Declaring a shared mutable variable volatile ensures the visibility of the latest updates on it, across other threads but does not guarantee the atomicity of composite operations. For example, the variable increment operation consisting of the sequence read-modify-write is not atomic even when the variable is declared volatile. In such cases, the java.util.concurrent utilities must be used to atomically manipulate the variable. If the utilities do not provide the required atomic methods, accesses to the variable must be explicitly synchronized.
Noncompliant Code Example
In this noncompliant code example, the field itemsInInventory can be accessed by multiple threads. However, when a thread is updating the value of itemsInInventory, it is possible for other threads to read the original value, before the update. This is because the post decrement operator is non-atomic.
| Code Block | ||
|---|---|---|
| ||
private int itemsInInventory = 100;
public int removeItem() {
if(itemsInInventory > 0) {
return itemsInInventory--; // Returns new count of items in inventory
} else {
return 0;
}
}
|
Compliant Solution (1)
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Volatile variables are unsuitable when more than one load/store operation needs to be atomic. There is an alternative method to perform multiple operations atomically. This compliant solution shows a {{java.util.concurrent.atomic.AtomicInteger}} variable. According to the Java API \[[API 06|AA. Java References#API 06]\], Class {{AtomicInteger}} documentation: |
...
| Code Block | ||
|---|---|---|
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public class Sync {
private final AtomicInteger itemsInInventory = new AtomicInteger(100);
private int removeItem() {
for (;;) {
int old = itemsInInventory.get();
if (old > 0) {
int next = old - 1;
if (itemsInInventory.compareAndSet(old, next)) {
return next; //returns new count of items in inventory
}
} else {
return 0;
}
}
}
}
|
Compliant Solution (2)
This compliant solution uses method synchronization to synchronize access to shared variables. Consequently, access to itemsInInventory is mutually exclusive and consistent across object states.
...
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Synchronization is more expensive than using the optimized {{java.util.concurrent}} utilities and should only be used when the utilities do not contain the required method to carry out the atomic operation. When using explicit synchronization, the programmer must also ensure that two or more threads are not mutually accessible from a different set of two or more threads such that each thread holds a lock while trying to obtain another lock that is held by the other thread \[[Lea 00|AA. Java References#Lea 00]\]. Failure to follow this advice results in deadlocks ([CON11-J. Avoid deadlock by requesting locks in the proper order]). |
Risk Assessment
If access to shared, mutable variables is not synchronized, unexpected results may be produced. For example, there can be inadvertent information disclosure as one user may be able to receive information about other users.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
CON01- J | medium | probable | medium | P8 | L2 |
Automated Detection
TODO
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
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\[[API 06|AA. Java References#API 06]\] Class AtomicInteger \[[JLS 05|AA. Java References#JLS 05]\] [Chapter 17, Threads and Locks|http://java.sun.com/docs/books/jls/third_edition/html/memory.html], section 17.4.5 Happens-before Order, section 17.4.3 Programs and Program Order, section 17.4.8 Executions and Causality Requirements \[[Tutorials 08|AA. Java References#Tutorials 08]\] [Java Concurrency Tutorial|http://java.sun.com/docs/books/tutorial/essential/concurrency/index.html] \[[Lea 00|AA. Java References#Lea 00]\] Sections, 2.2.7 The Java Memory Model, 2.2.5 Deadlock, 2.1.1.1 Objects and locks \[[Bloch 08|AA. Java References#Bloch 08]\] Item 66: Synchronize access to shared mutable data \[[Daconta 03|AA. Java References#Daconta 03]\] Item 31: Instance Variables in Servlets \[[JavaThreads 04|AA. Java References#JavaThreads 04]\] Section 5.2 Atomic Variables \[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 667|http://cwe.mitre.org/data/definitions/667.html] "Insufficient Locking", [CWE ID 413|http://cwe.mitre.org/data/definitions/413.html] "Insufficient Resource Locking", [CWE ID 366|http://cwe.mitre.org/data/definitions/366.html] "Race Condition within a Thread", [CWE ID 567|http://cwe.mitre.org/data/definitions/567.html] "Unsynchronized Access to Shared Data" |
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