weakness easier. */

  sleep(10);
  exit(0);

}

int main (int argc, char* argv[]) {

  logMessage = strdup(argv[1]);
  /* Register signal handlers. */

  signal(SIGHUP, handler);
  signal(SIGTERM, handler);
  /* artificially increase the size of the timing window to make demonstration of this weakness easier. */

  sleep(10);

}

The handler function uses global state (globalVar and logMessage), and it can be called by both the SIGHUP and SIGTERM signals. An attack scenario might follow these lines:

At this point, the state of the heap is uncertain, because malloc is still modifying the metadata for the heap; the metadata might be in an inconsistent state. The SIGTERM-handler call to free() is assuming that the metadata is inconsistent, possibly causing it to write data to the wrong location while managing the heap. The result is memory corruption, which could lead to a crash or even code execution, depending on the circumstances under which the code is running.

Note that this is an adaptation of a classic example as originally presented by Michal Zalewski [REF-360]; the original example was shown to be exploitable for code execution.

Also note that the strdup(argv[1]) call contains a potential buffer over-read (CWE-126) if the program is called without any arguments, because argc would be 0, and argv[1] would point outside the bounds of the array.

Example Two

The following code registers a signal handler with multiple signals in order to log when a specific event occurs and to free associated memory before exiting.

#include <signal.h>
#include <syslog.h>
#include <string.h>
#include <stdlib.h>

void *global1, *global2;
char *what;
void sh (int dummy) {

  syslog(LOG_NOTICE,"%s\n",what);
  free(global2);
  free(global1);
  /* Sleep statements added to expand timing window for race condition */

  sleep(10);
  exit(0);

}

int main (int argc,char* argv[]) {

  what=argv[1];
  global1=strdup(argv[2]);
  global2=malloc(340);
  signal(SIGHUP,sh);
  signal(SIGTERM,sh);
  /* Sleep statements added to expand timing window for race condition */

  sleep(10);
  exit(0);

}

However, the following sequence of events may result in a double-free (CWE-415):

This is just one possible exploitation of the above code. As another example, the syslog call may use malloc calls which are not async-signal safe. This could cause corruption of the heap management structures. For more details, consult the example within "Delivering Signals for Fun and Profit" [REF-360].

See Also