gsc – Gramine Shielded Containers

Synopsis

gsc COMMAND [OPTIONS] …

Description

Docker containers are widely used to deploy applications in the cloud. Using Gramine Shielded Containers (GSC) we provide the infrastructure to deploy Docker containers protected by Intel SGX enclaves using the Gramine Library OS.

The gsc tool transforms a Docker image into a new image (called gsc-<image-name>) which includes the Gramine Library OS, manifest files, Intel SGX related information, and executes the application inside an Intel SGX enclave using the Gramine Library OS. It follows the common Docker approach to first build an image and subsequently run a container of an image. At first a Docker image has to be graminized via the gsc build command. When the graminized image should run within an Intel SGX enclave, the image has to be signed via a gsc sign-image command. Subsequently, the image can be run using docker run.

NOTE: As part of the gsc build step, GSC generates the manifest file with a list of trusted files (files with integrity protection). This list contains hashes of all files present in the original Docker image. Therefore, GSC’s manifest creation capability depends on packaging of the original Docker image: if the original Docker image is bloated (contains unnecessary files), then the generated manifest will also be bloated. Though this doesn’t worsen security guarantees of Gramine/GSC, it may affect startup performance. Please exercise care in pulling in only the dependencies truly required for your Docker image.

Prerequisites

The installation descriptions of prerequisites are for Ubuntu 18.04 and may differ when using a different Ubuntu version or Linux distribution.

Software packages

Please install the docker.io, python3, python3-pip packages. In addition, install the Docker client, Jinja2, TOML, and YAML python packages via pip. GSC requires Python 3.6 or later.

sudo apt-get install docker.io python3 python3-pip
pip3 install docker jinja2 tomli tomli-w pyyaml
pip3 install toml  # for compatibility with Gramine v1.3 or lower

SGX software stack

To run with Intel SGX, please install the corresponding software stack as described in https://gramine.readthedocs.io/en/stable/devel/building.html.

Host configuration

To create Docker images, the user must have access to Docker daemon.

Warning

Please use this step with caution. By granting the user access to the Docker group, the user may acquire root privileges via docker run.

sudo adduser $USER docker

Create a configuration file called config.yaml or specify a different configuration file via gsc option. Please see the documentation on configuration options below and use the config.yaml.template as reference.

Command line arguments

--help

Display usage.

gsc build – build graminized image

Builds an unsigned graminized Docker image of an application image called gsc-<IMAGE-NAME>-unsigned by compiling Gramine or relying on a prebuilt Gramine image.

gsc build [OPTIONS] <IMAGE-NAME> <APP.MANIFEST>

-b <buildtype>, --buildtype <buildtype>

Use <buildtype> value release, debug or debugoptimized to compile Gramine in the corresponding mode. Default: release.

-d, --debug

Compile Gramine with debug flags and debug output.

Deprecated since version 1.5: Use --buildtype.

-L

Compile Gramine with Linux PAL in addition to Linux-SGX PAL. If configured to use a prebuilt Gramine image, the image has to support this option.

--insecure-args

Allow untrusted arguments to be specified at docker run. Otherwise any arguments specified during docker run are ignored.

--no-cache

Disable Docker’s caches during gsc build. This builds the unsigned graminized image from scratch.

--rm

Remove intermediate Docker images created by gsc build, if the image build is successful.

--build-arg

Set build-time variables during gsc build (same as docker build –build-arg).

-c

Specify configuration file. Default: config.yaml.

IMAGE-NAME

Name of the application Docker image.

APP.MANIFEST

Manifest file (Gramine configuration).

gsc sign-image – signs a graminized image

Signs the enclave of an unsigned graminized Docker image and creates a new Docker image called gsc-<IMAGE-NAME>. gsc sign-image always removes intermediate Docker images, if successful or not, to ensure the removal of the signing key in them.

gsc sign-image [OPTIONS] <IMAGE-NAME> <KEY-FILE>

-c

Specify configuration file. Default: config.yaml

-p

Provide passphrase for the enclave signing key (if applicable)

-D, --define

Set image sign-time variables during gsc sign.

--remove-gramine-deps

Remove Gramine dependencies that are not needed at runtime. This may have a negative side effect of removing even those dependencies that are actually needed by the original application. Use with care!

IMAGE-NAME

Name of the application Docker image

KEY-FILE

Used to sign the Intel SGX enclave

gsc build-gramine – build Gramine-only Docker image

Builds a base Docker image including the Gramine sources and compiled runtime. This base image can be used as input for gsc build via configuration parameter Gramine.Image.

gsc build-gramine [OPTIONS] <IMAGE-NAME>

-b <buildtype>, --buildtype <buildtype>

Use <buildtype> value release, debug or debugoptimized to compile Gramine in the corresponding mode. Default: release.

-d, --debug

Compile Gramine with debug flags and debug output.

Deprecated since version 1.5: Use --buildtype.

-L

Compile Gramine with Linux PAL in addition to Linux-SGX PAL. Allows gsc build commands to include the Linux PAL using -L.

--no-cache

Disable Docker’s caches during gsc build-gramine. This builds the unsigned graminized image from scratch.

--rm

Remove intermediate Docker images created by gsc build-gramine, if the image build is successful.

--build-arg

Set build-time variables during gsc build-gramine (same as docker build –build-arg).

-c

Specify configuration file. Default: config.yaml

-f

Stop after Dockerfile is created and do not build the Docker image.

IMAGE-NAME

Name of the resulting Gramine Docker image

gsc info-image – retrieve information about graminized Docker image

Retrieves Intel SGX relevant information about the graminized Docker image such as the MRENCLAVE and MRSIGNER measurements for each application in the Docker image.

Synopsis:

gsc info-image <IMAGE-NAME>

IMAGE-NAME

Name of the graminized Docker image

Using Gramine’s trusted command line arguments

Most executables aren’t designed to run with attacker-controlled arguments. Allowing an attacker to control executable arguments can break the security of the resulting enclave.

gsc build uses the existing Docker image’s entrypoint and cmd fields to identify the trusted arguments. These arguments are stored in trusted_argv. This file is only generated when --insecure-args is not specified. As a result any arguments specified during docker run are ignored.

To be able to provide arguments at runtime, the image build has to enable this via the option --insecure-args.

Stages of building graminized SGX Docker images

The build process of a graminized Docker image from image <image-name> follows three main stages and produces an image named gsc-<image-name>. gsc build-gramine performs only the first stage, gsc build performs the first two stages, and finally gsc sign-image performs the last stage.

1. Building Gramine. The first stage builds Gramine from sources based on the provided configuration (see config.yaml) which includes the distribution (e.g., Ubuntu 18.04), Gramine repository, and the Intel SGX driver details. This stage can be skipped if gsc build uses a pre-built Gramine Docker image.
2. Graminizing the application image. The second stage copies the important Gramine artifacts (e.g., the runtime and signer tool) from the first stage (or if the first stage was skipped, it pulls a prebuilt Docker image defined via the configuration file). It then prepares image-specific variables such as the executable path and the library path, and scans the entire image to generate a list of trusted files. GSC excludes files and paths starting with /boot, /dev, .dockerenv, .dockerinit, /etc/mtab, /etc/rc, /proc, /sys, and /var, since checksums are required which either don’t exist or may vary across different deployment machines. GSC combines these variables and list of trusted files into a new manifest file. In a last step the entrypoint is changed to launch the apploader.sh script which generates an Intel SGX token (only if needed, on non-FLC platforms) and starts the gramine-sgx loader. Note that the generated image (gsc-<image-name>-unsigned) cannot successfully load an Intel SGX enclave, since essential files and the signature of the enclave are still missing (see next stage).
3. Signing the Intel SGX enclave. The third stage uses Gramine’s signer tool to generate SIGSTRUCT files for SGX enclave initialization. This tool also generates an SGX-specific manifest file. The required signing key is provided by the user via the gsc sign-image command and copied into this Docker build stage. The generated image is called gsc-<image-name> and includes all necessary files to start an Intel SGX enclave.

In the future we plan to provide prebuilt Gramine images for popular cloud-provider offerings.

Generating a signed graminized Docker image

The last stage combines the graminized Docker image with the signed enclave and manifest files. Therefore it copies the SIGSTRUCT files and the SGX-specific manifest file from the previous stage into the graminized Docker image from the second stage.

Configuration

GSC is configured via a configuration file called config.yaml or specified as a gsc option. A template configuration file is provided in config.yaml.template.

Distro

Defines Linux distribution to be used to build Gramine in. This distro should match the distro underlying the application’s Docker image; otherwise the results may be unpredictable. Currently supported distros are Ubuntu 18.04, Ubuntu 20.04, Ubuntu 21.04, Debian 10, Debian 11 and CentOS 8. Default value is ubuntu:18.04.

Registry

Defines the registry and repository where the Linux distribution image is located. Only needed if the image in Distro requires to be prepended with this information.

Gramine.Repository

Source repository of Gramine. Default value: https://github.com/gramineproject/gramine.git.

Gramine.Branch

Use this release/branch of the repository. Default value: v1.4.

Gramine.Image

Builds graminized Docker image based on a prebuilt Gramine Docker image. These images are prepared via gsc build-gramine and will be provided for popular cloud-provider environments. Gramine.Repository and Gramine.Branch are ignored in case Gramine.Image is specified.

SGXDriver.Repository

Source repository of the Intel SGX driver. Default value: “” (in-kernel driver).

SGXDriver.Branch

Use this branch of the repository. Default value: “” (in-kernel driver).

Run graminized Docker images

Execute docker run command via Docker CLI and provide gsgx and isgx/sgx devices and the PSW/AESM socket. Additional Docker options and executable arguments may be supplied to the docker run command.

Warning

Forwarding devices to a container lowers security of the host. GSC should never be used as a sandbox for applications (i.e. it only shields the app from the host but not vice versa).

docker run [OPTIONS] gsc-<IMAGE-NAME> [<ARGUMENTS>]

OPTIONS

docker run options. Common options include -it (interactive with terminal), -d (detached), --device (forward device). Please see Docker manual for details.

IMAGE-NAME

Name of original image (without GSC build).

ARGUMENTS

Arguments to be supplied to the executable launching inside the Docker container and Gramine. Such arguments may only be provided when --insecure-args was specified during gsc build.

Execute with Linux PAL instead of Linux-SGX PAL

When specifying -L during GSC gsc build, you may select the Linux PAL at Docker run time instead of the Linux-SGX PAL by specifying the environment variable GSC_PAL as an option to the docker run command. When using the Linux PAL, it is not necessary to sign the image via a gsc sign-image command.

GSC_PAL

This environment variable specifies the pal loader.

GSC requires a custom seccomp profile while running with Linux PAL, which has to be specified at Docker run time. There are two options:

1. Pass unconfined to run the container without the default seccomp profile. This option is generally considered insecure, since this results in containers running with unrestricted system calls (all system calls are allowed which increases the attack surface of the Linux Kernel).

2. Pass the custom seccomp profile https://github.com/gramineproject/gramine/blob/master/scripts/docker_seccomp.json.

   With this option, Docker containers restrict themselves to a rather narrow set of allowed system calls, keeping the attack surface of the Linux kernel small. All the necessary capabilities required for GSC to function are still enabled.

docker run ... --env GSC_PAL=Linux --security-opt seccomp=<profile> gsc-<image-name> ...

Example

Warning

Example below relies on insecure arguments to be able to run Python with arbitrary arguments. This is not intended for production environments.

The example below shows how to graminize the public Docker image of Python3. This example assumes that all prerequisites are installed and configured.

1. Create a configuration file:

   cp config.yaml.template config.yaml
   # Manually adopt config.yaml to the installed Intel SGX driver and desired
   # Gramine repository/version.
   

2. Generate the signing key (if you don’t already have a key):

   openssl genrsa -3 -out enclave-key.pem 3072
   

3. Pull public Python image from Dockerhub:

   docker pull python
   

4. Graminize the Python image using gsc build:

   ./gsc build --insecure-args python test/generic.manifest
   

5. Sign the graminized Docker image using gsc sign-image:

   ./gsc sign-image python enclave-key.pem
   

6. Retrieve SGX-related information from graminized image using gsc info-image:

   ./gsc info-image gsc-python
   

7. Test the graminized Docker image (change --device=/dev/sgx_enclave to your version of the Intel SGX driver if needed):

   docker run --device=/dev/sgx_enclave \
      -v /var/run/aesmd/aesm.socket:/var/run/aesmd/aesm.socket \
      gsc-python -c 'print("HelloWorld!")'
   

8. You can also start a Bash interactive session in the graminized Docker image (useful for debugging):

   docker run --device=/dev/sgx_enclave \
      -v /var/run/aesmd/aesm.socket:/var/run/aesmd/aesm.socket \
      -it --entrypoint /bin/bash gsc-python
   

Limitations

This document focuses on the most important limitations of GSC. Issue #13 provides the complete list of known limitations and serves as a discussion board for workarounds.

Operating System dependency

GSC relies on Gramine to execute Linux applications inside Intel SGX enclaves and the installation of prerequisites depends on package manager and package repositories. Docker images based on Ubuntu and CentOS are supported by GSC. GSC can simply be extended to support other distributions by providing a set of templates for this distribution in templates/.

Trusted data in Docker volumes

Data mounted as Docker volumes at runtime is not included in the general search for trusted files during the image build. As a result, Gramine denies access to these files, since they are neither allowed nor trusted files. This will likely break applications using files stored in Docker volumes.

Workaround

Trusted files can be added to image-specific manifest file (first argument to gsc build command) at build time. This workaround does not allow these files to change between build and run, or over multiple runs. This only provides integrity for files and not confidentiality.

Allowing dynamic file contents via Gramine protected files

Docker volumes can include Gramine protected files. As a result Gramine can open these protected files without knowing the exact contents as long as the protected file was configured in the manifest. The complete and secure use of protected files may require additional steps.

Integration of Docker Secrets

Docker Secrets are automatically pulled by Docker and the results are stored either in environment variables or mounted as files. GSC is currently unaware of such files and hence, cannot mark them trusted. Similar to trusted data, these files may be added to the manifest.

Access to files in excluded paths

The manifest generation excludes all files and paths starting with /boot , /dev, .dockerenv, .dockerinit, /etc/mtab, /etc/rc, /proc, /sys, and /var from the list of trusted files. If your application relies on some files in these directories, you must manually add them to the manifest:

sgx.trusted_files = [ "file:file1", "file:file2" ]
or
sgx.allowed_files = [ "file:file3", "file:file4" ]

Issues with hostname and DNS

If your application queries the hostname or DNS information, you must manually add the following option to the manifest:

sys.enable_extra_runtime_domain_names_conf = true

For more information on this option, refer to https://gramine.readthedocs.io/en/stable/manifest-syntax.html#domain-names-configuration.