> For the complete documentation index, see [llms.txt](https://ir0nstone.gitbook.io/notes/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://ir0nstone.gitbook.io/notes/binexp/kernel/smep.md). # SMEP Supervisor Memory Execute Protection 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 This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation by asking a question. Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters: ``` GET https://ir0nstone.gitbook.io/notes/binexp/kernel/smep.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case. 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.