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bubblewrap kernel requirements
==============================
bubblewrap can be used by various parts of the system to run
partially-trusted programs in a sandboxed environment where their impact
on system security is reduced. For example:
- Flatpak uses bubblewrap to run partially-trusted, user-installable
apps in a sandboxed environment.
- The glycin library uses bubblewrap to run image loaders in a sandboxed
environment, so that if there are security flaws in an image decoder
used by a thumbnailer, the process of generating thumbnails for a
maliciously crafted image cannot be used to attack the rest of the
system.
bubblewrap can also be used to run trusted programs in a different
environment, for example with different shared libraries available:
- Flatpak uses bubblewrap to run apps with a predictable library stack
that does not match the rest of the system, even if those apps are
trusted and so do not need to be sandboxed for security.
- Steam uses bubblewrap to run some games with a predictable library
stack that does not match the rest of the system.
The necessary capabilities to do this can be obtained in one of two
ways:
- On kernels where unprivileged users can create new user namespaces,
bubblewrap's bwrap executable can be an ordinary unprivileged program.
This is the case by default on all modern versions of Debian and Ubuntu.
- On kernels where this is not possible, bubblewrap will not work unless
the /usr/bin/bwrap executable is setuid root. Some bubblewrap and Flatpak
features will not work in this configuration for security reasons.
Custom and third-party kernels
------------------------------
If you compile your own kernel, you will need at least
CONFIG_NAMESPACES=y and preferably CONFIG_USER_NS=y.
If you do not have CONFIG_UTS_NS=y, CONFIG_IPC_NS=y, CONFIG_USER_NS=y,
CONFIG_PID_NS=y and CONFIG_NET_NS=y, then the corresponding bubblewrap
features will not work.
Configuring kernel.unprivileged_userns_clone
--------------------------------------------
This Debian-specific sysctl parameter controls whether unprivileged
users are allowed to create new user namespaces.
The default is 1 for all modern Debian and Ubuntu kernels.
If it is set to 0, some attacks against the kernel are made more difficult,
which can increase security. However, some user-space software will not
be able to create a sandboxed environment or will have to rely on a
setuid version of bubblewrap to create a sandboxed environment, which
reduces security. The value of this sysctl parameter is a trade-off
between different security risks.
If this parameter is set to 0, bubblewrap and Flatpak will not work unless
bwrap is made setuid root (see "Making bubblewrap setuid root" below).
You can view the current setting with:
cat /proc/sys/kernel/unprivileged_userns_clone
and temporarily set it to 0 or 1 (until the next reboot) with a command
like:
sudo sysctl -w kernel.unprivileged_userns_clone=1
It can be set permanently by using a file /etc/sysctl.d/*.conf:
this requires either systemd as pid 1, or the procps package.
For details please see sysctl.d(5) on systemd systems, or sysctl.conf(5)
on systems using a non-default init.
Configuring the maximum number of namespaces per user
-----------------------------------------------------
The number of user namespaces per user is limited. The default limit
depends on the amount of RAM available.
Setting this limit to 0 is the recommended way to disable user namespace
creation if this is required as a security hardening measure. bubblewrap
will not work with this limit set to 0, unless it is setuid root (see
"Configuring whether bubblewrap is setuid root" below).
The limit is given by the user.max_user_namespaces sysctl parameter.
You can view the current setting with:
cat /proc/sys/user/max_user_namespaces
and temporarily set it to a value (until the next reboot) with a
command like:
sudo sysctl -w user.max_user_namespaces=1000
To set it to a value during system startup, create a file in /etc/sysctl.d
containing a line like this:
user.max_user_namespaces=1000
Configuring whether bubblewrap is setuid root
---------------------------------------------
To use bubblewrap with kernel.unprivileged_userns_clone set to 0
or user.max_user_namespaces set to 0, it is necessary to make the bwrap
executable setuid root. This gives it the necessary capabilities to set
up containers even when run by an otherwise unprivileged user, and was the
configuration normally used in Debian 10.
This can be a security risk: if there are bugs in bubblewrap, it might be
possible for an unprivileged user to get root privileges by running a
setuid version of the bwrap executable. CVE-2020-5291 and CVE-2016-8659
are examples of bugs that had this effect in the past. However, it allows
the kernel to be configured to disallow creation of user namespaces by
unprivileged users, which prevents attacks like CVE-2016-3135 from being
carried out against the kernel. This is a trade-off between different
security risks.
To avoid other attacks, some Flatpak and bubblewrap features are not
available when bwrap is setuid root, and the absence of those features
is known to break some Flatpak apps. For example, the Flatpak app for
the Chromium web browser will not work with a setuid bwrap executable.
To check whether the bwrap executable will be made setuid root after
the next upgrade, use this command:
dpkg-statoverride --list /usr/bin/bwrap
To force the bwrap executable to be setuid root, use these commands:
sudo dpkg-statoverride --quiet --remove /usr/bin/bwrap
sudo dpkg-statoverride --update --add root root 4755 /usr/bin/bwrap
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