A Practical Attack Framework for Precise Enclave Execution Control

SGX-Step is an open-source framework to facilitate side-channel attack research on Intel x86 processors in general and Intel SGX platforms in particular. SGX-Step consists of an adversarial Linux kernel driver and a small user-space operating system library that allows to configure untrusted page table entries and/or x86 APIC timer interrupts completely from user space. SGX-Step has been leveraged in our own research, as well as by independent researchers, to enable 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).

License. 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.

SGX-Step release	Comments
v1.4.0	Privileged interrupt/call gates (Plundervolt).
v1.3.0	Transient-execution support (Foreshadow).
v1.2.0	User-space interrupt handling and deterministic zero-step filtering (Nemesis).
v1.1.0	IA32 support.
v1.0.0	User-space page table manipulation and APIC timer single-stepping.

Publications. SGX-Step has been employed by several independent research groups and has enabled a new line of high-resolution SGX attacks. A full up-to-date list of known projects using SGX-Step is included at the bottom of this README. A copy of the original paper is available here.

@inproceedings{vanbulck2017sgxstep,
    title     = {{SGX-Step}: A Practical Attack Framework for Precise Enclave Execution Control},
    author    = {Van Bulck, Jo and Piessens, Frank and Strackx, Raoul},
    booktitle = {2nd Workshop on System Software for Trusted Execution {(SysTEX)}},
    publisher = {{ACM}},
    pages     = {4:1--4:6},
    month     = Oct,
    year      = 2017,
}

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.

Source code overview

This repository is organized as follows:

.
├── app        -- Collection of sample client applications using SGX-Step to
│                 attack different victim enclave scenarios.
├── doc        -- Papers and reference material.
├── kernel     -- Minimal dynamically loadable Linux kernel driver to export
│                 physical memory to user space and bootstrap `libsgxstep`.
├── libsgxstep -- Small user-space operating system library that implements the
│                 actual SGX-Step functionality, including x86 page-table and
│                 APIC timer manipulations.
└── sdk        -- Bindings to use SGX-Step with different SGX SDKs and libOSs.

Building and running

0. System requirements

SGX-Step requires an SGX-capable Intel processor, and an off-the-shelf Linux kernel. Our original evaluation was performed on i7-6500U/6700 CPUs, running Ubuntu 18.04 with a stock Linux 4.15.0 kernel. More recent Linux kernels and distributions are also supported. 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.
nmi_watchdog=0	Suppress the kernel NMI watchdog.
isolcpus=1	Affinitize the victim process to an isolated CPU core.
nosmap nosmep	Disable Supervisor Mode Access/Execution Prevention (to allow SGX-Step to execute ring-0 IRQ handlers on user pages).
clearcpuid=514	Disable User-Mode Instruction Prevention (on newer CPUs).
pti=off	Disable Kernel Page-Table Isolation (to avoid kernel panics with user IRQ handlers).
rcuupdate.rcu_cpu_stall_suppress=1	Disable the kernel's read-copy update (RCU) CPU stall detector (to avoid warnings when single-stepping for a long time without calling the kernel's timer interrupt handler.)
msr.allow_writes=on	Suppress kernel warning messages for model-specific register (MSR) writes by SGX-Step.
vdso=0	Only on recent Linux kernels: disable vdso_sgx_enter_enclave library (not compatible with AEP interception patches).
dis_ucode_ldr	Optionally disable CPU microcode updates (recent transient-execution attack mitigations may necessitate re-calibrating the single-stepping interval).

Pass the desired boot parameters to the kernel as follows:

$ sudo vim /etc/default/grub
  # Add the following line: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nox2apic iomem=relaxed no_timer_check nosmep nosmap clearcpuid=514 pti=off isolcpus=1 nmi_watchdog=0 rcupdate.rcu_cpu_stall_suppress=1 msr.allow_writes=on vdso=0"
$ sudo update-grub && reboot
  # to inspect the boot parameters of the currently running kernel, execute:
$ cat /proc/cmdline

Finally, to improve overall execution time stability, you may opt to additionally disable C-States and SpeedStep technology in the BIOS configuration.

1. 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/
$ ./install_SGX_driver.sh              # tested on Ubuntu 18.04/20.04/22.04
$ make clean load

Note (/dev/sgx_enclave). SGX-Step supports both the legacy Intel /dev/isgx out-of-tree driver that should work on all platforms, as well as well as the upstream /dev/sgx_enclave driver for platforms with recent Linux kernels >5.11 plus hardware support for flexible-launch control. The install_SGX_driver.sh script should automatically detect whether an in-tree /dev/sgx_enclave driver is available, and, if not, build and load the out-of-tree /dev/isgx driver via the git submodule that points to an unmodified v2.14 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.

2. 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 v2.16 and apply our patches.

$ cd sdk/intel-sdk/
$ ./install_SGX_SDK.sh                 # tested on Ubuntu 18.04/20.04
$ source /opt/intel/sgxsdk/environment # add to ~/.bashrc to preserve across terminal sessions
$ sudo service aesmd status            # stop/start aesmd service if needed

The above install scripts are tested on Ubuntu 18.04 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.

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.

First, check the APIC and interrupt-descriptor table setup:

$ cd app/idt
$ make run    # fires interrupts in an infinite loop to stress-test stability; exit with CTRL-C

For example, to build and run an elementary example application to test page table manipulation features and SDK patches:

$ cd app/aep-redirect
$ make run

To test timer single-stepping functionality, try for example building and running the strlen attack from the paper for a benchmark enclave that processes the secret string 100 repeated times:

$ 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.

Note (stability). In order to avoid the Linux kernel getting stuck or panicking, SGX-Step should automatically restore the interrupt-descriptor table and local APIC timer after exiting the libsgxstep process. You can check if the APIC timer is still firing on all cores as follows:

$ watch -n0.1 "cat /proc/interrupts | grep 'Local timer interrupts'"

Calibrating the single-stepping interval

The table below lists currently supported Intel CPUs, together with their single-stepping APIC timer interval (libsgxstep/config.h). Note that the exact single-stepping interval may depend on the microcode version of the processor when recent transient-execution attack mitigations are in place to flush microarchitectural buffers on enclave entry/exit. Some different microcode versions are provided for reference in the table below.

Model name	CPU	Base frequency	ucode (date)	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
Kaby Lake R	i7-8650U	1.9 GHz	0xca (2019-10-03)	54
Coffee Lake R	i9-9900K	3.6 GHz	?	21
Ice Lake	i5-1035G1	1.00 GHz	0x32 (2019-07-05)	135
Ice Lake	i5-1035G1	1.00 GHz	0xb0 (2022-03-09)	255

Note (calibration). Currently, the easiest way to configure a reliable timer interval is to use the app/bench benchmarking tool with a long NOP slide and gradually increase/decrease SGX_STEP_TIMER_INTERVAL. You can probably start around 20 and then execute NUM=100 make parse to get a summary of single-steps, zero-steps, and multi-steps for a NOP slide of 100 instructions (once you have a more or less stable interval you can switch to longer slides). Too many zero-steps indicate that you have to increase the timer interval, whereas multi-steps demand lowering the timer interval. Btw, don't worry when there's some zero-steps left, as long as you make progress, you can always deterministically filter out zero-steps by looking at the enclave's code PTE accessed bit (which is only set when the instruction actually retires and a single-step occured).

Note (microcode). Another word of caution relates to recent Foreshadow/ZombieLoad/RIDL/etc microcode mitigations that flush leaky uarch buffers on enclave entry/exit. Be aware that when these mitigations are enabled, the timer interval will have to be increased as enclave entry takes longer (e.g., on my i7-8650U CPU I found the single-step timer interval goes up to 54 with recent ucode, from only 34 with pre-Foreshadow ucode). The additional flushing operations may furthermore somewhat increase the variance of enclave entry time, which implies that you might have to configure the timer more conservatively with more zero-steps (which can be deterministically filtered out as explained above).

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 https://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.

The following is a list of known projects that use SGX-Step. Feel free to open a pull request if your project uses SGX-Step but is not included below.

Title	Publication details	Source code	SGX-Step features used
Cache-timing attack against HQC	IACR23	-	Single-stepping, PTE A/D
FaultMorse: An automated controlled-channel attack via longest recurring sequence	ComSec23	link	Page fault
On (the Lack of) Code Confidentiality in Trusted Execution Environments	arXiv22	-	Single-stepping
AEPIC Leak: Architecturally Leaking Uninitialized Data from the Microarchitecture	USEC22	link	Single-Stepping, PTE A/D
MoLE: Mitigation of Side-channel Attacks against SGX via Dynamic Data Location Escape	ACSAC22	-	Single-Stepping, page fault, transient execution
WIP: Interrupt Attack on TEE-Protected Robotic Vehicles	NDSS22	-	Single-stepping, multi-stepping
Towards Self-monitoring Enclaves: Side-Channel Detection Using Performance Counters	NordSec22	-	Page fault, LVI.
ENCLYZER: Automated Analysis of Transient Data Leaks on Intel SGX	SEED22	link	Page-table manipulation
Side-Channeling the Kalyna Key Expansion	CT-RSA22	-	Single-Stepping, PTE A/D
Rapid Prototyping for Microarchitectural Attacks	USENIX22	link	Single-stepping, page fault, PTE A/D, etc.
Util::Lookup: Exploiting Key Decoding in Cryptographic Libraries	CCS21	link	Single-Stepping, PTE A/D
SmashEx: Smashing SGX Enclaves Using Exceptions	CCS21	-	Single-Stepping
Online Template Attacks: Revisited	CHES21	link	Single-stepping, page fault, PTE A/D
Aion Attacks: Manipulating Software Timers in Trusted Execution Environment	DIMVA21	-	Single-stepping, interrupts(?)
Platypus: Software-based Power Side-Channel Attacks on x86	S&P21	link	Single-stepping, zero-stepping
CrossTalk: Speculative Data Leaks Across Cores Are Real	S&P21	-	Single-stepping, page fault
Frontal Attack: Leaking Control-Flow in SGX via the CPU Frontend	USEC21	link	Single-stepping interrupt latency, PTE A/D
SpeechMiner: A Framework for Investigating andMeasuring Speculative Execution Vulnerabilities	NDSS20	link	Page-table manipulation
From A to Z: Projective coordinates leakage in the wild	CHES20	-	Page fault
LVI: Hijacking Transient Execution through Microarchitectural Load Value Injection	S&P20	link	Single-stepping, page-table manipulation
CopyCat: Controlled Instruction-Level Attacks on Enclaves	USEC20	-	Single-stepping, page fault, PTE A/D
When one vulnerable primitive turns viral: Novel single-trace attacks on ECDSA and RSA	CHES20	-	Single-stepping, page fault, PTE A/D
Big Numbers - Big Troubles: Systematically Analyzing Nonce Leakage in (EC)DSA Implementations	USEC20	-	Page fault
Plundervolt: Software-based Fault Injection Attacks against Intel SGX	S&P20	link	Privileged interrupt/call gates, MSR
Bluethunder: A 2-level Directional Predictor Based Side-Channel Attack against SGX	CHES20	-	Single-stepping
Fallout: Leaking Data on Meltdown-resistant CPUs	CCS19	-	PTE A/D
A Tale of Two Worlds: Assessing the Vulnerability of Enclave Shielding Runtimes	CCS19	link	Single-stepping, page fault, PTE A/D
ZombieLoad: Cross-Privilege-Boundary Data Sampling	CCS19	link	Single-stepping, zero-stepping, page-table manipulation
SPOILER: Speculative Load Hazards Boost Rowhammer and Cache Attacks	USEC19	-	Single-stepping interrupt latency
Nemesis: Studying Microarchitectural Timing Leaks in Rudimentary CPU Interrupt Logic	CCS18	link	Single-stepping interrupt latency, page fault, PTE A/D
Foreshadow: Extracting the Keys to the Intel SGX Kingdom with Transient Out-of-Order Execution	USEC18	link	Single-stepping, zero-stepping, page-table manipulation
Single Trace Attack Against RSA Key Generation in Intel SGX SSL	AsiaCCS18	-	Page fault
Off-Limits: Abusing Legacy x86 Memory Segmentation to Spy on Enclaved Execution	ESSoS18	link	Single-stepping, IA32 segmentation, page fault
SGX-Step: A Practical Attack Framework for Precise Enclave Execution Control	SysTEX17	link	Single-stepping, page fault, PTE A/D