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Race condition

Need​

Ensuring proper synchronization and ordering of inputs to prevent race conditions

Context​

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

Description​

Non compliant code​

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​

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

Compliant code​

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​

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