qusal/salt/sys-git

sys-git

Git operations through Qrexec in Qubes OS.

Table of Contents

• Description
• Alternatives comparison
• Security
• Installation
• Access control
• Usage
  • Initialize the server repository
  • Prepare the client
• Credits

Description

Setup a Git server called "sys-git", an offline Git Server that can be accessed from client qubes via Qrexec. Access control via Qrexec policy can restrict access to certain repositories, set of git actions for Fetch, Push and Init. This is an implementation of split-git.

Alternatives comparison

The following alternatives will be compared against each other and this implementation:

• Rudd-O/git-remote-qubes
• QubesOS-contrib/qubes-app-split-git
• qubes-os.org/doc/development-workflow/#git-connection-between-vms

	sys-git	git-remote-qubes	qubes-app-split-git	git-connection-between-vms
Codebase Size	Small	Large	Large	Small
Custom Protocol	True	True	True	False
Path	Repository	Absolute	Repository	Repository
Repository restriction	True	False	True	True
No hanging	True	True	True	False
Fetch	True	True	True (only tags)	True
Push	True	True	False	True
Init	True	False	False	False
Validates git communication	False	False	True	False
Verifies tag signature	False	False	True	False

Security

It is not possible to filter Git's stdout from a Qrexec call as it is used by the local running git process, we rely on Git's parsing and filtering for remote operations. A remote can send up to 4 bytes of UTF-8 character to it's stdout as packet information during the initial server client negotiation, the client will display the characters on stderr with an error message containing the character. Git only filters for control characters but other characters that are valid UTF-8 such as multibyte are not filtered. The same characters can be present in the git log. In reality, there are many other ways the remote can make the client display a refname with attacker controlled data with a much larger byte size, this cannot be solved while the remote helper does not verify each received reference.

A remote helper that validates the data received can increase the security by not printing untrusted data, which is the case with qubes-app-split-git, but unfortunately it demands signed tags and doesn't work for normal git operations with signed commits and branches, as the later can't be signed. A fork of the aforementioned project might be the future of this helper.

Even if the transport is secure, the tool that renders the information of your recently acquired repository can contain bugs that result in local code execution and remote code execution. In the end, if you don't trust the origin, don't use it.

Installation

• Top

sudo qubesctl top.enable sys-git
sudo qubesctl --targets=tpl-sys-git,sys-git state.apply
sudo qubesctl top.disable sys-git

• State

sudo qubesctl state.apply sys-git.create
sudo qubesctl --skip-dom0 --targets=tpl-sys-git state.apply sys-git.install
sudo qubesctl --skip-dom0 --targets=sys-git state.apply sys-git.configure

Installation on the client template:

sudo qubesctl --skip-dom0 --targets=tpl-dev state.apply sys-git.install-client

Access control

Default policy: any qube can ask via the @default target if you allow it to Fetch from, Push to and Init on sys-git.

Recommended usage:

• Init: Argument useful when allowing a qube to always create a repository on the server.
• Fetch: Fetch can be allowed by less trusted qubes.
• Push: Push should only be made by trusted qubes.

Allow qube dev to Fetch from sys-git, but ask to Push and Init:

qusal.GitFetch * dev @default allow target=sys-git
qusal.GitPush  * dev @default ask   target=sys-git default_target=sys-git
qusal.GitInit  * dev @default ask   target=sys-git default_target=sys-git
qusal.GitFetch * dev @anyvm   deny
qusal.GitPush  * dev @anyvm   deny
qusal.GitInit  * dev @anyvm   deny

Allow qube untrusted to Fetch repo if using target name sys-git but deny Push and Init to any other qube:

qusal.GitFetch +repo untrusted sys-git ask target=sys-git default_target=sys-git
qusal.GitFetch *     untrusted @anyvm  deny
qusal.GitPush  *     untrusted @anyvm  deny
qusal.GitInit  *     untrusted @anyvm  deny

Deny Fetch, Push and Init from any qube to any other qube:

qusal.GitFetch *     @anyvm @anyvm deny
qusal.GitPush  *     @anyvm @anyvm deny
qusal.GitInit  *     @anyvm @anyvm deny

Usage

Initialize the server repository

There are a few constraints regarding repositories:

• Must be created under /home/user/src in sys-git;
• Names must have only letters, numbers, hyphen, underscore and dot. Must not begin or end with dot, hyphen and underscore.

In sys-git, create bare repositories under /home/user/src.

From the server:

git init --bare ~/src/X.git

You must use the .git prefix to indicate a bare repository.

Or from the client, if the qusal.GitInit policy allows:

cd ~/path/to/repo
git init-qrexec

Prepare the client

Qrexec protocol is supported with the following URL format: qrexec://<QUBE>/<REPO>, where the <QUBE> field can be a literal name or token and the <REPO> field is the name of the repository that exists on sys-git under /home/user/src.

Clone an existing repository:

git clone qrexec://@default/qubes-doc

Or Initialize a new repository:

git init qubes-doc
cd qubes-doc

Add a remote using the Qrexec protocol:

git remote add sg qrexec://@default/qubes-doc

Test fetching from the newly added remote:

git fetch sg

Make changes to the git repository as you normally would on any system.

Push to the server and set it as the default upstream:

git push -u sg main

Following pushes will be simpler:

git push

Credits

• Unman