Many container runtime tools like systemd-nspawn, docker, etc. focus on providing infrastructure for system administrators and orchestration tools (e.g. Kubernetes) to run containers.

These tools are not suitable to give to unprivileged users, because it is trivial to turn such access into to a fully privileged root shell on the host.

There is an effort in the Linux kernel called user namespaces which attempts to allow unprivileged users to use container features. While significant progress has been made, there are still concerns about it.

Bubblewrap is a setuid implementation of a subset of user namespaces. (Emphasis on subset)

It inherits code from xdg-app helper which in turn distantly derives from linux-user-chroot.

The maintainers of this tool believe that it does not, even when used in combination with typical software installed on that distribution, allow privilege escalation. It may increase the ability of a logged in user to perform denial of service attacks, however.

In particular, bubblewrap uses PR_SET_NO_NEW_PRIVS to turn off setuid binaries, which is the traditional way to get out of things like chroots.

This program can be shared by all container tools which perform non-root operation, such as:

• xdg-app
• rpm-ostree unprivileged

We would also like to see this be available in Kubernetes/OpenShift clusters. Having the ability for unprivileged users to use container features would make it significantly easier to do interactive debugging scenarios and the like.

bubblewrap works by creating a new, completely empty, mount namespace where the root is on a tmpfs that is invisible from the host, and will be automatically cleaned up when the last process exists. You can then use commandline options to construct the root filesystem and process environment and command to run in the namespace.

A simple example is

bwrap --ro-bind / / bash

This will create a read-only bind mount of the host root at the sandbox root, and then start a bash.

Another simple example would be a read-write chroot operation:

bwrap --bind /some/chroot/dir / bash

A more complex example is to run a with a custom (readonly) /usr, but your own (tmpfs) data, running in a PID and network namespace:

bwrap --ro-bind /usr /usr \
   --dir /tmp \
   --proc /proc \
   --dev /dev \
   --ro-bind /etc/resolv.conf /etc/resolv.conf \
   --symlink usr/lib /lib \
   --symlink usr/lib64 /lib64 \
   --symlink usr/bin /bin \
   --symlink usr/sbin /sbin \
   --chdir / \
   --unshare-pid \
   --unshare-net \
   --dir /run/user/$(id -u) \
   --setenv XDG_RUNTIME_DIR "/run/user/`id -u`" \
   /bin/sh

The goal of bubblewrap is to run an application in a sandbox, where it has restricted access to parts of the operating system or user data such as the home directory.

bubblewrap always creates a new mount namespace, and the user can specify exactly what parts of the filesystem should be visible in the sandbox. Any such directories you specify mounted nodev by default, and can be made readonly.

Additionally you can use these kernel features:

User namespaces (CLONE_NEWUSER): This hides all but the current uid and gid from the sandbox. You can also change what the value of uid/gid should be in the sandbox.

IPC namespaces (CLONE_NEWIPC): The sandbox will get its own copy of all the different forms of IPCs, like SysV shared memory and semaphores.

PID namespaces (CLONE_NEWPID): The sandbox will not see any processes outside the sandbox. Additionally, bubblewrap will run a trivial pid1 inside your container to handle the requirements of reaping children in the sandbox. .This avoids what is known now as the Docker pid 1 problem.

Network namespaces (CLONE_NEWNET): The sandbox will not see the network. Instead it will have its own network namespace with only a loopback device.

UTS namespace (CLONE_NEWUTS): The sandbox will have its own hostname.

Seccomp filters: You can pass in seccomp filters that limit which syscalls can be done in the sandbox. For more information, see Seccomp.

The name bubblewrap was chosen to convey that this tool runs as the parent of the application (so wraps it in some sense) and creates a protective layer (the sandbox) around it.

(Bubblewrap cat by dancing_stupidity)