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

From: Taylor Blau <me@ttaylorr.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: "Emily Shaffer" <emilyshaffer@google.com>,
	"Taylor Blau" <me@ttaylorr.com>,
	"Glen Choo" <chooglen@google.com>,
	"Git List" <git@vger.kernel.org>,
	justin@justinsteven.com,
	"Johannes Schindelin" <Johannes.Schindelin@gmx.de>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Derrick Stolee" <derrickstolee@github.com>,
	"brian m. carlson" <sandals@crustytoothpaste.net>,
	"Randall S. Becker" <rsbecker@nexbridge.com>
Subject: Re: Bare repositories in the working tree are a security risk
Date: Thu, 21 Apr 2022 14:54:43 -0400	[thread overview]
Message-ID: <YmGoc8gwB3W/cfHM@nand.local> (raw)
In-Reply-To: <xmqq4k2mi88p.fsf@gitster.g>

On Thu, Apr 21, 2022 at 11:47:34AM -0700, Junio C Hamano wrote:
> Emily Shaffer <emilyshaffer@google.com> writes:
>
> > Ah, another thing I forgot to mention. There has been a little
> > discussion in the past about isolating "safe" parts of config (and
> > gitdir) from "unsafe" parts, e.g. "which configs and embedded scripts
> > are executables", to help better protect from zipfile-type attacks,
> > which are very similar to this kind of attack. I wonder if it makes
> > sense to consider that sort of thing as a needswork for this bugfix?
> > e.g. "/* NEEDSWORK: Only ignore unsafe configs and hooks instead of
> > ignoring the entire embedded config and hooks in the future */"?
>
> There have been such discussions in the past and they all went
> nowhere because such safe-listing fundamentally does not work.  What
> you consider "safe" today may turn out to be "unsafe" and in a later
> version of Git will stop honoring it, and for those who depended on
> it being listed as "safe", such a security fix will be a regression.
>
> Disabling the whole thing if the file can be tainted is the only
> sensible way forward without promising users that they will be hurt
> by such changes/regressions in the future, I would think.

I assume when Junio says "safe-listing" he is talking about your "which
configs and embedded scripts are executables". I have tossed that idea
around in my own head for a little while, and in addition to the points
that Junio refers to, I think that this could be confusing for users.

I worry about forcing the user to consider which parts of their
config and hooks are being read/ignored, and then re-interpret their
meaning in light of that.

That seems like it would be an unnecessarily tricky position to put
users in, and I think we could get around it by either reading the
config/hooks, or ignoring them entirely.

(It also seems error-prone to me: just trying to list which parts of our
config could lead to command-execution is challenging for me at least.
In addition to the ongoing maintenance cost, a clever attacker would
almost certainly be able to spot some obscure piece of config that we
didn't consider and then use it in their attack.)

Thanks,
Taylor