...
If however, one follows a fine-grained locking approach by using member locks, deadlocks can still arise unless it is ensured that each and every thread always requests locks in the same order.
Noncompliant Code Example
This noncompliant code example shows a subtle deadlock issue that manifests itself when synchronization is implemented incorrectly. Assume that an attacker has two bank accounts and is capable of requesting two amount transfer operations in succession. This corresponds to two threads as shown in main(). Objects a and b are constructed such that the amount will be transferred from a to b by first depositing the amount from a to b and then zeroing out a (and vice-versa during the call to b.deposit()).
...
| Code Block | ||
|---|---|---|
| ||
class BadLocks {
private int balance; // Total amount
private BadLocks(int balance) {
this.balance = balance;
}
private synchronized void withdraw(BadLocks bl) {
System.out.println("Withdrawing amount...");
this.balance = 0;
}
private synchronized void deposit(BadLocks bl) {
System.out.println("Depositing amount...");
bl.balance += this.balance;
bl.withdraw(bl);
}
public static void main(String[] args) throws Exception {
final BadLocks a = new BadLocks(5000);
final BadLocks b = new BadLocks(6000);
// These two threads correspond to two malicious requests triggered by the attacker
Thread t1 = new Thread(new Runnable() {
public void run() {
a.deposit(b);
}
});
Thread t2 = new Thread(new Runnable() {
public void run() {
b.deposit(a);
}
});
t1.start();
t2.start();
}
}
|
Compliant Solution
To be compliant, do not request a lock when it is already held by another object where that object is waiting for the requesting object to release its own lock. This can be achieved by invoking the withdraw() method without qualifying it with the bl parameter.
| Code Block | ||
|---|---|---|
| ||
withdraw(bl); |
Noncompliant Code Example
In this noncompliant code example, a deadlock occurs if the transfer() method acquires the monitors in decreasing numeric order, while the sumHelper() method uses an increasing numeric order to acquire its locks.
| Code Block | ||
|---|---|---|
| ||
class Stocks implements FundConstants {
static int[] balances = new int[noOfStocks];
static Object[] locks = new Object[noOfStocks];
static {
for (int n = 0; n < noOfStocks; n++) {
balances[n] = 10000;
locks[n] = new Object();
}
}
static void transfer(Transfer t) {
int lo, hi;
if (t.fundFrom > t.fundTo) { // Acquires the monitors in decreasing numeric order
lo = t.fundFrom;
hi = t.fundTo;
} else {
lo = t.fundTo;
hi = t.fundFrom;
}
synchronized (locks[lo]) {
synchronized (locks[hi]) {
balances[t.fundFrom] -= t.amount;
balances[t.fundTo] += t.amount;
}
}
}
static int sumHelper(int next) {
synchronized (locks[next]) {
if (next == (noOfStocks - 1)) {
return balances[next];
} else {
return balances[next] + sumHelper(next + 1);
}
}
}
static void checkSystem() {
int actual = 0;
actual = sumHelper(0);
System.out.println("The actual balance is " + actual);
}
}
|
Compliant Solution
To implement a fine-grained locking strategy, request a separate lock for each position in the balances array. As it is not possible to lock on primitive types, a direct lock on the items in the balances array cannot be obtained. Instead, an array of raw Objects (locks) can be used.
...
| Code Block | ||
|---|---|---|
| ||
class Stocks implements FundConstants {
static int[] balances = new int[noOfStocks];
static Object[] locks = new Object[noOfStocks];
static {
for (int n = 0; n < noOfStocks; n++) {
balances[n] = 10000;
locks[n] = new Object();
}
}
static void transfer(Transfer t) {
int lo, hi;
if (t.fundFrom < t.fundTo) {
lo = t.fundFrom;
hi = t.fundTo;
} else {
lo = t.fundTo;
hi = t.fundFrom;
}
synchronized (locks[lo]) {
synchronized (locks[hi]) {
balances[t.fundFrom] -= t.amount;
balances[t.fundTo] += t.amount;
}
}
}
static int sumHelper (int next) {
synchronized (locks[next]) {
if (next == (noOfStocks - 1)) {
return balances[next];
} else {
return balances[next] + sumHelper(next + 1);
}
}
}
static void checkSystem() {
int actual = 0;
actual = sumHelper(0);
System.out.println("The actual balance is " + actual);
}
}
|
Risk Assessment
Fine-grained locking may result in deadlocks if some thread does not always request locks in the same order as others.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
CON34- J | low | likely | high | P3 | L3 |
Automated Detection
TODO
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
| Wiki Markup |
|---|
\[[JLS 05|AA. Java References#JLS 05]\] [Chapter 17, Threads and Locks|http://java.sun.com/docs/books/jls/third_edition/html/memory.html] \[[SDN 08|AA. Java References#SDN 08]\] [Sun Developer Network Tech tips|http://java.sun.com/developer/TechTips/2000/tt0328.html] \[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 412|http://cwe.mitre.org/data/definitions/412.html] "Unrestricted Lock on Critical Resource" |
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