CWE-413 资源加锁不恰当
Improper Resource Locking
结构: Simple
Abstraction: Base
状态: Draft
被利用可能性: unkown
基本描述
The software does not lock or does not correctly lock a resource when the software must have exclusive access to the resource.
扩展描述
When a resource is not properly locked, an attacker could modify the resource while it is being operated on by the software. This might violate the software's assumption that the resource will not change, potentially leading to unexpected behaviors.
相关缺陷
适用平台
Language: {'cwe_Class': 'Language-Independent', 'cwe_Prevalence': 'Undetermined'}
常见的影响
| 范围 | 影响 | 注释 |
|---|---|---|
| ['Integrity', 'Availability'] | ['Modify Application Data', 'DoS: Instability', 'DoS: Crash, Exit, or Restart'] |
可能的缓解方案
Architecture and Design
策略:
Use a non-conflicting privilege scheme.
['Architecture and Design', 'Implementation']
策略:
Use synchronization when locking a resource.
示例代码
例
The following function attempts to acquire a lock in order to perform operations on a shared resource.
bad C
/ access shared resource */
pthread_mutex_unlock(mutex);
}
However, the code does not check the value returned by pthread_mutex_lock() for errors. If pthread_mutex_lock() cannot acquire the mutex for any reason the function may introduce a race condition into the program and result in undefined behavior.
In order to avoid data races correctly written programs must check the result of thread synchronization functions and appropriately handle all errors, either by attempting to recover from them or reporting it to higher levels.
good C
result = pthread_mutex_lock(mutex);
if (0 != result)
/ access shared resource */
return pthread_mutex_unlock(mutex);
}
例
This Java example shows a simple BankAccount class with deposit and withdraw methods.
bad Java
// variable for bank account balance
private double accountBalance;
// constructor for BankAccount
public BankAccount() {
}
// method to deposit amount into BankAccount
public void deposit(double depositAmount) {
double newBalance = accountBalance + depositAmount;
accountBalance = newBalance;
}
// method to withdraw amount from BankAccount
public void withdraw(double withdrawAmount) {
double newBalance = accountBalance - withdrawAmount;
accountBalance = newBalance;
}
// other methods for accessing the BankAccount object
...
}
However, the deposit and withdraw methods have shared access to the account balance private class variable. This can result in a race condition if multiple threads attempt to call the deposit and withdraw methods simultaneously where the account balance is modified by one thread before another thread has completed modifying the account balance. For example, if a thread attempts to withdraw funds using the withdraw method before another thread that is depositing funds using the deposit method completes the deposit then there may not be sufficient funds for the withdraw transaction.
To prevent multiple threads from having simultaneous access to the account balance variable the deposit and withdraw methods should be synchronized using the synchronized modifier.
good Java
// synchronized method to deposit amount into BankAccount
public synchronized void deposit(double depositAmount) {
}
// synchronized method to withdraw amount from BankAccount
public synchronized void withdraw(double withdrawAmount) {
}
...
}
An alternative solution is to use a lock object to ensure exclusive access to the bank account balance variable. As shown below, the deposit and withdraw methods use the lock object to set a lock to block access to the BankAccount object from other threads until the method has completed updating the bank account balance variable.
good Java
// lock object for thread access to methods
private ReentrantLock balanceChangeLock;
// condition object to temporarily release lock to other threads
private Condition sufficientFundsCondition;
// method to deposit amount into BankAccount
public void deposit(double amount) {
// set lock to block access to BankAccount from other threads
balanceChangeLock.lock();
try {
balance = newBalance;
// inform other threads that funds are available
sufficientFundsCondition.signalAll();
} catch (Exception e) {...}
finally {
balanceChangeLock.unlock();
}
}
// method to withdraw amount from bank account
public void withdraw(double amount) {
// set lock to block access to BankAccount from other threads
balanceChangeLock.lock();
try {
// temporarily unblock access
// until sufficient funds are available
sufficientFundsCondition.await();
}
double newBalance = balance - amount;
balance = newBalance;
} catch (Exception e) {...}
finally {
balanceChangeLock.unlock();
}
}
...
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