Race condition - Scala

Need

Ensuring proper synchronization and ordering of inputs to prevent race conditions

Context

1. Usage of Scala for building scalable and high-performance applications
2. Usage of play.api.mvc for handling HTTP requests and responses in Play Framework
3. Usage of global execution context for Scala concurrent operations
4. Usage of scala.concurrent.Future for asynchronous programming in Scala

Description

Insecure Code Example

import play.api.mvc._
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future

class HomeController extends Controller {

  var sharedResource: Int = 0

  def raceCondition() = Action.async {
    Future {
      Thread.sleep(1000)
      sharedResource += 1
    }
    Future.successful(Ok("Shared Resource Value: " + sharedResource))
  }
}

The above code represents a race condition vulnerability in a Play Framework application. The `raceCondition` method in the `HomeController` class is vulnerable to race conditions. Here's a detailed walkthrough of the code: 1. `sharedResource` is a shared mutable state that is accessed by multiple threads. 2. The `raceCondition` method is an asynchronous action that increments the `sharedResource` after a delay of 1 second. 3. After that, it immediately returns the current value of `sharedResource` without waiting for the increment operation to complete. When multiple requests are made to the `raceCondition` endpoint at the same time, they all get the same initial value of `sharedResource`. They all increment `sharedResource` after 1 second, but the returned value is the initial value, not the incremented value. This is because the increment operation and the read operation are not synchronized, leading to a race condition. This race condition can lead to inconsistent and unexpected behavior, as the value of `sharedResource` depends on the timing and order of requests, which is not predictable or under the control of the program.

Steps

1. Identify the critical section of code where the shared resource is being accessed and modified.
2. Implement a synchronization mechanism to ensure that only one thread can access the critical section at a time.
3. Use locks or synchronized blocks to protect the shared resource from concurrent access.
4. Ensure that the synchronization mechanism is applied consistently across all code paths that access the shared resource.
5. Test the application to verify that the race condition vulnerability has been resolved.

Secure Code Example

import play.api.mvc._
import scala.concurrent.{ExecutionContext, Future}
import java.util.concurrent.atomic.AtomicInteger

class HomeController extends Controller {

  val sharedResource: AtomicInteger = new AtomicInteger(0)

  def raceCondition()(implicit ec: ExecutionContext) = Action.async {
    Future {
      Thread.sleep(1000)
      sharedResource.incrementAndGet()
    }.map { _ =>
      Ok("Shared Resource Value: " + sharedResource.get())
    }
  }
}

The original code had a race condition vulnerability due to the shared mutable state `sharedResource`. Multiple threads could access and modify this shared resource concurrently, leading to unpredictable behavior and incorrect results. The fixed code uses `AtomicInteger` for the shared resource. `AtomicInteger` is a class in `java.util.concurrent.atomic` package that provides atomic operations on underlying int value. The `incrementAndGet()` method is used which atomically increments the current value by one and returns the updated value. This operation happens atomically and is thread-safe, which means it eliminates the race condition vulnerability. The `get()` method is used to fetch the current value of the shared resource. This method is also thread-safe. The `Future` block is modified to map the result to the `Ok` response. This ensures that the response is created after the Future operation is completed, providing the correct value of the shared resource. The `ExecutionContext` is now an implicit parameter to the `raceCondition` method. This allows for better control over the threading context where the Future operations are executed. This solution ensures that the shared resource is accessed and modified in a thread-safe manner, eliminating the race condition vulnerability.

References

Last updated

2023/09/18