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 | Changelog features |
|---|---|
| v1.5.0 | Stabilization fixes (KPTI, kernel ISR mapping); (experimental) Gramine port; IPI support. |
| 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,
}
Demo. The app/memcmp directory contains a small demo application that
illustrates the power of SGX-Step attacks by single-stepping a sample enclave
that contains subtle, non-constant-time memcmp password comparison logic.
As opposed to traditional, notoriously noisy timing attacks, SGX-Step can
deterministically brute-force the password character-per-character in linear
time:
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.
- The local APIC timer interrupt arrives within an enclaved instruction.
- 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.
- At this point, any attack-specific, spy code can easily be plugged in.
- 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 (using Linux's
/proc/self/meminterface and theEDBGRDinstruction). - Thereafter, we configure the local APIC timer for the next interrupt
by writing into the initial-count memory-mapped I/O register, just before
executing (6)
ERESUME.
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.
SGX-Step is a universal execution control framework that enables the precise interleaving of victim enclave instructions with arbitrary attacker code. Some of the main use cases of the SGX-Step framework are summarized in the figure below (see also the bottom of this README for an up-to-date list of publications using SGX-Step).
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. |
clearcpuid=308,295,514 |
Disable supervisor mode access prevention (SMAP, bits 295), supervisor mode execution prevention (SMEP, bits 308) and user-mode instruction prevention (UMIP, bits 514) features. |
pti=off |
Disable Kernel Page-Table Isolation (to avoid kernel panics with user IRQ handlers). |
rcupdate.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:
# if you don't have vim, use nano instead
$ sudo vim /etc/default/grub
# Add the following line: GRUB_CMDLINE_LINUX_DEFAULT="quiet splash nox2apic iomem=relaxed no_timer_check clearcpuid=308,295,514 pti=off isolcpus=1 nmi_watchdog=0 rcupdate.rcu_cpu_stall_suppress=1 msr.allow_writes=on vdso=0"
$ sudo update-grub && rebootTo check that the currently running kernel is configured correctly, execute:
$ sudo ./check_sys.sh
.. Checking recommended SGX-Step parameters [OK]
.. Checking unknown kernel parameters [OK]
.. Checking CPU features [OK]
.. Checking kernel page-table isolation [OK]Finally, to improve overall execution time stability, you may opt to additionally disable C-States and SpeedStep technology in the BIOS configuration.
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 20.04/22.04
$ make clean loadNote (/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.
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 v2.23 and apply our patches.
$ cd sdk/intel-sdk/
$ ./install_SGX_SDK.sh # tested on Ubuntu 20.04/22.04
$ source /opt/intel/sgxsdk/environment # add to ~/.bashrc to preserve across terminal sessions
$ sudo service aesmd status # stop/start aesmd service if neededThe above install scripts are tested on Ubuntu 22.04 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 (outdated and not maintained!) for building 32-bit versions of the SGX SDK and SGX-Step can be found in README-m32.md.
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-CFor example, to build and run an elementary example application to test page table manipulation features and SDK patches:
$ cd app/aep-redirect
$ make runTo 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 executableThe 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 (see also
app/apic for detailed microbenchmarks assessing the accuracy of
various APIC timer modes). Configure a suitable value in /app/bench/main.c.
We established precise timer intervals for our evaluation platforms (see table
below) by tweaking and observing the NOP microbenchmark enclave instruction
pointer trace results, as further outlined below.
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'"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 | i7-9700 | 3 GHz | 0xf4 (2022-07-31) | 26 |
| Coffee Lake R | i9-9900K | 3.6 GHz | ? | 21 |
| Coffee Lake HR | i7-9750H | 2.6 GHz | 0xf4 (2023-02-23) | 37 |
| 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.
Note (filtering out zero-steps). Important: do not worry when there are 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). Thus, after configuring a conservative timer interval that always precludes multi-steps, SGX-Step can achieve noiseless single-stepping at a perfect, instruction-level granularity.
Note (extending the interrupt "landing window").
As clarified in the root-cause analysis below, the slower the page table walk
to resolve the (code) address of the first enclave instruction following
ERESUME, the longer the interrupt "landing window" and, hence, the more
reliable SGX-Step's single-stepping rate will be. For instance, we found that,
on top of clearing the enclave's PMD accessed bit, the landing window can be
even further extended by flushing one or more of the unprotected page-table
entries from the CPU cache before ERESUME, effectively forcing the CPU to
wait for slow memory during the page-table walk. Thus, when you cannot find a
reliable timer-interval configuration, make sure to (i) clear the enclave's
code PTE/PMD "accessed" bit and (ii) flush (CLFLUSH) one or more enclave
page-table entries in the AEP handler.
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).
A detailed root-cause analysis of how exactly SGX-Step
succeeds in reliably interrupting the first (possibly very short!) enclave
instruction following the notoriously complex ERESUME instruction is described in the
AEX-Notify paper.
We found that the key to SGX-Step's success lies in its use of the "accessed"
(A) bit. Specifically, SGX-Step always clears the A-bit in the victim en
clave's page-middle directory (PMD) before arming the APIC to fire a one-shot
interrupt. The A-bit is only ever set by the processor when at least one
instruction is executed by the enclave and can, hence, be used to
deterministically distinguish between zero-steps versus single-steps.
Crucially, as the processor's page-miss handler is optimized for the common
fast path and uses a much slower "microcode assist" to handle the less frequent
and more complex case where a PMD or PTE needs to be modified, this assist has
the effect of prolonging the execution of the first enclave instruction
following ERSUME by several hundreds of cycles. This "assist window" thus
effectively opens a spacious landing space for the coarse-grained, normally
distributed APIC timer interrupt to arrive with high accuracy.
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 aboveHave 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 |
|---|---|---|---|
| TeeJam: Sub-Cache-Line Leakages Strike Back | CHES24 | Github (full) | Single-stepping interrupt latency, PTE A/D |
| AEX-Notify: Thwarting Precise Single-Stepping Attacks through Interrupt Awareness for Intel SGX Enclaves | USEC23 | GitHub (SGX SDK mitigation) | Single-Stepping, PTE A/D |
| BunnyHop: Exploiting the Instruction Prefetcher | USEC23 | GitHub (non-SGX PoC) | Single-stepping, PTE A/D |
| Downfall: Exploiting Speculative Data Gathering | USEC23 | GitHub (non-SGX PoC) | Single-stepping, zero-stepping |
| All Your PC Are Belong to Us: Exploiting Non-control-Transfer Instruction BTB Updates for Dynamic PC Extraction | ISCA23 | - | Single-stepping |
| Cache-timing attack against HQC | CHES23 | - | Single-stepping, PTE A/D |
| FaultMorse: An automated controlled-channel attack via longest recurring sequence | ComSec23 | GitHub (post processing) | 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 | GitHub (full) | 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 | AutoSec22 | - | 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 | GitHub (full) | Page-table manipulation |
| Side-Channeling the Kalyna Key Expansion | CT-RSA22 | - | Single-Stepping, PTE A/D |
| Rapid Prototyping for Microarchitectural Attacks | USENIX22 | GitHub (full) | Single-stepping, page fault, PTE A/D, etc. |
| Util::Lookup: Exploiting Key Decoding in Cryptographic Libraries | CCS21 | GitHub (full) | Single-Stepping, PTE A/D |
| SmashEx: Smashing SGX Enclaves Using Exceptions | CCS21 | - | Single-Stepping |
| Online Template Attacks: Revisited | CHES21 | Zenodo (simulation) | 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 | GitHub (simulated PoC) | 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 | GitHub (full, artifact evaluated) | Single-stepping interrupt latency, PTE A/D |
| PThammer: Cross-User-Kernel-Boundary Rowhammer through Implicit Accesses | - | MICRO20 | Page table walk |
| SpeechMiner: A Framework for Investigating andMeasuring Speculative Execution Vulnerabilities | NDSS20 | GitHub (full) | Page-table manipulation |
| Déjà Vu: Side-Channel Analysis of Mozilla's NSS | CCS20 | - | Page fault |
| From A to Z: Projective coordinates leakage in the wild | CHES20 | - | Page fault |
| LVI: Hijacking Transient Execution through Microarchitectural Load Value Injection | S&P20 | GitHub (PoC) | 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 | GitHub (full) | 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 | GitHub (full) | Single-stepping, page fault, PTE A/D |
| ZombieLoad: Cross-Privilege-Boundary Data Sampling | CCS19 | GitHub (PoC) | 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 | GitHub (full) | 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 | GitHub (PoC) | 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 (full, artifact evaluated) | Single-stepping, IA32 segmentation, page fault |
| SGX-Step: A Practical Attack Framework for Precise Enclave Execution Control | SysTEX17 | GitHub (full) | Single-stepping, page fault, PTE A/D |
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