1. Practical Timing Side Channel Attacks Against Kernel Space ASLR
Fall 2014
Instructor: Kun Sun, Ph.D.
A little background about myself.

2. Outline
Background
Side Channel Attack
Kernel Space ASLR
Use timing-based side channel to defeat kernel space ASLR (A heavy engineering work)
How to prevent/mitigate this side channel attack?

3. Outline
Background
Side Channel Attack
Kernel Space ASLR
Use timing-based side channel to defeat kernel space ASLR
How to prevent/mitigate this side channel attack?

4. Side Channel Side channel vs. Covert channel
Side channel: incidentally, unintentionally
Covert channel: colluding, intentionally
A side channel is a communication channel based on the use of system resources not normally intended for communication between the subjects (processes) in the system

5. Side Channel Types Computer systems abound with side channels
Storage Channels
use system variables and attributes (other than time) to signal information
classic example is resource exhaustion channel
Timing Channels
vary the amount of time required to complete a task to signal information
classic example is load sensing channel
Side channels are typically noisy but information theory techniques can be used to achieve error- free communication

6. Resource Exhaustion Channel
Given 5MB pool of dynamically allocated Memory
High Process
bit = 1  request 5MB of memory
bit = 0  request 0MB of memory
Low Process
request 5MB of memory
if allocated then bit = 0 otherwise bit = 1

7. Load Sensing Channel High Process Low Process
bit = 1  enter computation intensive loop
bit = 0  go to sleep
Low Process
perform a task with known computational requirements
if completed quickly then bit = 0 otherwise bit =1

8. Mitigating Side Channel
Audit for storage-based
Time normalization for timing-based
Slow it down, reduce the transmission date rate

9. Outline
Background
Side Channel Attack
Kernel Space ASLR
Use timing-based side channel to defeat kernel space ASLR
How to prevent/mitigate this side channel attack?

10. ASLR Deployment Windows user space and kernel space ASLR since vista.
Pax patches in Linux (not in kernel)
MacOS since 10.5
Mobile OSes including Android and iOS

11. User Space ASLR vs. Kernel Space ASLR
Shacham et al. showed that user mode ASLR on 32-bit architectures only leaves 16 bit of randomness, which is not enough to defeat brute-force attacks.
However, such brute-force attacks are not applicable for kernel space ASLR.
An attacker wants to exploit a vulnerability in kernel code, a wrong offset will cause a complete crash of the system and thus an attacker has only one attempt to perform an exploit.

12. Windows Kernel Space ASLR
For both 32-bit and 64 bit Vista, Windows 7, and Windows 8.
Obtain the info by reverse-engineering
During boot process, Windows loader allocates kernel_region for the kernel image and the hardware abstraction layer (HAL)
The entropy is 6 bit, 32*2=64=26

13. Outline
Background
Side Channel Attack
Kernel Space ASLR
Use timing-based side channel to defeat kernel space ASLR
How to prevent/mitigate this side channel attack?

14. Timing-based Side Channel Attack
Target at how a local attacker with restricted rights can mount a timing-based side channel attack against the memory management system to deduce information about the privileged kernel space ASLR.

15. Attack Model System is protected by ASLR and
Local adversary can run user-space applications, but cannot execute privileged kernel mode code.

16. Challenges on Exploiting the Attack
Timing attacks against a modern CPU are complicated.
Performance optimizations including hardware prefetching, speculative execution, multi-core architectures, or branch prediction.
Target on modern CPU and new OS
Previous work focused on older processors without such optimization.
Instruction Set Architecture: Defined as part of the processor architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external IO.
Kernel related registers: Task register (TR), Interrupt Descriptor Table Register (IDTR), Global Descriptor Table Register (GDTR), Local Descriptor Table Register (LDTR)

17. TLB and Cache
Translation lookaside buffer: convert virtual address to physical address
Virtually indexed, physically tagged

18. Intel i7 Memory Architecture plus Clock Latency
For Physical address extension (PAE) and 64 bit mode: page map level 4 (PML4) and Page directory point (PDP)

19. General Approach Two steps:
Set the system in a specific state from user mode.
Measure the duration of a certain memory access operation. The time span of this operation then (possibly) reveals certain information about the kernel space layout.
One example:
Cache a system call handler by executing sys call.
Access a designated set of user space addresses and execute the system call again.
If the system call takes longer than expected, the access of user space addresses evicted the system call handler code and reveals address info of system code.

20. Handling Noise Circumvent performance optimizations
E.g., test memory in non-consecutive order to circumvent memory pre- fetching.
OS Scheduler, shared cache
Two steps requires 5000 clock cycles
OS time slides several ms
Guarantee to find cases with No Interrupts between two steps.

21. Three Attacks Attacks on L1/L2/L3 caches and TLB/PS caches
Cache probing
Double page fault
Cache preloading

22. Cache Probing
Based on multiple memory addresses mapped into the same cache set and compete for available slots
n-way set associative mode.
all available slots are grouped into sets of the size n and each memory chunk can be stored in all slots of one particular set.
E.g., in 32-bit address, a typical L3 cache of 8 MB is 16-way set associative. It consists of (8, 192∗1, 024)/64 = 131, 072 single slots that are grouped into 131, 072/16 = 8, 192 different sets.
Reverse-engineering Sandybridge Hash Function for L3 parallel access among multiple cores

23. Double Page Fault Based on the observation on Intel CPUs
When the page translation was successful, but the access permission check fails (e.g., when kernel space is accessed from user mode), a TLB entry is indeed created.
Do not work on AMD CPUs
Can be easily fixed by hardware redesign.
The double page fault timings yield an estimation for the allocation map of the kernel
space, but do not determine at which concrete base addresses
the kernel and drivers are loaded to.

24. Double Page Fault
First page fault: for each kernel space page p, access p from user mode and a page fault will be handled by the operating system and forwarded to the exception handler of the process.
When p refers to an allocated page, the MMU creates a TLB entry for p although the succeeding permission check fails.
When p refers to an unallocated page, since the translation fails, the MMU does not create a TLB entry for p.
Second page fault: accessing p again, if p refers to an allocated kernel page, the page fault will be delivered faster due to the inherent TLB hit.

25. Cache Preloading
Double page fault estimates the allocation map of the kernel space, but do not determine at which concrete base addresses the kernel and drivers are loaded to.
3 steps in Cache Preloading:
flush all caches (i.e., address translation and instruction/data caches) to start with a clean state.
preload the address translation caches by indirectly calling into kernel code, for example by issuing a sysenter operation.
Intentionally generate a page fault by jumping to some kernel space address and measure the time. If the faulting address lies in the same memory range as the preloaded kernel memory  a shorter time

26. Testbed Tested on 32-/64-bit systems running Windows 7/Linux
Tested on different CPUs + VM:
Intel i7-870 (Nehalem/Bloomfield, Quad-Core)
Intel i7-950 (Nehalem/Lynnfield, Quad-Core)
Intel i (Sandybridge, Quad-Core)
AMD Athlon II X3 455 (Triple-Core)
VMWare Player on Intel i7-870 (with VT- x)

27. Results

28. Outline
Background
Side Channel Attack
Kernel Space ASLR
Use timing-based side channel to defeat kernel space ASLR (A heavy engineering work)
How to prevent/mitigate this side channel attack?

29. Mitigation Approaches
Root cause: memory hierarchy is a shared resource
Also, no enough randomization: 64 different slots for windows kernel
Option one: split caches for user and kernel.
Shortcoming: checking overhead, maximum cache size reduced
Option two: suppress the creation of TLB entries on successful address translation if an access violation happens
Option three: Timing normalization; modify the execution time of the page fault handler

30. Future works Test MacOS Test Android
Obtain the physical address of a certain memory location from user mode.

31. Questions?