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* Design doc for securing the Virtio interface

Signed-off-by: Bo Zhang (ACC) <>

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SGX-LKL-OE (Open Enclave Edition)

WARNING: This branch contains an experimental port of SGX-LKL to use Open Enclave as an enclave abstraction layer. This is an ongoing research project. Various features are under development and there are several known bugs.

Build Status

The SGX-LKL project is designed to run existing unmodified Linux binaries inside of Intel SGX enclaves. The goal of the project is to provide the necessary system support for complex applications (e.g., TensorFlow, PyTorch, and OpenVINO) and programming language runtimes (e.g., Python, the DotNet CLR and the JVM). SGX-LKL can run these applications in SGX enclaves without modifications or reliance on the untrusted host OS. Known incompatibilities are documented in

The SGX-LKL project includes several components:

  • A launcher and host interface modelled after a lightweight VM interface. This is documented in
  • A port of Linux to run in this environment, using the Linux Kernel Library (LKL) (
  • A port of the musl standard C library to run on top of this version of Linux.

For frequently asked questions, please see the FAQ.

SGX-LKL uses the Linux Kernel Library (LKL) ( to provide a mature POSIX implementation within an enclave. A modified version of the musl standard C library ( is available to applications inside the enclave.

SGX-LKL supports in-enclave user-level threading, signal handling, and file and network I/O. System calls are handled within the enclave by LKL, and the host is used only for access to I/O resources.

SGX-LKL can be run in hardware mode, when it requires an Intel SGX compatible CPU, and also in software simulation mode, when it runs on any Intel CPU without hardware security guarantees.

A. Installing SGX-LKL-OE

SGX-LKL-OE is distributed as Debian package. This package is alpha quality and not meant for production.

The SGX-LKL-OE package contains the runtime, tools, and all its dependencies and can be run on any Linux distribution.

To use development releases (updated on every commit to master), run:

echo "deb [trusted=yes] ./" | sudo tee /etc/apt/sources.list.d/azure-clc.list

To use stable releases (manually published), run:

echo "deb [trusted=yes] ./" | sudo tee /etc/apt/sources.list.d/azure-clc.list

Now, install with:

sudo apt update
# or: sgx-lkl-nonrelease (-release variant will follow)
sudo apt install sgx-lkl-debug

To make the SGX-LKL commands available from any directory, add an entry to the PATH environment variable:


Finally, setup the host environment by running:


SGX-LKL works most performant with a Linux kernel that has support for userspace FSGSBASE instructions. Otherwise, support for thread local storage (TLS) must use emulated instructions, which reduces performance. SGX-LKL outputs a message on start-up if the currently running Linux kernel does not support FSGSBASE instructions.

FSGSBASE support is not part of the mainline Linux kernel yet. Azure VMs run on Linux kernels with FSGSBASE support based on a proposed Linux kernel patch. To apply the latest patch version to non-Azure systems you may follow the instructions here.

B. Building SGX-LKL-OE from source

SGX-LKL has been tested on Ubuntu Linux 18.04 and with a gcc compiler version of 7.4 or above. Older compiler versions may lead to compilation and/or linking errors.

  1. Install the SGX-LKL build dependencies:
sudo apt-get install make gcc g++ bc python xutils-dev bison flex libgcrypt20-dev libjson-c-dev automake autopoint autoconf pkgconf libtool libcurl4-openssl-dev libprotobuf-dev libprotobuf-c-dev protobuf-compiler protobuf-c-compiler libssl-dev
  1. Clone the SGX-LKL git repository:
git clone --branch oe_port --recursive
cd sgx-lkl
  1. Install the Open Enclave build dependencies:
cd openenclave
sudo scripts/ansible/
sudo ansible-playbook scripts/ansible/oe-contributors-setup.yml

Note that the above also installs the Intel SGX driver on the host.

If running on an Azure Confidential Computing (ACC) VM, which offers SGX support, the last line above should be replaced by:

sudo ansible-playbook scripts/ansible/oe-contributors-acc-setup-no-driver.yml
  1. Build SGX-LKL in the source tree:

DEBUG build (with debug functionality, no compiler optimisations)

To build SGX-LKL with debug symbols and without compiler optimisations, run the following command in the SGX-LKL source tree

make DEBUG=true

Note that, on the first invocation, this initialises all git submodules, including a clone of the LKL library, which downloads several GBs of data.

You will then find the build files under build/.

NON-RELEASE build (no debug symbols, with compiler optimisations)

To build SGX-LKL with compiler optimisations and without debug symbols, run:


RELEASE build (not yet supported by SGX-LKL-OE)

SGX-LKL has a RELEASE build, which make the resulting enclave library secure by removing any insecure debug funcationlity and enforcing security features such as attestestation.

To build SGX-LKL in release mode, run:

    make RELEASE=true
  1. To install SGX-LKL on the host system, use the following command:
sudo -E make install

SGX-LKL is installed under /opt/sgx-lkl by default. To change the install prefix, use PREFIX, e.g.:

make install PREFIX="${PWD}/install"

To uninstall SGX-LKL, run

sudo make uninstall

This removes SGX-LKL specific artefacts from the installation directory as well as cached artefacts of sgx-lkl-disk (stored in ~/.cache/sgxlkl).

  1. To make the SGX-LKL commands available from any directory, add an entry to the PATH environment variable:
  1. Finally, setup the host environment by running:

This has to be done after each reboot. It configures the host networking to forward packets from SGX-LKL instances.

C. Running applications with SGX-LKL

To run applications with SGX-LKL, they must be provided as part of a Linux disk image. Since SGX-LKL is built using the musl libc library, applications must have been dynamically linked against musl. Currently, applications linked against glibc are not supported by SGX-LKL. The simplest way to run applications with SGX-LKL is to use prebuilt binaries for Alpine Linux, which uses musl libc as its default C standard library.

1. Running existing sample applications

The SGX-LKL source tree contains sample applications under 'samples/'. Most sample applications can be run in hardware SGX mode by going to the corresponding directory and execute the following command:

make run-hw

To run an application in software mode without SGX support, execute:

make run-sw

2. Creating SGX-LKL disk images with sgx-lkl-disk

While it is possible to create disk images manually, SGX-LKL comes with a helper tool sgx-lkl-disk. It can be used to create, check, mount, and unmount SGX-LKL disk images.

To see all options, run:

sgx-lkl-disk --help

The tool has been tested on Ubuntu 18.04. sgx-lkl-disk will need superuser rights for some operations, e.g. temporarily mounting/unmounting disk images.

Creating Alpine-based disk images

To create a disk image, use the create action, which expects the disk image size to be specified via --size=<SIZE> and the disk image file name. It also requies the the source of the image.

To build an image with one or more applications available in the Alpine package repository, use the --alpine=<pkgs> flag. The following example creates an image with Redis installed:

sgx-lkl-disk create --size=50M --alpine="redis" sgxlkl-disk.img

Redis can then be run as follows:

SGXLKL_TAP=sgxlkl_tap0 sgx-lkl-run-oe --hw-debug ./sgxlkl-disk.img /usr/bin/redis-server --bind

To create and run a disk image with Memcached, execute:

sgx-lkl-disk create --size=50M --alpine="memcached" sgxlkl-disk.img
SGXLKL_TAP=sgxlkl_tap0 sgx-lkl-run-oe --hw-debug ./sgxlkl-disk.img /usr/bin/memcached --listen= -u root --extended=no_drop_privileges -vv

If you need to add extra data to the disk image, the parameter --copy=<path> can be used to copy files from the host to the disk image. The following example creates a disk image with the Alpine Python package together with a custom Python application:

# When --copy points to a directory, the contents of the directory are copied
# to the root of the file system.
tree my-python-root
> my-python-root
> ├── app
> │   ├──
> │   └──

sgx-lkl-disk create --size=100M --alpine="python" --copy=./my-python-root sgxlkl-disk.img
# Run with
sgx-lkl-run-oe --hw-debug ./sgxlkl-disk.img /usr/bin/python /app/

Creating Docker-based disk images

The sgx-lkl-disk tool can also build disk images from Dockerfiles with the --docker flag, e.g. when an application needs to be compiled manually. Note that SGX-LKL applications still need to be linked against musl libc, so a good starting point is an Alpine Docker base image.

To build an SGX-LKL disk image from a Dockerfile, run:

sgx-lkl-disk create --size=100M --docker=MyDockerfile sgxlkl-disk.img

Creating plain disk images

If all that is needed is a plain disk image based on files existing on the host, the --copy flag can be used on its own:

sgx-lkl-disk create --size=50M --copy=./my-root sgxlkl-disk.img

Disk encryption

SGX-LKL supports disk encryption via the dm-crypt subsystem in the Linux kernel. Typically encryption for a disk can be setup via the cryptsetup tool. The sgx-lkl-disk tool provides an --encrypt option to simplify this process. To create an encrypted disk image with default options run:

sgx-lkl-disk create --size=50M --encrypt --key-file --alpine="" sgxlkl-disk.img.enc
# Run with
SGXLKL_HD_KEY=./sgxlkl-disk.img.enc.key sgx-lkl-run-oe --hw-debug ./sgxlkl-disk.img.enc /bin/echo "Hello World"

In this example, sgx-lkl-disk automatically generates a 512-byte key file, uses "AES-XTS Plain 64" as a cipher/mode and "SHA256" for hashing. The cipher and hash algorithm is stored as metadata in a LUKS header on disk. The tool provides a number of options to customise this (see sgx-lkl-disk --help for more information).

Disk integrity protection

To provide disk/data integrity, SGX-LKL supports both dm-verity (read-only) and dm-integrity (read/write). These can be combined with disk encryption (dm-integrity can currently only be used together with --encrypt).

To create a read-only encrypted disk image with integrity protection via dm-verity, run:

sgx-lkl-disk create --size=50M --encrypt --key-file --verity --alpine="" sgxlkl-disk.img.enc.vrt
# Run with
SGXLKL_HD_VERITY=./sgxlkl-disk.img.enc.vrt.roothash SGXLKL_HD_KEY=./sgxlkl-disk.img.enc.vrt.key sgx-lkl-run-oe ./sgxlkl-disk.img.enc.vrt /bin/echo "Hello World"

To create an encrypted and integrity-protected disk that uses HMAC-SHA256 for authenticated encryption and supports both reads and writes, run:

# --integrity requires a host kernel version 4.12 or greater and cryptsetup version 2.0.0 or greater
sgx-lkl-disk create --size=50M --encrypt --key-file --integrity --alpine=""
# Run with
SGXLKL_HD_KEY=./ sgx-lkl-run-oe ./ /bin/echo "Hello World"

sgx-lkl-disk relies on cryptsetup for setting up encryption and integrity protection. For more information on cryptsetup and dm-crypt/dm-verity/dm-integrity, see

3. Running applications from the Alpine Linux repository

Alpine Linux uses musl as its standard C library. SGX-LKL supports a large number of unmodified binaries available through the Alpine Linux repository. For an example on how to create the corresponding disk image and how to run the application, samples/miniroot can be used as a template.

Build the disk image by running:


This creates an Alpine mini root disk image that can be passed to sgx-lkl-run-oe. can be modified to specify APKs that should be part of the disk image. After creating the disk image, applications can be run on top of SGX-LKL using sgx-lkl-run-oe. Using Redis as an example (the APK redis is listed in the example file in samples/miniroot), redis-server can be launched as follows:

SGXLKL_TAP=sgxlkl_tap0 sgx-lkl-run-oe --hw-debug ./sgxlkl-miniroot-fs.img /usr/bin/redis-server --bind

The readme file in samples/miniroot contains more detailed information on how to build custom disk images manually.

4. OpenJDK Java Virtual Machine (JVM)

A simple Java HelloWorld example application is available in samples/jvm/helloworld-java. Building the example requires curl and a Java 8 compiler on the host system. On Ubuntu, install these by running:

sudo apt-get install curl openjdk-8-jdk

To build the disk image, run:

cd samples/jvm/helloworld-java

This compiles the HelloWorld Java example, create a disk image with an Alpine mini root environment, add a JVM, and add the HelloWorld.class file.

To run the HelloWorld java program on top of SGX-LKL inside an enclave, run"

sgx-lkl-java ./sgxlkl-java-fs.img HelloWorld

The command sgx-lkl-java is a simple wrapper around sgx-lkl-run-oe, which sets some common JVM arguments in order to reduce its memory footprint. It can be found in the tools/ directory. For more complex applications, SGX-LKL or JVM arguments may have to be adjusted, e.g. to increase the size of the JVM heap/metaspace/code cache, or to enable networking support by providing a TAP/TUN interface via SGXLKL_TAP.

If the application runs successfully, you should see an output like this:

OpenJDK 64-Bit Server VM warning: Can't detect initial thread stack location - find_vma failed
Hello world!

The warning is caused by the fact that the JVM is trying to receive information about the process's virtual memory regions from /proc/self/maps. While SGX-LKL generally supports the /proc file system in-enclave, /proc/self/maps is currently not populated by SGX-LKL. This does not affect the functionality of the JVM.

5. Cross-compiling applications for SGX-LKL

For applications with a complex build process and/or a larger set of dependencies, it is easiest to use the unmodified binaries from the Alpine Linux repository as described in the previous section. However, it is also possible to cross-compile applications on non-musl based Linux distributions (e.g. Ubuntu) and create a minimal disk image that only contains the application and its dependencies. An example of how to cross-compile a C application and create the corresponding disk image can be found in samples/helloworld. To build the disk image and execute the application with SGX-LKL run:

make sgxlkl-disk.img
sgx-lkl-run-oe --hw-debug sgxlkl-disk.img /app/helloworld

Run the following command in samples/miniroot to see a number of other applications you should be able to execute. Keep in mind that SGX-LKL currently does not support the fork() system call, so multi-process applications will not work.

sgx-lkl-run-oe --hw-debug ./sgxlkl-miniroot-fs.img /bin/ls /usr/bin

E. Configuring SGX-LKL-OE parameters

1. Enclave size

To be added

2. Enclave signing

To be added

3. Other configuration options

SGX-LKL-OE has a number of other configuration options e.g. for configuring the in-enclave scheduling, network configuration, or debugging/tracing. To see all options, run:

sgx-lkl-run-oe --help

Note that for the debugging options to have an effect, SGX-LKL must be built with DEBUG=true.

F. Remote attestation

To be added

G. Debugging SGX-LKL-OE and applications

See the Debugging page for details.