POS47-C. Do not use threads that can be cancelled asynchronously

In threading, pthreads have the option of being set to cancel immediately or defer until a specific cancellation point. Canceling asynchronously (immediately) is dangerous, however, because most threads are in fact not safe to cancel immediately.

The IEEE standards page states that:

only functions that are cancel-safe may be called from a thread that is asynchronously cancelable.

Canceling asynchronously would follow the same route as passing a signal in to the thread to kill it, thus posing similarities to POS44-C. Do not use signals to terminate threads, which is strongly related to SIG02-C. Avoid using signals to implement normal functionality. These expand on the dangers of canceling a thread suddenly as this can create a data race condition.

Noncompliant Code Example

In this noncompliant code example the worker thread is doing something as simple as swapping a and b repeatedly.

This code uses one lock. The global_lock mutex is used to ensure that the worker thread and main thread do not collide in accessing the a and b variables.

The worker thread repeatedly exchanges the values of a and b, until it is cancelled by the main thread. The main thread then prints out the current values of a and b. Ideally one should be 5 and the other should be 10.

volatile int a = 5, b = 10;

/* Lock to enable threads to access a and b safely */
pthread_mutex_t global_lock = PTHREAD_MUTEX_INITIALIZER;

void* worker_thread(void* dummy) {
  int i, c;
  int result;

  if ((result = pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS,&i)) != 0) {
    /* handle error */
  }

  while (1) {
    if ((result = pthread_mutex_lock(&global_lock)) != 0) {
      /* handle error */
    }
    c = b;
    b = a;
    a = c;
    if ((result = pthread_mutex_unlock(&global_lock)) != 0) {
      /* handle error */
    }
  }
  return NULL;
}


int main(void) {
  int result;
  pthread_t worker;

  if ((result = pthread_create( &worker, NULL, worker_thread, NULL)) != 0) {
    /* handle error */
  }

  /* .. Do stuff...meanwhile worker thread runs for some time */

  /* since we don't know when the character is read in, the program could continue at any time */
  if ((result = pthread_cancel(worker)) != 0) {
    /* handle error */
  }
  /* pthread_join waits for the thread to finish up before continuing */
  if ((result = pthread_join(worker, 0)) != 0) {
    /* handle error */
  }

  if ((result = pthread_mutex_lock(&global_lock)) != 0) {
    /* handle error */
  }
  printf("a: %i | b: %i", a, b);
  if ((result = pthread_mutex_unlock(&global_lock)) != 0) {
    /* handle error */
  }

  return 0;
}

However, this program is subject to a race condition, because an asynchronous cancel can happen at any time. If the worker thread is cancelled while the global_lock mutex is held, then it is never actually released. In this case the main thread will wait forever trying to acquire the global_lock, and the program will deadlock

It is also possible that the main thread cancels the worker thread before it has invoked pthread_setcanceltype(). If this happens, then the cancellation will be delayed until the worker thread calls pthread_setcanceltype().

Noncompliant Code Example

In this example, the worker thread arranges to release the global_lock mutex if it gets interrupted.

void release_global_lock(void* dummy) {
  int result;
  if ((result = pthread_mutex_unlock(&global_lock)) != 0) {
    /* handle error */
  }
}

void* worker_thread(void* dummy) {
  int i, c;
  int result;

  if ((result = pthread_setcanceltype(PTHREAD_CANCEL_ASYNCHRONOUS,&i)) != 0) {
    /* handle error */
  }

  while (1) {
    if ((result = pthread_mutex_lock(&global_lock)) != 0) {
      /* handle error */
    }
    pthread_cleanup_push( release_global_lock, NULL);
    c = b;
    b = a;
    a = c;
    pthread_cleanup_pop(1);
  }
  return NULL;
}

The global variables are still subject to a race condition, because an asynchronous cancel can happen at any time. For instance, the worker thread could be cancelled right before the last line (a = c) and thereby lose the old value of b. Consequently the main thread might print that a and b have the same value.

The program is still subject to the race condition where the main thread cancels the worker thread before it has invoked pthread_setcanceltype(). If this happens, then the cancellation will be delayed until the worker thread calls pthread_setcanceltype().

Furthermore, while less likely, the program can still deadlock if the worker thread gets cancelled after the global_lock is acquired but before pthread_cleanup_push() is invoked. In this case, the worker thread is cancelled while holding global_lock, and the program will deadlock.

Compliant Solution

From IEEE standards page:

The cancelability state and type of any newly created threads, including the thread in which main() was first invoked, shall be PTHREAD_CANCEL_ENABLE and PTHREAD_CANCEL_DEFERRED respectively.

Since the default condition according to the IEEE standards for POSIX is PTHREAD_CANCEL_DEFERRED, it is not necessary to invoke pthread_setcanceltype() in the compliant solution.

void* worker_thread(void* dummy) {
  int c;
  int result;

  while (1) {
    if ((result = pthread_mutex_lock(&global_lock)) != 0) {
      /* handle error */
    }
    c = b;
    b = a;
    a = c;
    if ((result = pthread_mutex_unlock(&global_lock)) != 0) {
      /* handle error */
    }

    /* now we're safe to cancel, creating cancel point */
    pthread_testcancel();
  }
  return NULL;
}

Since this code limits cancellation of the worker thread to the end of the while loop, the worker thread can preserve the data invariant that a != b. Consequently, the program might print that a is 5 and b is 10, or that a is 10 and b is 5, but they will always be revealed to have different values when the worker thread is cancelled.

The other race conditions that plague the noncompliant code examples are not possible here. Because the worker thread does not modify its cancel-type, it may not be cancelled before being improperly initialized. And since it cannot be cancelled while the global_lock mutex is held, there is no possibility of deadlock, and the worker thread does not need to register any cleanup handlers.

Risk Assessment

Incorrectly using threads that asynchronously cancel may result in silent corruption, resource leaks and, in the worst case, unpredictable interactions.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Other Languages

In Java, similar reasoning resulted in the deprecation of Thread.stop() and appears in the Java Secure Coding Standard as CON24-J. Do not use Thread.stop() to terminate threads .

Bibliography

[MKS] pthread_cancel() Man Page
[Open Group 04] Threads Overview

[!CERT C Secure Coding Standard^button_arrow_left.png!]      [!CERT C Secure Coding Standard^button_arrow_up.png!]      [!CERT C Secure Coding Standard^button_arrow_right.png!|]