[RFC] tag-ref and tag object binding

[Santiago Torres <santiago@nyu.edu>]

Hello everyone.

I've done some further research on the security properties of git
metadata and I think I've identified something that might be worth
discussing. In this case, The issue is related to the refs that point to
git tag objects. Specifically, the "loose" nature of tag refs might
possibly trick people into installing the wrong revision (version?) of a
file.

To elaborate, the ref of a tag object can be moved around in the same
way a branch can be moved around (either manually or by using git
commands). If someone with write access can modify where this ref points
to, and points it to another valid tag (e.g., an older, vulnerable
version), then many tools that work under the assumption of static tags
might mistakenly install/pull the wrong reivision of source code. I've
verified that this is possible to pull off in package managers such as
PIP, rubygems, gradle(maven), as well as git submodules tracking tags.

In order to stay loyal to the way files in the .git directory are
ordered, I don't think that making the ref file (or packed refs) files
differently is an option. However, I think that it could be possible to
store the "origin ref" in the git tag object, so tools can verify that
they are looking at the appropriate tag. There might also be a simpler
solution to this, and I would appreciate any feedback.

What do you guys think?

Thanks!
Santiago.