Microsoft Windows Kernel - Device Contexts and NtGdiSelectBitmap Use-After-Free (MS15-115)

Source: https://code.google.com/p/google-security-research/issues/detail?id=505

The attached testcase triggers a use-after-free condition in win32k. The attached debugger output was triggered on Windows 7 with Special Pool enabled on win32k.sys. 
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Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/38796.zip