A Practical Attack Framework for Precise Enclave Execution Control

This repository contains the source code of the SGX interrupt attack framework accompanying our SysTEX 2017 paper.

Jo Van Bulck, Frank Piessens, and Raoul Strackx. 2017. SGX-Step: A Practical Attack Framework for Precise Enclave Execution Control. In Proceedings of the 2nd Workshop on System Software for Trusted Execution (SysTEX '17).

SGX-Step is free software, licensed under GPLv3. The SGX-Step logo is derived from Eadweard Muybridge's iconic public domain "Sallie Gardner at a Gallop" photo series, which, like our enclave single-stepping goal, breaks down the galloping horse dynamics into a series of individual photo frames to reveal overall horse gait properties.

Abstract

Protected module architectures such as Intel SGX hold the promise of protecting sensitive computations from a potentially compromised operating system. Recent research convincingly demonstrated, however, that SGX's strengthened adversary model also gives rise to to a new class of powerful, low-noise side-channel attacks leveraging first-rate control over hardware. These attacks commonly rely on frequent enclave preemptions to obtain fine-grained side-channel observations. A maximal temporal resolution is achieved when the victim state is measured after every instruction. Current state-of-the-art enclave execution control schemes, however, do not generally achieve such instruction-level granularity.

This paper presents SGX-Step, an open-source Linux kernel framework that allows an untrusted host process to configure APIC timer interrupts and track page table entries directly from user space. We contribute and evaluate an improved approach to single-step enclaved execution at instruction-level granularity, and we show how SGX-Step enables several new or improved attacks. Finally, we discuss its implications for the design of effective defense mechanisms.

Overview

Crucial to the design of SGX-Step, as opposed to previous enclave preemption proposals, is the creation of user-space virtual memory mappings for physical memory locations holding page table entries, as well as for the local x86 APIC memory-mapped I/O configuration registers. This allows an untrusted, attacker-controlled host process to easily (i) configure the APIC timer one-shot/periodic interrupt source, (ii) trigger inter-processor interrupts, and (iii) track or modify enclave page table entries directly from user-space.

The above figure summarizes the sequence of hardware and software steps when interrupting and resuming an SGX enclave through our framework.

1. The local APIC timer interrupt arrives within an enclaved instruction.
2. The processor executes the AEX procedure that securely stores execution context in the enclave’s SSA frame, initializes CPU registers, and vectors to the kernel-level interrupt handler.
3. Our /dev/sgx-step loadable kernel module registered itself in the APIC event call back list to make sure it is called on every timer interrupt. At this point, any attack-specific, kernel-level spy code can easily be plugged in. Furthermore, to enable precise evaluation of our approach on attacker-controlled debug enclaves, SGX-Step can optionally be instrumented to retrieve the stored instruction pointer from the interrupted enclave’s SSA frame using the EDBGRD instruction.
4. The kernel returns to the user space AEP trampoline. We modified the untrusted runtime of the official SGX SDK to allow easy registration of a custom AEP stub.
5. At this point, any attack-specific user mode spy code can again easily be run, before the single-stepping adversary configures the APIC timer for the next interrupt, just before executing (6) ERESUME.

Building and Running

0. System Requirements

SGX-Step requires an SGX-capable Intel processor, and an off-the-shelf Linux kernel. Our evaluation was performed on i7-6500U/6700 CPUs, running unmodified Linux versions 4.2.0/4.4.0. To make use of SGX-Step's single-stepping features, the local APIC device needs to be configured in memory-mapped xAPIC mode. The easiest way to do this is to pass the nox2apic Linux kernel parameter at boot time. We furthermore advise passing the iomem=relaxed, no_timer_check, and isolcpus parameters to respectively avoid too many warning messages in the kernel logs and affinitize the victim process to an isolated CPU core.

$ sudo vim /etc/default/grub
  # GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nox2apic iomem=relaxed no_timer_check isolcpus=1"
$ sudo update-grub && sudo reboot

Finally, in order to reproduce our experimental results, make sure to disable C-States and SpeedStep technology in the BIOS configuration. The table below lists currently supported Intel CPUs, together with their single-stepping APIC timer interval.

Model name	CPU	Base frequency	APIC timer interval
Dell Latitude 7490	i7-8650U	1.9 GHz	36
Dell Inspiron 13 7359	i7-6500U	2.5 GHz	25
Dell Optiplex 7040	i7-6700	3.4 GHz	19
Acer Aspire V15	i5-6200U	2.3 GHz	28

1. Patch and install SGX SDK

To enable easy registration of a custom Asynchronous Exit Pointer (AEP) stub, we modified the untrusted runtime of the official Intel SGX SDK. Proceed as follows to checkout linux-sgx v2.1.1 and apply our patches.

$ git submodule init
$ git submodule update
$ ./patch_sdk.sh

Now, follow the instructions in the linux-sgx project to build and install the Intel SGX SDK and PSW packages. You will also need to build and load an (unmodified) linux-sgx-driver SGX kernel module in order to use SGX-Step.

Note (local installation). The patched SGX SDK and PSW packages can be installed locally, without affecting a compatible system-wide 'linux-sgx' installation. For this, the example Makefiles support an SGX_SDK environment variable that points to the local SDK installation directory. When detecting a non-default SDK path (i.e., not /opt/intel/sgxsdk), the "run" Makefile targets furthermore dynamically link against the patched libsgx_urts.so untrusted runtime built in the local linux-sgx directory (using the LD_LIBRARY_PATH environment variable).

Note (32-bit support). Instructions for building 32-bit versions of the SGX SDK and sgx-step can be found in README-m32.md.

2. Build and load /dev/sgx-step

SGX-Step comes with a loadable kernel module that exports an IOCTL interface to the libsgxstep user-space library. The driver is mainly responsible for (i) hooking the APIC timer interrupt handler, (ii) collecting untrusted page table mappings, and optionally (iii) fetching the interrupted instruction pointer for benchmark enclaves.

To build and load the /dev/sgx-step driver, execute:

$ cd kernel
$ make clean load

Note (/dev/isgx). Our driver uses some internal symbols and data structures from the official Intel /dev/isgx driver. We therefore include a git submodule that points to an unmodified v2.1 linux-sgx-driver.

Note (/dev/mem). We rely on Linux's virtual /dev/mem device to construct user-level virtual memory mappings for APIC physical memory-mapped I/O registers and page table entries of interest. Recent Linux distributions typically enable the CONFIG_STRICT_DEVMEM option which prevents such use, however. Our /dev/sgx-step driver therefore includes an approach to bypass devmem_is_allowed checks, without having to recompile the kernel.

3. Build and run test applications

User-space applications can link to the libsgxstep library to make use of SGX-Step's single-stepping and page table manipulation features. Have a look at the example applications in the "app" directory.

For example, to build and run the strlen attack from the paper for a benchmark enclave that processes the secret string 100 repeated times, execute:

$ cd app/bench
$ NUM=100 STRLEN=1 make parse   # alternatively vary NUM and use BENCH=1 or ZIGZAG=1
$ # (above command defaults to the Dell Inspiron 13 7359 evaluation laptop machine;
$ # use DESKTOP=1 to build for a Dell Optiplex 7040 machine)
$ # use SGX_SDK=/home/jo/sgxsdk/ for a local SDK installation
$ # use M32=1 To produce a 32-bit executable

The above command builds libsgxstep, the benchmark victim enclave, and the untrusted attacker host process, where the attack scenario and instance size are configured via the corresponding environment variables. The same command also runs the resulting binary non-interactively (to ensure deterministic timer intervals), and finally calls an attack-specific post-processing Python script to parse the resulting enclave instruction pointer benchmark results.

Note (performance). Single-stepping enclaved execution incurs a substantial slowdown. We measured execution times of up to 15 minutes for the experiments described in the paper. SGX-Step's page table manipulation features allow to initiate single-stepping for selected functions only, for instance by revoking access rights on specific code or data pages of interest.

Note (timer interval). The exact timer interval value depends on CPU frequency, and hence remains inherently platform-specific. Configure a suitable value in /app/bench/main.c. We established precise timer intervals for our evaluation platforms (see table above) by tweaking and observing the NOP microbenchmark enclave instruction pointer trace results.

Using SGX-Step in your own projects

The easiest way to get started using the SGX-Step framwork in your own projects, is through git submodules:

$ cd my/git/project
$ git submodule add git@github.com:jovanbulck/sgx-step.git
$ cd sgx-step # Now build `/dev/sgx-step` and `libsgxstep` as described above

Have a look at the Makefiles in the app directory to see how a client application can link to libsgxstep plus any local SGX SDK/PSW packages.