Re: Bare repositories in the working tree are a security risk

From: "brian m. carlson" <sandals@crustytoothpaste.net>
To: Justin Steven <justin@justinsteven.com>
Cc: Glen Choo <chooglen@google.com>,
	git@vger.kernel.org, Emily Shaffer <emilyshaffer@google.com>,
	Taylor Blau <me@ttaylorr.com>
Subject: Re: Bare repositories in the working tree are a security risk
Date: Thu, 7 Apr 2022 22:10:00 +0000	[thread overview]
Message-ID: <Yk9hONuCIVIq6ieV@camp.crustytoothpaste.net> (raw)
In-Reply-To: <CAHZU0ySHqc7f9qB0+ZrMWHHJiWsS-_hsUzomwNrGNMTF6qwcOw@mail.gmail.com>

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On 2022-04-07 at 21:53:26, Justin Steven wrote:
> Hi all,
> 
> I'm the author of one of the articles linked in Glen's mail. Thank you
> Glen for summarising the problem beautifully and pushing this forward.
> 
> Brian said:
> > As mentioned elsewhere, git status doesn't work without a working tree.
> 
> This is correct. However, it is possible to embed a bare repo that has
> its own core.worktree which points to a directory within the
> containing repo, satisfying the requirement of having a working tree.
> This is covered in the article [1] and looks to be accounted for in
> Taylor's reproducer script which admittedly I haven't run.
> 
> > Instead, I'd rather see us avoid executing any program from the config
> > or any hooks in a bare repository without a working tree (except for
> > pushes).  I think that would avoid breaking things while still improving
> > security.
> 
> Due to the fact that the embedded bare repo can be made to have a
> working tree, this won't be an effective fix.

Then we'd probably be better off just walking up the entire hierarchy
and excluding worktrees from embedded bare repositories, or otherwise
restricting the config we read.  That will probably mean we'll need to
walk the entire directory hierarchy to see if it's embedded (or at least
to the root of the device) in such a case, but that should be relatively
uncommon.

I'd definitely like to see us make a security improvement here, but I
also would like to avoid us breaking a lot of repositories, especially
since we lack alternatives.

If git fast-import could 100% correctly round-trip all commits and
repositories, I would be much more open to blocking this in fsck after a
deprecation period, but as it stands that's not possible.  Perhaps
improving that would be a suitable way forward.
-- 
brian m. carlson (he/him or they/them)
Toronto, Ontario, CA

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