Secure Operating Systems

1. Secure Operating Systems Lesson 0x12h: Return to User

2. Where are we? • Done! Yay! Code Complete! • But there’s always more  • So, let’s look at a new trend in OS exploitation: ret2usr

3. Exploiting the Kernel • When we exploit the kernel, it can be hard to actually gain control • In particular, NULL pointer dereference has often been thought of as unexploitable… • Fortunately (?) that’s not true: return to user!!!

4. Underlying Vuln • Think about memory layout for a little bit… • Right… the kernel can still write to user space (any part of user space) • This means that ‘nuisance’ attacks like NULL pointer dereference can be deadly in Kernel space (i.e. deadly == not just a DoS attack)

5. What happens… • Imagine we can get a struct inside the kernel to be dereferenced, and this struct contains a function pointer which the kernel will use… • Boom! The pointer is now in memory which is valid in user mode (somewhere around –x000000nn, typically) • Aside: this is really confused deputy all over again

6. Example Exploit • Let’s look at the step-by-step vuln in the paper… • Discussion: turtles all the way down?

7. SMEP • Supervisor Mode Execution Prevention • Prevent code execution of user-mode code page in CPL=0 • Note: does not prevent modification (read and write) • Does not raise a #GP but a #PF • Can use kernel mode ROP to avoid this…

8. Questions & Comments • What do you want to know?