保护异常处理

更新:2007 年 11 月

在 Visual C++ 和 Visual Basic 中,在堆栈上部的筛选器表达式在任何 finally 语句之前运行。与该筛选器关联的 catch 块在 finally 语句之后运行。有关更多信息,请参见使用用户筛选的异常。本节讨论上述顺序的安全含义。请看下面的伪代码示例,其中演示了筛选语句和 finally 语句的运行顺序。

void Main() 
{
    try 
    {
        Sub();
    } 
    except (Filter()) 
    {
        Console.WriteLine("catch");
    }
}
bool Filter () {
    Console.WriteLine("filter");
    return true;
}
void Sub() 
{
    try 
    {
        Console.WriteLine("throw");
        throw new Exception();
    } 
    finally 
    {
        Console.WriteLine("finally");
    }
}                      

该代码将输出下面的内容。

Throw
Filter
Finally
Catch

该筛选器在 finally 语句之前运行,这样,任何会更改状态的做法都可能引发安全问题,因为其他代码的执行可能会利用这种状态更改。例如:

try 
{
    Alter_Security_State();
    // This means changing anything (state variables,
    // switching unmanaged context, impersonation, and 
    // so on) that could be exploited if malicious 
    // code ran before state is restored.
    Do_some_work();
} 
finally 
{
    Restore_Security_State();
    // This simply restores the state change above.
}

该伪代码允许位于堆栈较上部的筛选器运行任意代码。可能会产生类似效果的其他操作示例有:临时模拟另一个标识;设置跳过某种安全检查的内部标志;更改与线程关联的区域性。建议采纳的解决方案是引入一个异常处理程序,用于将代码对线程状态的更改与调用方的筛选器块分开。然而,重要的是必须正确地引入异常处理程序,否则,就无法解决这一问题。下面的示例切换用户界面区域性,但任何一种线程状态更改都可采用类似的方式公开。

YourObject.YourMethod()
{
   CultureInfo saveCulture = Thread.CurrentThread.CurrentUICulture;
   try {
      Thread.CurrentThread.CurrentUICulture = new CultureInfo("de-DE");
      // Do something that throws an exception.
}
   finally {
      Thread.CurrentThread.CurrentUICulture = saveCulture;
   }
}
Public Class UserCode
   Public Shared Sub Main()
      Try
         Dim obj As YourObject = new YourObject
         obj.YourMethod()
      Catch e As Exception When FilterFunc
         Console.WriteLine("An error occurred: '{0}'", e)
         Console.WriteLine("Current Culture: {0}", 
Thread.CurrentThread.CurrentUICulture)
      End Try
   End Sub

   Public Function FilterFunc As Boolean
      Console.WriteLine("Current Culture: {0}", Thread.CurrentThread.CurrentUICulture)
      Return True
   End Sub

End Class

在这种情况下,正确的修复方法是将现有的 try/finally 块包装到 try/catch 块中。仅仅将 catch-throw 子句引入到现有的 try/finally 块中不能修复该问题,如下面的示例所示。

YourObject.YourMethod()
{
    CultureInfo saveCulture = Thread.CurrentThread.CurrentUICulture;

    try 
    {
        Thread.CurrentThread.CurrentUICulture = new CultureInfo("de-DE");
        // Do something that throws an exception.
    }
    catch { throw; }
    finally 
    {
        Thread.CurrentThread.CurrentUICulture = saveCulture;
    }
}

这样不能修复上述问题,原因在于:在 FilterFunc 获得控制之前,没有运行 finally 语句。

下面的示例通过确保在向调用方的异常筛选器块提供异常之前执行 finally 子句,修复了上述问题。

YourObject.YourMethod()
{
    CultureInfo saveCulture = Thread.CurrentThread.CurrentUICulture;
    try  
    {
        try 
        {
            Thread.CurrentThread.CurrentUICulture = new CultureInfo("de-DE");
            // Do something that throws an exception.
        }
        finally 
        {
            Thread.CurrentThread.CurrentUICulture = saveCulture;
        }
    }
    catch { throw; }
}

请参见

其他资源

代码安全维护指南