# SMEP If [ret2usr ](/notes/binexp/kernel/kernel-rop-ret2usr.md)is analogous to ret2shellcode, then SMEP is the new [NX](/notes/binexp/stack/no-execute.md). SMEP is a primitive protection that ensures [any code executed in kernel mode is located in kernel space](https://wiki.osdev.org/Supervisor_Memory_Protection), and it does this based on the User/Supervisor bit in page tables. This means a simple ROP back to our own shellcode no longer works. To bypass SMEP, we have to use gadgets located in the kernel to achieve what we want to (without switching to userland code). In older kernel versions we could [use ROP to disable SMEP entirely](/notes/binexp/kernel/smep/kernel-rop-disabling-smep.md), but this has been patched out. This was possible because SMEP is determined by the [20th bit of the CR4 register](https://wiki.osdev.org/CPU_Registers_x86#CR4), meaning that if we can control CR4 we can disable SMEP from messing with our exploit. We can enable SMEP in the kernel by controlling the respective QEMU flag (`qemu64` is not notable): ``` -cpu qemu64,+smep ``` Sometimes it will be enabled by default, in which case you need to us `nosmep`. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://ir0nstone.gitbook.io/notes/binexp/kernel/smep.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.