Answer to quick puzzle about security and synchronization

Raymond Chen

As many people quickly figured out, the reason why the the WaitForSingleObject returns immediately is that the call is failing. The reason is that the second process opened the handle with EVENT_MODIFY_STATE access, which grants permission to call the SetEvent function, the ResetEvent function, and the fatally flawed PulseEvent function, but it doesn’t include SYNCHRONIZE access, which is necessary if you intend to synchronize on the object (i.e., wait on it).

The fix is for Process B to ask for SYNCHRONIZE access instead of EVENT_MODIFY_STATE.

The fact that it’s happening in a second process is a red herring. You can put this code in the same process and it will fail/succeed in the same way:

HANDLE hEventA = CreateEvent(NULL, FALSE, TRUE, TEXT("MyNamedEvent"));
HANDLE hEventB = OpenEvent(EVENT_MODIFY_STATE, FALSE, TEXT("MyNamedEvent"));
WaitForSingleObject(hEventB, INFINITE); // fails

Indeed, the fact that the object is named is a red herring. It has nothing to do with named/unnamed objects.

HANDLE hEventA = CreateEvent(NULL, FALSE, TRUE, NULL);
DuplicateHandle(GetCurrentProcess(), hEventA,
                GetCurrentProcess(), &hEventB,
                EVENT_MODIFY_STATE, FALSE, 0);
WaitForSingleObject(hEventB, INFINITE); // fails

In all three cases, the fix is to change EVENT_MODIFY_STATE to SYNCHRONIZE.


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