Debugging walkthrough: Diagnosing an NX exception

Raymond Chen

Raymond

A colleague of mine asked for help debugging a strange failure. Execution halted because the CPU detected that it was trying to execute data.

ABC!__PchSym_ (ABC+0x67be4)
user32!UserCallWinProcCheckWow+0x140
user32!DispatchClientMessage+0xa2
user32!__fnDWORD+0x2d
ntdll!KiUserCallbackDispatcherContinue
user32!ZwUserPeekMessage+0xa
user32!PeekMessageW+0x7f
explorerframe!CExplorerFrame::FrameMessagePump+0x5b
explorerframe!BrowserThreadProc+0x5e
explorerframe!BrowserNewThreadProc+0x3a
explorerframe!CExplorerTask::InternalResumeRT+0x12
explorerframe!CRunnableTask::Run+0xc9
shell32!CShellTaskThread::ThreadProc+0x284
shell32!CShellTaskThread::s_ThreadProc+0x2b
SHCore!_WrapperThreadProc+0x15f
kernel32!BaseThreadInitThunk+0xd
ntdll!RtlUserThreadStart+0x1d

EXCEPTION_RECORD:  (.exr -1)
ExceptionAddress: 00007ffcfd197be4 (ABC+0x67be4)
   ExceptionCode: c0000005 (Access violation)
  ExceptionFlags: 00000000
NumberParameters: 2
   Parameter[0]: 0000000000000008
   Parameter[1]: 00007ffcfd197be4
Attempt to execute non-executable address 00007ffcfd197be4

My colleague suspected that a return address got overwritten by some function deeper in the stack, and that caused the instruction pointer to jump to a random module, and the victim module was ABC.

I looked at the crash dump, and came to a different conclusion. The stack is just fine. The problem is that a DLL got unloaded:

0:067> lm
...
Unloaded modules:
...
00007ffc`fd140000 00007ffc`fd1ee000   DEF.dll
...

After DEF.dll got unloaded, ABC.DLL got loaded into the same location.

0:067> .reload /unl DEF.dll
WARNING: DEF overlaps ABC

The problem is that DEF.dll unloaded before destroying all its windows. And then its window received a message (in this case, WM_ACTIVATE­APP, but you were not expected to know this since it wasn’t in the stack trace). The window manager called the window procedure, which now points into the middle of ABC.dll. The debugger is correctly reporting that execution halted in the middle of ABC.dll.

The next step is to engage the people responsible for DEF.dll to figure out why they leaked a window.

Exercise: What command would be useful at this point to help the DEF.dll identify the window that they leaked?

Raymond Chen
Raymond Chen

Follow Raymond   

0 comments

Comments are closed.