Sandboxing pre-commit pre-commit is a tool that is often a ...

2026-10-27T02:59:34Z

Sandboxing pre-commit

pre-commit is a tool that is often a
prerequisite for contributing to Python upstream projects. It installs as a git
pre-commit hook and runs various checks on commit, using tools installed in a
set of virtualenvs that it automatically manages.
In other words, it is a Remote Code Execution engine running with the
permissions of your own user, and with read and write access to all your
information (including, say, ~/.ssh or ~/.netrc). If you are lucky, and for
as long as you're lucky, it will perform useful tasks on your code.
The recent eMudhra certificate situation, where I learnt how Python, as well as
several other ecosystems, likes to embed CA certificate bundles in
virtualenvs, caused my
tolerance for this to lower to a point where I felt the need to limit
pre-commit's reach.
However, I didn't want to end up with another containerized OS tree that it's
then my responsibility to manage, and I tried to go the
bubblewrap way, so that the
environment where pre-commit runs will get the same security updates as the
rest of my system.
The result is a pre-commit wrapper script
that:
Runs in a private filesystem, with no access to user's files or system
configuration
Runs in a filesystem where everything is read-only, except for the git
working directory
Even the .git directory is readonly, so that changes in the code can be
reliably detected via git diff
Runs without network unless explicitly requested by the user
Here's a breakdown of the bwrap options I used so far.
Set up usual filesystem expectations:
--ro-bind /usr /usr \
--symlink usr/bin /bin \
--symlink usr/lib64 /lib64 \
--symlink usr/lib /lib \
--proc /proc \
--dev /dev \
Make the working directory and pre-commit's environments storage available and
writable:
--bind ${CURDIR} "$GUEST_DIR" \
--bind ~/.cache/pre-commit "/home/${USER}/.cache/pre-commit" \
--chdir "$GUEST_DIR" \
Forward git's configuration:
--ro-bind /etc/gitconfig /etc/gitconfig \
--ro-bind ~/.gitconfig /home/${USER}/.gitconfig \
Make .git writable (pre-commit needs to stash, for example), but use a tmpfs
overlay so that the writes don't persist:
--overlay-src "$CURDIR/.git" \
--tmp-overlay "$GUEST_DIR/.git" \
Remove access to everything except the above. Prevent keystroke injection at
the terminal. Propagate ^C.
--unshare-all \
--new-session \
--die-with-parent
If network is needed:
--share-net \
--ro-bind /etc/resolv.conf /etc/resolv.conf \
--ro-bind /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
To run it, you can set up the wrapper script to run with a higher priority in
$PATH, but you need to edit .git/hooks/pre-commit to remove the first
branch of the if that, frustratingly, tries to run it as python3 -m
pre_commit.
I'm looking forward to a time where it becomes stadard practice to manage user
commands this way: why should a desktop solitaire game have access to ~/.ssh?
Or network? Why is it still normal, in 2026, to run a Desktop system like that?

https://www.ezini.eu/blog/2026/python/sandboxing-pre-commit

#runs #user #code #python #curdir #make #codeexecution #remotecode #set #remove

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Nostr notes by Enrico's blog (Unofficial)

Sandboxing pre-commit pre-commit is a tool that is often a ...