POS00-C. Avoid race conditions with multiple threads

When multiple threads can read or modify the same data, use synchronization techniques to avoid software flaws that can lead to security vulnerabilities. Concurrency problems can often result in abnormal termination or denial of service, but it is possible for them to result in more serious vulnerabilities.

Noncompliant Code Example

Assume this simplified code is part of a multithreaded bank system. Threads call credit() and debit() as money is deposited into and withdrawn from the single account. Because the addition and subtraction operations are not atomic, it is possible that two operations can occur concurrently, but only the result of one would be saved. For example, an attacker can credit the account with a sum of money and make a large number of small debits concurrently. Some of the debits might not affect the account balance because of the race condition, so the attacker is effectively creating money.

int account_balance;

void debit(int amount) {
  account_balance -= amount;
}

void credit(int amount) {
  account_balance += amount;
}

Compliant Solution

This solution uses a mutex to make credits and debits atomic operations. All credits and debits will now affect the account balance, so an attacker cannot exploit the race condition to steal money from the bank. The mutex is created with the pthread_mutex function. In addition, the volatile keyword is used so prefetching does not occur (see DCL34-C. Use volatile for data that cannot be cached).

#include <pthread.h>

volatile int account_balance;
pthread_mutex_t account_lock = PTHREAD_MUTEX_INITIALIZER;

void debit(int amount) {
  pthread_mutex_lock(&account_lock);
  account_balance -= amount;
  pthread_mutex_unlock(&account_lock);
}

void credit(int amount) {
  pthread_mutex_lock(&account_lock);
  account_balance += amount;
  pthread_mutex_unlock(&account_lock);
}

Risk Assessment

Race conditions caused by multiple threads concurrently accessing and modifying the same data can lead to abnormal termination and denial-of-service attacks, or data integrity violations.

Related Vulnerabilities

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

References

[[Dowd 06]] Chapter 13, "Synchronization and State"
[[Seacord 05a]] Chapter 7, "File I/O"

50. POSIX (POS)      50. POSIX (POS)