Alpine Linux and Docker Nautilus Protect Crate Images

Filed under
May 10, 2016

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CC-BY-SA 2.0, Michael Bentley

Crate is a database, a place where you store your most valuable
business assets. For this reason, it is important to detect and
mitigate any vulnerabilities as soon as possible. We value security
highly. Using open source libraries enables us to closely monitor any
security advisories disclosed. While building and packaging Crate, we
only use the latest versions with no known vulnerabilities.

Base Level Security: Signed and Stamped

Since Crate generally plays well with containers, many users like and
download our Docker images. As containerization is a new technology,
many security experts have questioned how to ensure that the code
running inside containers is untampered with and doesn’t include
backdoors or other malware. This is an excellent question to ask, and
we have made significant effort to address it.

We already have an Official
in the
Docker Hub, a curated repository of useful software components that is
actively reviewed by a specialized team at Docker, Inc. To be able to
provide and maintain Official Images, we adhere to certain standards,
answer support questions, and update our image whenever necessary. For
example, Docker’s engineers helped us to improve checking the GPG
signatures of some extra packages we still need to build at the
moment, since not all of Docker’s dependencies are available as
upstream packages from Alpine at the moment.

As a result of the peer review, our Official Image is digitally
signed. You can enforce checking this signature when you use docker
pull crate
with export DOCKER_CONTENT_TRUST=1 enabled. This way you
are guaranteed to download an untampered image.

Nevertheless, we are delighted that Docker takes an extra effort to
detect even more security issues in containers: As part of the Docker
Security Scanning Project, formerly known as
the container
vendor scans each layer of our Official Image and notifies us on any
findings. They check shared libraries and other important artifacts,
calculate a hash value of them and look them up in a known-goods and
known-bads database. If they find a potential vulnerable component,
they list it on their website together with some hints and a CVE

Extra Level: Layers Scanned by Nautilus

This feature has been available to Official Repo partners like us
since DockerCon EU in November 2015. For us as a publisher on Docker
Hub, it is valuable to have this kind of insight. To make our official
crate image a part of the Docker Security Scanning project, we had
to make a number of changes. We switched from the Java 8 base image
(derived from Debian) to version 3.3 of Alpine Linux and installed the
OpenJDK 8 ourselves. Alpine Linux is a recommended foundation of
thoroughly scanned images with Nautilus. When we started, Docker
reported 9 critical and 5 major issues in 5 out of 125 scanned
components. A component is typically an artifact like a shared library
or binary installed by a package manager:

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Nautilus reported issues in 5 out of 125 scanned components. Luckily
all of them had been easy to fix.

All the reported issues came from Debian libraries and Java
packages. Docker lists their
CVE-numbers for
reference. Though we suspect some of them are false positives, it’s
worth verifying them. For example, Nautilus reported two issues with
Oracle Java SE, but the image just contained an OpenJDK package:

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With deep insight from the scan it’s easy to single out vulnerable
libraries and fix them.

The issues were all resolved as soon as we switched to Alpine Linux
and applied a few changes to our publicly maintained Dockerfile on
. We
had to adapt the package names slightly, since we now use the
package manager of Alpine Linux, which feels much like the APT system
we used before, but has different naming conventions. This was easy
because Crate just requires a Java 1.8 environment and some extra
libraries, most available in the APK package manager.

Build Environment for a Special Library

One exception was the Sigar
which we use
to provide platform independent statistics on system load, pick up
disk utilization or network throughput from the operating system. We
feed that information to our
for cluster infrastructure management.

The upstream Sigar code expects to be compiled in a glibc-based build
environment, but Alpine Linux comes with a libc-musl, which claims
to have a lean footprint and is easier to
. Unfortunately the
upstream libsigar doesn’t work with libc-musl out of the box, nor
is there a version of libsigar in the Alpine Linux APK repositories
for version 3.3. Building the library required some small, but tricky
patches, most notably setting up an Ant environment from its source
since that wasn’t available in Alpine 3.3, too.

Thankfully Natanael Copa, an Alpine Linux
maintainer and Docker employee, got in touch to help us patch the
Sigar upstream code and compile it with libc-musl. Currently we do
this in several stages. In lines 18 to 39 we download, verify, and
compile Ant; lines 40 to 42 apply the patch to the upstream libsigar
and lines 43 to 55 build and package the library into Alpines package

The remaining code from line 58 just downloads the platform
independent Crate code, installs Java (for the Crate core) and Python
(for Crash, the Crate shell client) and checks the integrity of all
downloads with a cryptogrphic GPG signature. The Docker folks kindly
pointed out verifying signatures is more secure if we bake the actual
GPG fingerprint into the image instead of just using the key ID.

Two Birds With One Stone

While working on the fixes we discussed with Alpine developers having
actual APK packages for the upcoming Alpine version 3.4. Both Jakub
Jirutka and Natanael Copa agreed to have packages for Ant and Sigar,
respectively. Now we’re prepared and once the Alpine team releases
their new version, we are able to strip our Dockerfile even
more. The result is a 148 MB image down from 181 MB we had before the
switch. More importantly, all previous issues are gone! But, wait!
Once we’ve been down to zero, the nasty CVE-2016-3443 showed up
(together with some of its ugly siblings). Fortunately the Alpine team
is already working on the fix for OpenJDK and we’ll let you know as
soon as possible.

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33 MB leaner and (almost) without issues. The Crate Official Image
with improved security components.

Docker named its security project originally after the Nautilus, an
old marine mollusk that lives in a spiral shell. It has up to ninety
tentacles probing its environment. The shell comprises of a sequence
of confined chambers that relate to the layered components of a Docker
image. As Docker announced the service officially to its users, they
renamed it to Docker Security Scanning.

Switching to Alpine as an base image had several advantages for
us: It has a smaller memory footprint, we benefit from enhanced
security thanks to the Nautilus project and reduced the size of the
image by 33 MB. Pretty amazing for a giant shrimp!

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