git_superproject: tell git that superproject is bare

The superproject is initialized as a bare repo in Superproject:_Init().
That means that later operations must treat it as a bare repository,
specifying the gitdir and setting 'bare' appropriately when launching
GitCommand()s. It's also OK not to specify cwd here because GitCommand()
will drop cwd if bare == True anyways.

With this change, it's possible to run `repo init` and `repo sync` with the
Git config 'safe.bareRepository' set to 'explicit'. This config strengthens
Git's security posture against embedded bare repository attacks like
https://github.com/justinsteven/advisories/blob/main/2022_git_buried_bare_repos_and_fsmonitor_various_abuses.md.

Bug: b/227257481
Change-Id: I954a64c6883d2ca2af9c603e7076fd83b52584e9
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/389794
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Jason R. Coombs <jaraco@google.com>
Tested-by: Emily Shaffer <emilyshaffer@google.com>
Reviewed-by: Emily Shaffer <emilyshaffer@google.com>
Commit-Queue: Jason R. Coombs <jaraco@google.com>

1 files changed, 7 insertions, 5 deletions

diff --git a/git_superproject.py b/git_superproject.py
index b80f0130..fca0e6f7 100644
--- a/git_superproject.py
+++ b/git_superproject.py
@@ -69,9 +69,9 @@ class UpdateProjectsResult(NamedTuple):
 class Superproject:
     """Get commit ids from superproject.
 
-    Initializes a local copy of a superproject for the manifest. This allows
-    lookup of commit ids for all projects. It contains _project_commit_ids which
-    is a dictionary with project/commit id entries.
+    Initializes a bare local copy of a superproject for the manifest. This
+    allows lookup of commit ids for all projects. It contains
+    _project_commit_ids which is a dictionary with project/commit id entries.
     """
 
     def __init__(
@@ -235,7 +235,8 @@ class Superproject:
         p = GitCommand(
             None,
             cmd,
-            cwd=self._work_git,
+            gitdir=self._work_git,
+            bare=True,
             capture_stdout=True,
             capture_stderr=True,
         )
@@ -271,7 +272,8 @@ class Superproject:
         p = GitCommand(
             None,
             cmd,
-            cwd=self._work_git,
+            gitdir=self._work_git,
+            bare=True,
             capture_stdout=True,
             capture_stderr=True,
         )