The vulnerability exists due to a missing return value check within the win32k.sys driver. This driver is responsible for the kernel-mode part of the Windows subsystem. It handles window management and provides the Graphics Device Interface (GDI) among other things. The results of the analysis show that even with the presence of protection mechanisms like SMEP, with full control over a moderately large kernel structure there are enough possibilities to trigger a simple memory corruption in order to achieve the desired goal and be able to escalate privileges without overwriting any function pointers or executing any shell code at all.
Read the full analysis here - http://dl.packetstormsecurity.net/papers/attack/CVE-2014-4113.pdf