bubblewrap

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bubblewrap is a lightweight setuid sandbox application developed from Flatpak with a small installation footprint and minimal resource requirements. While the application package is named bubblewrap, the actual executable binary and manpage reference is bwrap. bubblewrap is expected to anchor the sandbox mechanism of the Tor Browser (Linux) in the future. Notable features include support for cgroup/IPC/mount/network/PID/user/UTS namespaces and seccomp filtering. Note that bubblewrap drops all capabilities within a sandbox and that child tasks cannot gain greater privileges than its parent. Notable feature exclusions include the lack of explicit support for blacklisting/whitelisting file paths.

Warning: Unlike when using a separate user and a separate log-in session, bubblewrap not only exposes security vulnerabilities in the kernel but also in the window compositor. Users should be aware that running untrustworthy code in bubblewrap is still not safe.

Installation

Install bubblewrap or bubblewrap-gitAUR.

Note: The user namespace configuration item CONFIG_USER_NS is not set in the stock Arch kernel per FS#36969. This prevents the kernel from exposing user namespaces as a means to accomodate separate user information for separate virtualized services. An example would be running syslog in a namespace with a UID and GID different than that of the host system.

Configuration

bubblewrap can be called directly from the command-line and/or within shell scripts as part of a complex wrapper. Unlike applications such as Firejail which automatically set /var and /etc to read-only within the sandbox, bubblewrap makes no such operating assumptions. It is up to the user to determine which configuration options to pass in accordance to the application being sandboxed. bubblewrap does not automatically create user namespaces when running with setuid privileges and can accomodate typical environment variables including $HOME and $USER.

Usage examples

No-op

A no-op bubblewrap invocation is as follows:

$ bwrap --dev-bind / / bash

This will spawn a bash process which should behave exactly as outside a sandbox. If a sandboxed program misbehaves, you may want to start from the above no-op invocation, and work your way towards a more secure configuration step-by-step.

Bash

Create a simple Bash sandbox:

  • Determine available kernel namespaces
$ ls /proc/self/ns 
cgroup  ipc  mnt  net  pid  user uts
Note: The presence of user indicates that the kernel has exposed support for user namespaces with CONFIG_USER_NS=y
  • Bind as read-only the entire host / directory to / in the sandbox
  • Create a new user namespace and set the user ID to 256 and the group ID to 512
$ bwrap --ro-bind / / --unshare-user --uid 256 --gid 512 bash
bash-4.4$ id
uid=256 gid=512 groups=512,65534(nobody)
bash-4.4$ ls -l /usr/bin/bash
-rwxr-xr-x 1 nobody nobody 811752 2017-01-01 04:20 /usr/bin/bash

dhcpcd

Create a simple dhcpcd sandbox:

  • Determine available kernel namespaces
$ ls /proc/self/ns 
cgroup  ipc  mnt  net  pid  uts
Note: The absence of user indicates that the kernel has been built with CONFIG_USER_NS=n or is user namespace restricted.
  • Bind as read-write the entire host / directory to / in the sandbox
  • Mount a new devtmpfs filesystem to /dev in the sandbox
  • Create new IPC and control group namespaces
  • Create a new UTS namespace and set dhcpcd as the hostname
# /usr/bin/bwrap --bind / / --dev /dev --unshare-ipc --unshare-cgroup --unshare-uts --hostname dhcpcd /usr/bin/dhcpcd -q -b

Unbound

Create a more granular and complex Unbound sandbox:

  • Bind as read-only the system /usr directory to /usr in the sandbox
  • Create a symbolic link from the system /usr/lib directory to /lib64 in the sandbox
  • Bind as read-only the system /etc directory to /etc in the sandbox
  • Create empty /var and /run directories within the sandbox
  • Mount a new devtmpfs filesystem to /dev in the sandbox
  • Create new IPC and PID and control group namespaces
  • Create a new UTS namespace and set unbound as the hostname
# /usr/bin/bwrap --ro-bind /usr /usr --symlink usr/lib /lib64 --ro-bind /etc /etc --dir /var --dir /run --dev /dev --unshare-ipc --unshare-pid --unshare-cgroup --unshare-uts --hostname unbound /usr/bin/unbound -d
Tip: See systemd#Editing provided units to enable the bubblewrapping of systemd unit files including unbound.service

Desktop

Leverage bubblewrap within desktop entries:

  • Bind as read-write the entire host / directory to / in the sandbox
  • Re-bind as read-only the /var and /etc directories in the sandbox
  • Mount a new devtmpfs filesystem to /dev in the sandbox
  • Create a tmpfs filesystem over the sandboxed /run directory
  • Disable network access by creating new network namespace
[Desktop Entry]
Name=nano Editor
Exec=bwrap --bind / / --dev /dev --tmpfs /run --unshare-net  st -e nano -o . %f
Type=Application
MimeType=text/plain;
Note: --dev /dev is required to write to /dev/pty
  • Example MuPDF desktop entry incorporating a mupdf.sh shell wrapper:
[Desktop Entry]
Name=MuPDF
Exec=mupdf.sh %f
Icon=application-pdf.svg
Type=Application
MimeType=application/pdf;application/x-pdf;
Note: Ensure that mupdf.sh is located within your executable PATH e.g. PATH=$PATH:$HOME/bwrap

MuPDF

The power and flexibility of bwrap is best revealed when used to create an environment within a shell wrapper:

  • Bind as read-only the host /usr/bin directory to /usr/bin in the sandbox
  • Bind as read-only the host /usr/lib directory to /usr/lib in the sandbox
  • Create a symbolic link from the system /usr/lib directory to /lib64 in the sandbox
  • Create a tmpfs filesystem overlaying /usr/lib/gcc in the sandbox
    • This effectively blacklists the contents of /usr/lib/gcc from appearing in the sandbox
  • Create a new tmpfs filesystem as the $HOME directory in the sandbox
  • Bind as read-only an .Xauthority file and Documents directory into the sandbox
    • This effectively whitelists the .Xauthority file and Documents directory with recursion
  • Create a new tmpfs filesystem as the /tmp directory in the sandbox
  • Whitelist the X11 socket by binding it into the sandbox as read-only
  • Clone and create private containers for all namespaces supported by the running kernel
    • If the kernel does not support non-privileged user namespaces, skip its creation and continue
  • Do not place network components into a private namespace
    • This allows for network access to follow URI hyperlinks
#!/bin/sh
#~/bwrap/mupdf.sh
(exec bwrap \
--ro-bind /usr/bin /usr/bin \
--ro-bind /usr/lib /usr/lib \
--symlink usr/lib /lib64 \
--tmpfs /usr/lib/gcc \
--tmpfs $HOME \
--ro-bind $HOME/.Xauthority $HOME/.Xauthority \
--ro-bind $HOME/Documents $HOME/Documents \
--tmpfs /tmp \
--ro-bind /tmp/.X11-unix/X0 /tmp/.X11-unix/X0 \
 --unshare-all \
--share-net \
/usr/bin/mupdf "$@")
Tip: Execute a shell wrapper substituting the existing executable with /usr/bin/sh to debug and verify the contents and filesystem structure of the sandbox.
$ bwrap \
--ro-bind /usr/bin /usr/bin \
--ro-bind /usr/lib /usr/lib \
--symlink usr/lib /lib64 \
--tmpfs /usr/lib/gcc \
--tmpfs $HOME \
--ro-bind $HOME/.Xauthority $HOME/.Xauthority \
--ro-bind $HOME/Desktop $HOME/Desktop \
--tmpfs /tmp \
--ro-bind /tmp/.X11-unix/X0 /tmp/.X11-unix/X0 \
--unshare-all \
--share-net \
 /usr/bin/sh
bash-4.4$ ls -AF
.Xauthority  Documents/

Perhaps the most important rule to consider when building a bubblewrapped filesystem is that commands are executed in the order they appear. From the MuPDF example above:

  • A tmpfs system is created followed by the bind mounting of an .Xauthority file and a Documents directory:
--tmpfs $HOME \
--ro-bind $HOME/.Xauthority $HOME/.Xauthority \
--ro-bind $HOME/Documents $HOME/Documents \
bash-4.4$ ls -a
.  ..  .Xauthority  Desktop
  • A tmpfs filesystem is created after the bind mounting of .Xauthority and overlays it so that only the Documents directory is visible within the sandbox:
--ro-bind $HOME/.Xauthority $HOME/.Xauthority \
--tmpfs $HOME \
--ro-bind $HOME/Desktop $HOME/Desktop \
bash-4.4$ ls -a
.  ..  Desktop

p7zip

Applications which have not yet been patched against known vulnerabilities constitute prime candidates for bubblewrapping:

  • Bind as read-only the host /usr/bin/7za executable path to the sandbox
  • Create a symbolic link from the system /usr/lib directory to /lib64 in the sandbox
  • Blacklist the sandboxed contents of /usr/lib/modules and /usr/lib/systemd with tmpfs overlays
  • Mount a new devtmpfs filesystem to /dev in the sandbox
  • Bind as read-write the host /sandbox directory to the /sandbox directory in the sandbox
    • 7za will only run in the host /sandbox directory and/or its subdirectories when called from the shell wrapper
  • Create new cgroup/IPC/network/PID/UTS namespaces for the application and its processes
    • If the kernel does not support non-privileged user namespaces, skip its creation and continue
    • Creation of a new network namespace prevents the sandbox from obtaining network access
  • Add a custom or an arbitrary hostname to the sandbox such as p7zip
  • Unset the XAUTHORITY environment variable to hide the location of the X11 connection cookie
    • 7za does not need to connect to an X11 display server to function properly
  • Start a new terminal session to prevent keyboard input from escaping the sandbox
#!/bin/sh
#~/bwrap/pz7ip.sh
(exec bwrap \
--ro-bind /usr/bin/7za /usr/bin/7za \
--symlink usr/lib /lib64 \
--tmpfs /usr/lib/modules \
--tmpfs /usr/lib/systemd \
--dev /dev \
--bind /sandbox /sandbox \
--unshare-all \
--hostname p7zip \
--unsetenv XAUTHORITY \
--new-session \
/usr/bin/7za "$@")
Note: /usr/bin/sh and /usr/bin/ls must reside in the executable path in order to traverse and verify the sandbox filesystem.
bwrap \
--ro-bind /usr/bin/7za /usr/bin/7za \
--ro-bind /usr/bin/ls /usr/bin/ls \
--ro-bind /usr/bin/sh /usr/bin/sh \
--symlink usr/lib /lib64 \
--tmpfs /usr/lib/modules \
--tmpfs /usr/lib/systemd \
--dev /dev \
--bind /sandbox /sandbox \
--unshare-all \
--hostname p7zip \
--unsetenv XAUTHORITY \
--new-session \
/usr/bin/sh
bash: no job control in this shell
bash-4.4$ ls -AF         
dev/  lib64@  usr/
bash-4.4$ ls -l /usr/lib/modules 
total 0
bash-4.4$ ls -l /usr/lib/systemd
total 0
bash-4.4$ ls -AF /dev
console  full  null  ptmx@  pts/  random  shm/  stderr@  stdin@  stdout@  tty  urandom  zero
bash-4.4$ ls -A /usr/bin
7za  ls  sh

Filesystem isolation

Warning: It is the bubblewrap user’s responsibility to update the filesystem trees regularly.

To further hide the contents of the file system (such as those in /var, /usr/bin and /usr/lib) and to sandbox even the installation of software, pacman can be made to install Arch packages into isolated filesystem trees.

In order to use pacman for installing software into the filesystem trees, you will need to install fakeroot and fakechroot.

Suppose you want to install the xterm package with pacman into an isolated filesystem tree. You should prepare your tree like this:

$ MYPACKAGE=xterm
$ mkdir -p ~/sandboxes/${MYPACKAGE}/files/var/lib/pacman
$ mkdir -p ~/sandboxes/${MYPACKAGE}/files/etc
$ cp /etc/pacman.conf ~/sandboxes/${MYPACKAGE}/files/etc/pacman.conf

You may want to edit ~/sandboxes/${MYPACKAGE}/files/etc/pacman.conf and adjust the pacman configuration used:

  • Remove any undesired custom repositories and IgnorePkg, IgnoreGroup, NoUpgrade and NoExtract settings that are needed only for the host system.
  • You may need to remove the CheckSpace option so pacman will not complain about errors finding the root filesystem for checking disk space.

Then install the base group along with the needed fakeroot into the isolated filesystem tree:

$ fakechroot fakeroot pacman -Syu \
    --root ~/sandboxes/${MYPACKAGE}/files \
    --dbpath ~/sandboxes/${MYPACKAGE}/files/var/lib/pacman \
    --config ~/sandboxes/${MYPACKAGE}/files/etc/pacman.conf \
    base fakeroot

Since you will be repeatedly calling bubblewrap with the same options, make an alias:

$ alias bw-install='bwrap                        \
     --bind ~/sandboxes/${MYPACKAGE}/files/ /    \
     --ro-bind /etc/resolv.conf /etc/resolv.conf \
     --tmpfs /tmp                                \
     --proc /proc                                \
     --dev /dev                                  \
     --chdir /                                   '

You will need to set up the locales:

$ nano -w ~/sandboxes/${MYPACKAGE}/files/etc/locale.gen
$ bw-install locale-gen

Then set up pacman’s keyring:

$ bw-install fakeroot pacman-key --init
$ bw-install fakeroot pacman-key --populate archlinux

Now you can install the desired xterm package.

$ bw-install fakeroot pacman -S ${MYPACKAGE}

If the pacman command fails here, try running the command for populating the keyring again.

Congratulations. You now have an isolated filesystem tree containing xterm. You can use bw-install again to upgrade your filesystem tree.

You can now run your software with bubblewrap. command should be xterm in this case.

$ bwrap                                          \
     --ro-bind ~/sandboxes/${MYPACKAGE}/files/ / \
     --ro-bind /etc/resolv.conf /etc/resolv.conf \
     --tmpfs /tmp                                \
     --proc /proc                                \
     --dev /dev                                  \
     --chdir /                                   \
     command

Note that some files can be shared between packages. You can hardlink to all files of an existing parent filesystem tree to reuse them in a new tree:

$ cp -al ~/sandboxes/${MYPARENTPACKAGE} ~/sandboxes/${MYPACKAGE}

Then proceed with the installation as usual by calling pacman from bw-install fakechroot fakeroot pacman ….

Troubleshooting

Using X11

Bind mounting the host X11 socket to an alternative X11 socket may not work:

--bind /tmp/.X11-unix/X0 /tmp/.X11-unix/X8 --setenv DISPLAY :8

A workaround is to bind mount the host X11 socket to the same socket within the sandbox:

--bind /tmp/.X11-unix/X0 /tmp/.X11-unix/X0 --setenv DISPLAY :0

Sandboxing X11

While bwrap provides some very nice isolation for sandboxed application, there is an easy escape as long as access to the X11 socket is available. X11 does not include isolation between applications and is completely insecure. The only solution to this is to switch to a wayland compositor with no access to the Xserver from the sandbox.

There are however some workarounds that use xpra or xephyr to run in a new X11 environment. This would work with bwrap as well.

To test X11 isolation, run 'xinput test <id>' where <id> is your keyboard id which you can find with 'xinput list' When run without additional X11 isolation, this will show that any application with X11 access can capture keyboard input of any other application, which is basically what a keylogger would do.

Opening URLs from wrapped applications

When a wrapped IRC or email client attempts to open a URL, it will usually attempt to launch a browser process, which will run within the same sandbox as the wrapped application. With a well-wrapped application, this will likely not work. The approach used by Firejail is to give wrapped applications all the privileges of the browser as well, however this implies a good amount of permission creep.

A better solution to this problem is to communicate opened URLs to outside the sandbox. This can be done using snapd-xdg-open as follows:

  1. Install snapd-xdg-open-gitAUR
  2. On your bwrap command line, add:
$ bwrap ... \
  --ro-bind /run/user/$UID/bus /run/user/$UID/bus \
  --ro-bind /usr/lib/snapd-xdg-open/xdg-open /usr/bin/xdg-open \
  --ro-bind /usr/lib/snapd-xdg-open/xdg-open /usr/bin/chromium \
  ...

The /usr/bin/chromium bind is only necessary for programs not using XDG conventions, such as Mozilla Thunderbird.

New Session

There is a security issue with TIOCSTI, (CVE-2017-522) which allows sandbox escape. To prevent this, bubblewrap has introduced the new option '--new-session' which calls setsid(). However this causes some behavioural issues that are hard to work with in some cases. For instance, it makes shell job control not work for the bwrap command.

It is recommended to use this if possible, but if not the developers recommend that the issue is neutralized in some other way, for instance using SECCOMP, which is what flatpak does: https://github.com/flatpak/flatpak/commit/902fb713990a8f968ea4350c7c2a27ff46f1a6c4

See also