<AWT Dev> [15] Review request for 8242498: Invalid "sun.awt.TimedWindowEvent" object leads to JVM crash

Anton Litvinov anton.litvinov at oracle.com
Thu Apr 16 16:34:36 UTC 2020

Hi Sergey,

The situation is not bad at all. We are sure that if, current opposite 
HWND is mapped to "Component" instead of the "Window", then 
"GetTopLevelParentForWindow()" will be definitely mapped to "Window", 
just because the "java.awt.Component" is not hanging independently on 
the screen and is always located in some instance of "java.awt.Window", 
which can be that frame, dialog, embedded frame etc. The available 
reproducer proves this. But I want to emphasize here again, that, 
normally when JDK is used normally, the current opposite HWND is mapped 
to "Window", it is mapped to "Component" only in case of this reproducer 
which does unusual actions with JDK.

What I am not sure, and what I do not want to check by this fix, is a 
normal case, when current opposite HWND is mapped to "Window" and we 
would substitute it for value returned from 
"GetTopLevelParentForWindow", I do not say that it will for sure return 
something else than HWND equal to current opposite HWND, but I do not 
want to check it through hypothetical regressions.

It is better to verify that "jobject jOpposite" is instance of "Window" 
before passing it to the constructor of "TimedWindowEvent(Window, int, 
Window opposite, int, int, long)", because it is called from native code 
through JNI, which is not able to detect this type discrepancy and to 
throw a corresponding Java exception. By JNI specification the native 
code is responsible for validation of the input arguments before passing 
them to Java code through JNI. And it would look very unusual if in Java 
code in constructor of "TimedWindowEvent" we start checking "opposite" 
which by source code is instance of "Window" for being instance of again 
"Window" class. What do you think about this?

Thank you,

On 16/04/2020 14:50, Sergey Bylokhov wrote:
> On 4/16/20 6:38 am, Anton Litvinov wrote:
>> I prefer to fix one problem at a time. There is no need to get 
>> "jobject jOpposite" from HWND returned by 
>> "GetTopLevelParentForWindow", when "jOpposite" got from the original 
>> "opposite" HWND is instance of "java.awt.Window". I am not going to 
>> explore all possible scenarios involving dialogs, modal dialogs, 
>> owned windows, owner windows, components in those windows and check 
>> if "GetTopLevelParentForWindow" will work in those scenarios, it is 
>> not related to this bug. Also I will repeat I will not change the 
>> existing behavior just for the reason of checking, if it cause any 
>> regression or does not cause it.
> So it means that after the fix, if the current opposite_HWND will be 
> map to the "Component" instead of the "Window", and we will start to 
> execute the new code and call GetTopLevelParentForWindow(), then we do 
> not sure how this method will work for the dialogs, modal dialogs, 
> owned windows, owner windows, components in those windows?
> Then probably it is better to check that in java? Just pass Component 
> to the TimedWindowEvent, check the type and try to find the parent if 
> it is not Window.
>> Why do you think that the fix does not work?
> I just tried to understand is it work or not.
>> Thank you,
>> Anton
>> On 15/04/2020 20:59, Sergey Bylokhov wrote:
>>> On 4/15/20 12:26 pm, Anton Litvinov wrote:
>>>> When JDK is used in a normal way and not as it is used in the 
>>>> reproducer, "opposite" is always HWND of a peer of 
>>>> "java.awt.Window" instance, thus for such cases there is no need to 
>>>> search for some parent HWND of the "opposite" HWND. I do not want 
>>>> to break behavior of JDK for these normal scenarios, by getting 
>>>> some parent HWND of the valid "opposite" HWND. I do not want to 
>>>> rely on expectation that for valid "opposite" HWND the method 
>>>> "AwtComponent::GetTopLevelParentForWindow" must return the same 
>>>> "opposite" HWND, I do not want to rely on this expectation and run 
>>>> the risk of using some incorrect HWND as opposite, when there was 
>>>> already valid "opposite" HWND in hands, and by this to break 
>>>> existing JDK behavior related to focus handling, because this 
>>>> edited "AwtWindow::SendWindowEvent" method is really involved in 
>>>> focus handling.
>>> That was the question, when GetTopLevelParentForWindow may return 
>>> wrong/incorrect HWND and why the same situation may not happen after 
>>> fix when we call GetTopLevelParentForWindow later.
>>>> Thank you,
>>>> Anton
>>>> On 15/04/2020 07:56, Sergey Bylokhov wrote:
>>>>> On 4/14/20 11:12 am, Anton Litvinov wrote:
>>>>>> I think that it is better not to change current well established 
>>>>>> old behavior in the method "AwtWindow::SendWindowEvent(jint, 
>>>>>> HWND, jint, jint)" - what is first to get "jobject" from the 
>>>>>> original "opposite" HWND and try to use it for construction of 
>>>>>> "TimedWindowEvent". I was trying to make a fix which will just 
>>>>>> extend the code and will address just this very unreal and very 
>>>>>> narrow scenario without running the risk of affecting or breaking 
>>>>>> wide range of other normal scenarios under which this code was 
>>>>>> working well from the moment of its creation. If we assign parent 
>>>>>> HWND to the "opposite" from the very beginning, then we will 
>>>>>> change the currently existing behavior in favor of addressing 
>>>>>> this almost unreal bug and may for some cases, which we do not 
>>>>>> know yet, start using parent HWND instead of a valid original 
>>>>>> "opposite" HWND whose target will be instance of "java.awt.Window".
>>>>>> In my opinion it is better not to omit first attempt to use 
>>>>>> original "opposite" with related to it "jobject" instance.
>>>>> If it is not safe to replace the opposite by the 
>>>>> GetTopLevelParentForWindow(1) in the first place then
>>>>> why it is safe to use it in the fix later(1)? What could go wrong 
>>>>> at step (1) and why it will work fine
>>>>> at step(2)?
>>>>>> Thank you,
>>>>>> Anton
>>>>>> On 14/04/2020 16:41, Sergey Bylokhov wrote:
>>>>>>> Hi, Anton.
>>>>>>> Probably it is possible to simplify the code a little bit:
>>>>>>> Can we just replace initial "opposite" by the top level HWND?
>>>>>>> + opposite = AwtComponent::GetTopLevelParentForWindow(opposite);
>>>>>>>   AwtComponent *awtOpposite = AwtComponent::GetComponent(opposite);
>>>>>>> and add only one check
>>>>>>>   jOpposite = awtOpposite->GetTarget(env);
>>>>>>> + if ((jOpposite != NULL) &&
>>>>>>> +   !env->IsInstanceOf(jOpposite, windowCls)) {
>>>>>>> +       env->DeleteLocalRef(jOpposite);
>>>>>>> +       jOpposite = NULL;
>>>>>>> +  }
>>>>>>> On 4/10/20 1:32 pm, Anton Litvinov wrote:
>>>>>>>> Hello,
>>>>>>>> Could you please review the following fix for the bug.
>>>>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8242498
>>>>>>>> Webrev: 
>>>>>>>> http://cr.openjdk.java.net/~alitvinov/8242498/jdk15/webrev.00
>>>>>>>> The bug is the JVM crash, which occurs because a not existing 
>>>>>>>> method is called on a Java object which is not an instance of 
>>>>>>>> the expected Java class that has such a method. Such 
>>>>>>>> discrepancy of the expected type and the type in runtime is 
>>>>>>>> possible, because the Java object, whose field value is set to 
>>>>>>>> the instance of the not expected Java class, is instantiated by 
>>>>>>>> AWT native code through JNI invocation. Since JNI does not 
>>>>>>>> validate arguments passed to Java class constructor and since 
>>>>>>>> AWT native code does not validate arguments prior to invoking 
>>>>>>>> Java class constructor through JNI, such invalid object is 
>>>>>>>> created.
>>>>>>>> The fact that in the method 
>>>>>>>> "java.awt.DefaultKeyboardFocusManager.dispatchEvent(AWTEvent)" 
>>>>>>>> in the case "WindowEvent.WINDOW_LOST_FOCUS" of switch operator 
>>>>>>>> the variable defined by the expression "Window oppositeWindow = 
>>>>>>>> we.getOppositeWindow();" in runtime is instance of 
>>>>>>>> "java.awt.Component" class instead of "java.awt.Window" class. 
>>>>>>>> The crash occurs during attempt to call the method 
>>>>>>>> "java.awt.Window.getTemporaryLostComponent()" on the object 
>>>>>>>> "oppositeWindow" which in runtime is "Component" instead of the 
>>>>>>>> expected "Window" object, and since the method 
>>>>>>>> "getTemporaryLostComponent()" does not exist in 
>>>>>>>> "java.awt.Component" class JVM cannot find this method and 
>>>>>>>> initiates the crash.
>>>>>>>> Transfer of the object of the incompatible type 
>>>>>>>> "java.awt.Component" instead of an object of "java.awt.Window" 
>>>>>>>> type as "opposite" argument to the constructor 
>>>>>>>> "TimedWindowEvent(Window source, int id, Window opposite, int 
>>>>>>>> oldState, int newState, long time)" of the class 
>>>>>>>> "sun.awt.TimedWindowEvent" through JNI invocation. This JNI 
>>>>>>>> invocation occurs in the C++ class method 
>>>>>>>> "AwtWindow::SendWindowEvent(jint, HWND, jint, jint)" in the 
>>>>>>>> file 
>>>>>>>> "src/java.desktop/windows/native/libawt/windows/awt_Window.cpp". 
>>>>>>>> The exact expression creating the instance of Java class 
>>>>>>>> "TimedWindowEvent" with the invalid value of the field 
>>>>>>>> "opposite" is following:
>>>>>>>> jobject event = env->NewObject(wClassEvent, wEventInitMID, 
>>>>>>>> target, id,
>>>>>>>>          jOpposite, oldState, newState, 
>>>>>>>> ::JVM_CurrentTimeMillis(NULL, 0));
>>>>>>>> THE FIX:
>>>>>>>> The fix changes "AwtWindow::SendWindowEvent(jint, HWND, jint, 
>>>>>>>> jint)" method in the file "awt_Window.cpp" to introduce the 
>>>>>>>> code which verifies that the Java object "jOpposite" is really 
>>>>>>>> instance of the class "java.awt.Window", and if it is not then 
>>>>>>>> the fix tries to get Java object corresponding to parent window 
>>>>>>>> of the original "opposite" HWND. And if this parent window 
>>>>>>>> object also is not instance of "java.awt.Window" class, then 
>>>>>>>> NULL value is passed to the constructor of "TimedWindowEvent" 
>>>>>>>> class.
>>>>>>>> Thank you,
>>>>>>>> Anton

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