Sgx-Step - A Practical Attack Framework For Precise Enclave Execution Control

SGX-Step is an open-source framework to facilitate side-channel attack research on Intel SGX platforms. SGX-Step consists of an adversarial Linux kernel driver and user space library that allow to configure untrusted page table entries and/or x86 APIC timer interrupts completely from user space. Our research results have demonstrated several new and improved enclaved execution attacks that gather side-channel observations at a maximal temporal resolution (i.e., by interrupting the victim enclave after every single instruction).

Abstract
Trusted execution environments 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.

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).

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 APIC memory-mapped I/O configuration registers and the x86 Interrupt Descriptor Table (IDT). This allows an untrusted, attacker-controlled host process to easily (i) track or modify enclave page table entries, (ii) configure the APIC timer one-shot/periodic interrupt source, (iii) trigger inter-processor interrupts, and (iv) register custom interrupt handlers completely within 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 (user space) interrupt handler registered in the IDT.
3. At this point, any attack-specific, spy code can easily be plugged in.
4. The library 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. Furthermore, to enable precise evaluation of our approach on attacker-controlled benchmark debug enclaves, SGX-Step can optionally be instrumented to retrieve the stored instruction pointer from the interrupted enclave’s SSA frame. For this, our /dev/sgx-step driver offers an optional IOCTL call for the privileged EDBGRD instruction.
5. Thereafter, we configure the local APIC timer for the next interrupt by writing into the initial-count MMIO register, 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 Ubuntu 16.04 with a stock Linux 4.15.0 kernel. We summarize Linux kernel parameters below.

Linux kernel parameter	Motivation
nox2apic	Configure local APIC device in memory-mapped I/O mode (to make use of SGX-Step's precise single-stepping features).
iomem=relaxed, no_timer_check	Suppress unneeded warning messages in the kernel logs.
isolcpus=1	Affinitize the victim process to an isolated CPU core.
dis_ucode_ldr	Disable CPU microcode updates (Foreshadow/L1TF mitigations may affect single-stepping interval).

Pass the desired boot parameters to the kernel as follows:
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 (libsgxstep/config.h).

Model name	CPU	Base frequency	APIC timer interval
Skylake	i7-6700	3.4 GHz	19
Skylake	i7-6500U	2.5 GHz	25
Skylake	i5-6200U	2.3 GHz	28
Kaby Lake R	i7-8650U	1.9 GHz	34
Coffee Lake R	i9-9900K	3.6 GHz	21

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 v2.6 and apply our patches.
The above install scripts are tested on Ubuntu 16.04 LTS. For other GNU/Linux distributions, please 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:
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:
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:
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.