git submodule: update=!command

karaj,

'man git-submodule' contains mention (in one place) that:

    Setting the key submodule.$name.update to !command
    will cause command to be run.

This is not documented in 'man gitmodules' (which documents the other possible values for the 'update' key) nor in 'man git-config' which also mentions the 'update' key (but refers readers to the two other pages).

This feature is scary. The idea that arbitrary code could be executed on my machine when I run innocent-looking git commands, based on the content of the .gitmodules file is enough to give pause to anybody.

Fortunately, it seems that (for now?) this is not really the case. 'git submodule init' will copy the values of the 'update' key from .gitmodules to your local git config, but only if they are one of "none", "checkout", "merge" or "rebase".

So, I guess I'm asking two things.

The first is a question about git's basic policy with respect to things like this. I hope that it's safe to assume that running 'git' commands on repositories downloaded from potentially-hostile places will never result in the authors of those repositories being able to run code on my machine.

If that is true then, the second request would be to spell this out more explicitly in the relevant documentation. I'm happy to write a patch to do that, if it is deemed appropriate.

Thanks in advance.

Cheers

On Tue, Mar 17, 2015 at 03:28:57PM -0400, Ryan Lortie wrote:

Definitely, our policy is that downloading a git repository should not result in arbitrary code being run. If there is a case of that, it would be a serious security bug.

I am not an expert on submodules, but I think the security module there is:

  1. You can do whatever you like in submodule.*.update entries in
     .git/config, including arbitrary code. Nobody but the user can
     write to it.

  2. The submodule code may migrate entries from .gitmodules into
     .git/config, but does so with an allow-known-good whitelist (see
     git-submodule.sh lines 622-637).

So AFAICT there's no bug here, and the system is working as designed. It might be worth mentioning that restriction in the submodule documentation, if only to prevent non-malicious people from wondering why adding "!foo" does not work in .gitmodules.

Yeah, spelling out the security model more explicitly would be good. There is also some subtlety around hooks. Doing:

  git clone user@host:/path/to/repo.git local

should never run code controlled by "repo.git" as "user@host". But doing:

  ssh user@host 'cd /path/to/repo.git && git log'

will respect the .git/config in repo.git, which may include arbitrary commands.

-Peff

On Tue, Mar 17, 2015, at 15:50, Jeff King wrote:

Please see the attached patch.

Cheers

From a6d70056f21f039db8676f72a1deca00c39121bd Mon Sep 17 00:00:00 2001
From: Ryan Lortie <desrt@desrt.ca>
Date: Tue, 17 Mar 2015 16:29:51 -0400
Subject: [PATCH] docs: clarify !command submodule update policy

Clarify that the !command submodule update policy is not available from .gitmodules files.

This should help calm any fears people have about the security implications of this otherwise scary-looking feature.

Signed-off-by: Ryan Lortie <desrt@desrt.ca>
---
 Documentation/git-submodule.txt | 10 +++++++---
 Documentation/gitmodules.txt    | 11 ++++++++---
 2 files changed, 15 insertions(+), 6 deletions(-)

A little late to this thread

On Wed, Mar 18, 2015 at 8:50 AM, Jeff King <peff@peff.net> wrote:

Which was always the intention of the !command feature. It's for users who want to use additional git porcelains that need some help dealing with submodule updates (e.g stgit).

At the time the !command feature and copying of update config from .gitmodules slid past each other on the list. But out of that I think we got a much better handling that provides security and version compatibility.

Ryan Lortie <desrt@desrt.ca> writes:

Yes, that is deliberate, because you cannot use !command in .gitmodules that is tracked for security reasons.

With more recent versions of Git, namely, the versions after 30a52c1d (Merge branch 'ms/submodule-update-config-doc' into maint, 2015-03-13), the documentation pages already have updated descriptions around this area.

On Tue, Mar 17, 2015, at 16:49, Junio C Hamano wrote:

sigh.

That's what I get for forgetting to type 'git pull' before writing a patch.

Sorry for the noise!

Cheers

Ryan Lortie <desrt@desrt.ca> writes:

Nothing to apologise or sigh about. You re-confirmed that the old documentation was lacking, which led to an earlier discussion which in turn led to Michal to update the documentation. If you check the output from

    git diff 30a52c1d^ 30a52c1d

and find it appropriately address the problem you originally had, that would be wonderful, and if you can suggest further improvement, that is equally good.

Thanks for participating in our effort to collectively make Git better.

kara,

On Tue, Mar 17, 2015, at 17:05, Junio C Hamano wrote:

Indeed, the new version of the docs looks much better. I'm particularly happy about the change to the format to make it easier to visually scan for the possible update modes.

Cheers

On Wed, Mar 18, 2015 at 10:05 AM, Junio C Hamano <gitster@pobox.com> wrote:

I think 30a52c1d could be improved with the following snippet from Ryans patch.

"For security reasons, this feature is not supported from the `.gitmodules` file"

Or something along those lines.

On Wed, Mar 18, 2015 at 8:43 PM, Chris Packham <judge.packham@gmail.com> wrote:

Which is actually down the bottom if I take the time to read the whole diff.