CWE-364: Signal Handler Race Condition

What is Signal Handler Race Condition?

• Overview: Signal Handler Race Condition (CWE-364) occurs when a program uses a signal handler that introduces a race condition, typically due to asynchronous actions that interrupt regular code execution, leading to unpredictable program state and behavior.

• Exploitation Methods:

• Attackers exploit this vulnerability by triggering signals at specific times to disrupt the execution flow, potentially causing memory corruption.
• Common attack patterns include causing a signal handler to execute during the use of non-reentrant functions or manipulating shared state between regular code and signal handlers.

• Security Impact:

• Direct consequences include memory corruption, denial of service, or arbitrary code execution.
• Potential cascading effects involve further exploitation such as privilege escalation or unauthorized data access.
• Business impact can be severe, leading to data breaches, system downtime, and loss of customer trust.

• Prevention Guidelines:

• Specific code-level fixes include avoiding non-reentrant functions in signal handlers and careful management of shared state.
• Security best practices involve using reentrant-safe functions and ensuring signal handlers are as minimal as possible.
• Recommended tools and frameworks include static analysis tools that can identify potential race conditions and adhering to guidelines provided by secure coding standards like CERT C/C++ Secure Coding Standards.

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Technical Details

Likelihood of Exploit: Medium

Affected Languages: C, C++

Affected Technologies: Not specified

Vulnerable Code Example

Certainly! Below is the revised content with improvements to address the issues identified:

// Include necessary headers
#include <stdio.h>
#include <signal.h>
#include <unistd.h>

// Global variable that will be modified by both the main program and the signal handler
volatile int shared_counter = 0;

// Signal handler function that modifies the global shared_counter
void signal_handler(int signum) {
    if (signum == SIGUSR1) {
        // This update is not atomic, leading to a race condition
        shared_counter += 1; // Vulnerability: Race condition due to non-atomic update
    }
}

int main() {
    // Register the signal handler
    signal(SIGUSR1, signal_handler);

    // Simulate some work in the main program
    for (int i = 0; i < 100; i++) {
        // Increment shared_counter in the main program
        shared_counter += 2; // Vulnerability: Concurrent modification without synchronization
        usleep(10000); // Simulate work by sleeping
    }

    printf("Final counter value: %d\n", shared_counter);
    return 0;
}

In this vulnerable example, the shared_counter is accessed and modified by both the main program and the signal handler without any synchronization, leading to a race condition. This is because the increment operation is not atomic, and concurrent access can lead to unpredictable results.

How to fix Signal Handler Race Condition?

Fixed Code Example

// Include necessary headers
#include <stdio.h>
#include <signal.h>
#include <unistd.h>

// Use sig_atomic_t to ensure atomic updates
volatile sig_atomic_t shared_counter = 0; // Ensures atomicity for signal handler updates

// Signal handler function
void signal_handler(int signum) {
    if (signum == SIGUSR1) {
        // Update using sig_atomic_t to ensure atomicity
        shared_counter += 1; // Safe: Atomic update with sig_atomic_t
    }
}

int main() {
    // Register the signal handler
    signal(SIGUSR1, signal_handler);

    // Simulate some work in the main program
    for (int i = 0; i < 100; i++) {
        // Increment shared_counter in the main program
        shared_counter += 2; // Safe: Atomic operations are ensured by sig_atomic_t
        usleep(10000); // Simulate work by sleeping
    }

    printf("Final counter value: %d\n", shared_counter);
    return 0;
}

In the fixed example, the type of shared_counter is changed to sig_atomic_t, ensuring that updates to the variable are atomic. This prevents race conditions between the main program and the signal handler. Always ensure that signal handlers perform minimal work and avoid modifying complex data structures or calling non-reentrant functions.