From: Emily Shaffer <firstname.lastname@example.org>
To: Taylor Blau <email@example.com>
Cc: "Glen Choo" <firstname.lastname@example.org>,
"Git List" <email@example.com>,
"Johannes Schindelin" <Johannes.Schindelin@gmx.de>,
"Ævar Arnfjörð Bjarmason" <firstname.lastname@example.org>,
"Derrick Stolee" <email@example.com>,
"Junio C Hamano" <firstname.lastname@example.org>,
"brian m. carlson" <email@example.com>,
"Randall S. Becker" <firstname.lastname@example.org>
Subject: Re: Bare repositories in the working tree are a security risk
Date: Thu, 21 Apr 2022 11:29:15 -0700 [thread overview]
Message-ID: <CAJoAoZkf6VuAOwX9j8Zc0x4HqJRJ5zQgqfmu+8Zs1kVx88dGpg@mail.gmail.com> (raw)
On Thu, Apr 21, 2022 at 11:25 AM Emily Shaffer <email@example.com> wrote:
> On Fri, Apr 15, 2022 at 6:28 PM Taylor Blau <firstname.lastname@example.org> wrote:
> > On Fri, Apr 15, 2022 at 05:41:59PM -0700, Glen Choo wrote:
> > > * We want additional protection on the client besides `git fsck`; there are
> > > a few ways to do this:
> > I'm a little late to chime into the thread, but I appreciate you
> > summarizing some of the approaches discussed so far. Let me add my
> > thoughts on each of these in order:
> > > 1. Prevent checking out an embedded bare repo.
> > > 2. Detect if the bare repo is embedded and refuse to work with it.
> > > 3. Detect if the bare repo is embedded and do not read its config/hooks, but
> > > everything else still 'works'.
> > > 4. Don't detect bare repos.
> > > 5. Only detect bare repos that are named `.git` .
> > >
> > > (I've responded with my thoughts on each of these approaches in-thread).
> > 1. Likely disrupts too many legitimate workflows for us to adopt
> > without designing some way to declare an embedded bare repository
> > is "safe".
> > 2. Ditto.
> > 3. This seems the most promising approach so far. Similar to (1), I
> > would also want to make sure we provide an easy way to declare a
> > bare repository as "safe" in order to avoid permanently disrupting
> > valid workflows that have accumulated over the past >15 years.
> I'd like to think a little more about how we want to determine that a
> bare repo isn't embedded - wantonly scanning up the filesystem for any
> gitdir above the current one is really expensive. When I tried that
> approach for the purposes of including some shared config between
> superproject and submodules, it slowed down the Git test suite by
> something like 3-5x. So, I suppose that "we think this is bare" means
> refs/ and objects/ present in a directory that isn't named '.git/',
> and then we hunt anywhere above us for another .git/? Of course being
> careful not to accept another bare repo as the "parent" gitdir.
> Does it work for submodules? I suppose it works for non-bare
> submodules - and for bare submodules, e.g.
> 'submodule-having-project.git/modules/some-submodule/' we can rely on
> the user to turn that flag on if they want their submodule's config
> and hooks to be noticed from the gitdir. (From
> 'worktree-for-submodule-having-project/some-submodule' there is a
> '.git' pointer, so I'd expect things to work normally that way,
> As long as we are careful to avoid searching the filesystem in *every*
> case, this seems like a pretty reasonable approach to me.
> > 4. Seems like this approach is too heavy-handed.
> > 5. Ditto.
> > > With that in mind, here's what I propose we do next:
> > >
> > > * Merge the `git fsck` patch  if we think that it is useful despite the
> > > potentially huge number of false positives. That patch needs some fixing; I'll
> > > make the changes when I'm back.
> > If there are lots of false positives, we should consider downgrading the
> > severity of the proposed `EMBEDDED_BARE_REPO` fsck check to "info". I'm
> > not clear if there is another reason why this patch would have a
> > significant number of false positives (i.e., if the detection mechanism
> > is over-zealous).
> > But if not, and this does detect only legitimate embedded bare
> > repositories, we should use it as a reminder to consider how many
> > use-cases and workflows will be affected by whatever approach we take
> > here, if any.
> > > * I'll experiment with (5), and if it seems promising, I'll propose this as an
> > > opt-in feature, with the intent of making it opt-out in the future. We'll
> > > opt-into this at Google to help figure out if this is a good default or not.
> > >
> > > * If that direction turns out not to be promising, I'll pursue (1), since that
> > > is the only option that can be configured per-repo, which should hopefully
> > > minimize the fallout.
> > Here's an alternative approach, which I haven't seen discussed thus far:
> > When a bare repository is embedded in another repository, avoid reading
> > its config by default. This means that most Git commands will still
> > work, but without the possibility of running any "executable" portions
> > of the config. To opt-out (i.e., to allow legitimate use-cases to start
> > reading embedded bare repository config again), the embedding repository
> > would have to set a multi-valued `safe.embeddedRepo` configuration. This
> > would specify a list of paths relative to the embedding repository's
> > root of known-safe bare repositories.
> > I think there are a couple of desirable attributes of this approach:
> > - It minimally disrupts bare repositories, restricting the change to
> > only embedded repositories.
> > - It allows most Git commands to continue working as expected (modulo
> > reading the config), hopefully making the population of users whose
> > workflows will suddenly break pretty small.
> > - It requires the user to explicitly opt-in to the unsafe behavior,
> > because an attacker could not influence the embedding repository's
> > `safe.embeddedRepo` config.
> > If we were going to do something about this, I would strongly advocate
> > for something that resembles the above. Or at the very least, some
> > solution that captures the attributes I outlined there.
> Nice - a more strict spin on proposal 3 from above, if I understand it
> right. Rather than allowing any and all bare repos, avoid someone
> sneaking in a malicious one next to legitimate ones by using an
> allowlist. Seems reasonable to me.
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 */"?
> - Emily
next prev parent reply other threads:[~2022-04-21 18:29 UTC|newest]
Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-06 22:43 Bare repositories in the working tree are a security risk Glen Choo
2022-04-06 23:22 ` [PATCH] fsck: detect bare repos in trees and warn Glen Choo
2022-04-07 12:42 ` Johannes Schindelin
2022-04-07 13:21 ` Derrick Stolee
2022-04-07 14:14 ` Ævar Arnfjörð Bjarmason
2022-04-14 20:02 ` Glen Choo
2022-04-15 12:46 ` Ævar Arnfjörð Bjarmason
2022-04-07 15:11 ` Junio C Hamano
2022-04-13 22:24 ` Glen Choo
2022-04-07 13:12 ` Ævar Arnfjörð Bjarmason
2022-04-07 15:20 ` Junio C Hamano
2022-04-07 18:38 ` Bare repositories in the working tree are a security risk John Cai
2022-04-07 21:24 ` brian m. carlson
2022-04-07 21:53 ` Justin Steven
2022-04-07 22:10 ` brian m. carlson
2022-04-07 22:40 ` rsbecker
2022-04-08 5:54 ` Junio C Hamano
2022-04-14 0:03 ` Junio C Hamano
2022-04-14 0:04 ` Glen Choo
2022-04-13 23:44 ` Glen Choo
2022-04-13 20:37 ` Glen Choo
2022-04-13 23:36 ` Junio C Hamano
2022-04-14 16:41 ` Glen Choo
2022-04-14 17:35 ` Junio C Hamano
2022-04-14 18:19 ` Junio C Hamano
2022-04-15 21:33 ` Glen Choo
2022-04-15 22:17 ` Junio C Hamano
2022-04-16 0:52 ` Taylor Blau
2022-04-15 22:43 ` Glen Choo
2022-04-15 20:13 ` Junio C Hamano
2022-04-15 23:45 ` Glen Choo
2022-04-15 23:59 ` Glen Choo
2022-04-16 1:00 ` Taylor Blau
2022-04-16 1:18 ` Junio C Hamano
2022-04-16 1:30 ` Taylor Blau
2022-04-16 0:34 ` Glen Choo
2022-04-16 0:41 ` Glen Choo
2022-04-16 1:28 ` Taylor Blau
2022-04-21 18:25 ` Emily Shaffer
2022-04-21 18:29 ` Emily Shaffer [this message]
2022-04-21 18:47 ` Junio C Hamano
2022-04-21 18:54 ` Taylor Blau
2022-04-21 19:09 ` Taylor Blau
2022-04-21 21:01 ` Emily Shaffer
2022-04-21 21:22 ` Taylor Blau
2022-04-29 23:57 ` Glen Choo
2022-04-30 1:14 ` Taylor Blau
2022-05-02 19:39 ` Glen Choo
2022-05-02 14:05 ` Philip Oakley
2022-05-02 18:50 ` Junio C Hamano
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