The program uses double-checked locking to access a resource without the overhead of explicit synchronization, but the locking is insufficient.
Extended Description
Double-checked locking refers to the situation where a programmer checks to see if a resource has been initialized, grabs a lock, checks again to see if the resource has been initialized, and then performs the initialization if it has not occurred yet. This should not be done, as is not guaranteed to work in all languages and on all architectures. In summary, other threads may not be operating inside the synchronous block and are not guaranteed to see the operations execute in the same order as they would appear inside the synchronous block.
Time of Introduction
Implementation
Applicable Platforms
Languages
Java
Common Consequences
Scope
Effect
Integrity
Other
Technical Impact: Modify application
data; Alter execution
logic
Demonstrative Examples
Example 1
It may seem that the following bit of code achieves thread safety
while avoiding unnecessary synchronization...
(Bad Code)
Example
Language: Java
if (helper == null) {
synchronized (this) {
if (helper == null) {
helper = new Helper();
}
}
}
return helper;
The programmer wants to guarantee that only one Helper() object is
ever allocated, but does not want to pay the cost of synchronization
every time this code is called.
Suppose that helper is not initialized. Then, thread A sees that
helper==null and enters the synchronized block and begins to
execute:
(Bad Code)
helper = new Helper();
If a second thread, thread B, takes over in the middle of this call
and helper has not finished running the constructor, then thread B may
make calls on helper while its fields hold incorrect values.
Potential Mitigations
Phase: Implementation
While double-checked locking can be achieved in some languages, it is
inherently flawed in Java before 1.5, and cannot be achieved without
compromising platform independence. Before Java 1.5, only use of the
synchronized keyword is known to work. Beginning in Java 1.5, use of the
"volatile" keyword allows double-checked locking to work successfully,
although there is some debate as to whether it achieves sufficient
performance gains. See references.
[REF-7] Mark Dowd, John McDonald
and Justin Schuh. "The Art of Software Security Assessment". Chapter 13, "Threading Vulnerabilities", Page
815.. 1st Edition. Addison Wesley. 2006.
Description
