Git commit signatures are recursive by design — that is, each signature covers not only the commit in question but also indirectly all past commits, via tree and parent commit hashes. This makes user-side commit verification much simpler, as the user needs only to verify the signature on the most recent commit; with the assumption that the developer making it has verified the earlier commit and so on. Sadly, this is usually not the case at the moment.
Most of the Gentoo developers do not really verify the base upon which they are making their commits. While they might verify the commits when pulling before starting to work on their changes, it is rather unlikely that they verify the correctness when they repeatedly need to rebase before pushing. Usually this does not cause problems as Gentoo Infrastructure is verifying the commit signatures before accepting the push. Nevertheless, the recent attack on our GitHub mirrors made me realize that if a smart attacker was able to inject a single malicious commit without valid signature, then a Gentoo developer would most likely make a signed commit on top of it without even noticing the problem.
In this article, I would like to shortly present my quick solution to this problem — app-portage/gverify. gverify is a trivial reimplementation of gkeys in <200 lines of code. It uses the gkeys seed data (yes, this means it relies on manual updates) combined with autogenerated developer keyrings to provide strict verification of commits. Unlike gkeys, it works out-of-the-box without root privileges and automatically updates the keys on use.
The package installs a gv-install tool that installs two hooks on your repo/gentoo.git working copy. Those are post-merge and pre-rebase hooks that verify the tip of upstream master branch, respectively every time merge on master is finished, and every time a rebase is about to be started. This covers the two main cases — git pull and git pull --rebase. The former causes a verbose error after the update, the latter prevents a rebase from proceeding.
While this is far from perfect, it seems reasonably good solution given the limitations of available git hooks. Most importantly, it should prevent the git pull --rebase -S && git push --sign loop from silently accepting a malicious commit. Currently the hook verifies the top upstream commit only; however, in the future I want to implement incremental verification of all new commits.