35598f46a5
When installing a flatpak with extra-data we execute the apply_extra script from the flatpak to extract the extra data files we created. This script runs with very little filesystem acces, but it does have write permissions to the location that will eventually be /app/extra in the finished flatpak. This is especially problematic for the systemwide install case, because the script is then run as root, so it could potentially create a setuid file there. Such a file would not be usable inside the sandbox (because setuid is disabled in the sandbox), but it could potentially be a problem if the user could be tricked into running the file directly on the host. This is the same behaviour as e.g. rpm or deb which both can install setuid files, but we want to guarantee that flatpak is better than that. The fix is to run the script with all capabilities dropped (bwrap --cap-drop ALL) which removes a bunch of possible attack vectors (for instance setting file capabilities). However, even without capabilities, it is possible for a user to make any file setuid to the same user, so we also need to canonicalize the permissions of all files generated by running the script. Additionally, while running the script we set the toplevel directory only be accessible to the user, meaning we will not temporarily leak any potential setuid files to other users. Note, this commit actually goes furthen than that and completely canonicalizes all the file permissions to be the same as those otherwise used by flatpak. For example we fix up cases where the script creates files writable or unreadable by non-root users. Closes: #2323 Approved by: alexlarsson
Flatpak is a system for building, distributing, and running sandboxed desktop applications on Linux.
See https://flatpak.org/ for more information.
Community discussion happens in #flatpak on Freenode and on the mailing list.
Read documentation for the flatpak commandline tools and for the libflatpak library API.
Contributing
Flatpak welcomes contributions from anyone! Here are some ways you can help:
- Fix one of the issues and submit a PR
- Update flatpak's translations and submit a PR
- Update flatpak's documentation, hosted at http://docs.flatpak.org and developed over in flatpak-docs
- Find a bug and submit a detailed report including your OS, flatpak version, and the steps to reproduce
- Add your favorite application to Flathub by writing a flatpak-builder manifest and submitting it
- Improve the Flatpak support in your favorite Linux distribution
Hacking
Flatpak uses a traditional autoconf-style build mechanism. To build just do
./configure [args]
make
make install
Dependencies you will need include: autoconf, automake, libtool, bison, gettext, gtk-doc, gobject-introspection, libcap, libarchive, libxml2, libsoup, gpgme, polkit, libXau, ostree, json-glib, appstream, libseccomp (or their devel packages).
Most configure arguments are documented in ./configure --help. However,
there are some options that are a bit more complicated.
Flatpak relies on a project called
Bubblewrap for the
low-level sandboxing. By default, an in-tree copy of this is built
(distributed in the tarball or using git submodules in the git
tree). This will build a helper called flatpak-bwrap. If your system
has a recent enough version of Bubblewrap already, you can use
--with-system-bubblewrap to use that instead.
Bubblewrap can run in two modes, either using unprivileged user
namespaces or setuid mode. This requires that the kernel supports this,
which some distributions disable. For instance, Debian and Arch
(linux kernel v4.14.5 or later), support user namespaces with the kernel.unprivileged_userns_clone sysctl enabled.
If unprivileged user namespaces are not available, then Bubblewrap must be built as setuid root. This is believed to be safe, as it is designed to do this. Any build of Bubblewrap supports both unprivileged and setuid mode, you just need to set the setuid bit for it to change mode.
However, this does complicate the installation a bit. If you pass
--with-priv-mode=setuid to configure (of Flatpak or Bubblewrap) then
make install will try to set the setuid bit. However that means you
have to run make install as root. Alternatively, you can pass
--enable-sudo to configure and it will call sudo when setting the
setuid bit. Alternatively you can enable setuid completely outside of
the installation, which is common for example when packaging Bubblewrap
in a .deb or .rpm.
There are some complications when building Flatpak to a different
prefix than the system-installed version. First of all, the newly
built Flatpak will look for system-installed flatpaks in
$PREFIX/var/lib/flatpak, which will not match existing installed
flatpaks. You can use --with-system-install-dir=/var/lib/flatpak
to make both installations use the same location.
Secondly, Flatpak ships with a root-privileged policykit helper for
system-installation, called flatpak-system-helper. This is dbus
activated (on the system-bus) and if you install in a non-standard
location it is likely that this will not be found by dbus and
policykit. However, if the system installation is synchronized,
you can often use the system installed helper instead - at least
if the two versions are close in versions.