setup.c: create `safe.bareRepository`

There is a known social engineering attack that takes advantage of the
fact that a working tree can include an entire bare repository,
including a config file. A user could run a Git command inside the bare
repository thinking that the config file of the 'outer' repository would
be used, but in reality, the bare repository's config file (which is
attacker-controlled) is used, which may result in arbitrary code
execution. See [1] for a fuller description and deeper discussion.

A simple mitigation is to forbid bare repositories unless specified via
`--git-dir` or `GIT_DIR`. In environments that don't use bare
repositories, this would be minimally disruptive.

Create a config variable, `safe.bareRepository`, that tells Git whether
or not to die() when working with a bare repository. This config is an
enum of:

- "all": allow all bare repositories (this is the default)
- "explicit": only allow bare repositories specified via --git-dir
  or GIT_DIR.

If we want to protect users from such attacks by default, neither value
will suffice - "all" provides no protection, but "explicit" is
impractical for bare repository users. A more usable default would be to
allow only non-embedded bare repositories ([2] contains one such
proposal), but detecting if a repository is embedded is potentially
non-trivial, so this work is not implemented in this series.

[1]: https://lore.kernel.org/git/kl6lsfqpygsj.fsf@chooglen-macbookpro.roam.corp.google.com
[2]: https://lore.kernel.org/git/5b969c5e-e802-c447-ad25-6acc0b784582@github.com

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 54 insertions, 0 deletions

diff --git a/t/t0035-safe-bare-repository.sh b/t/t0035-safe-bare-repository.sh
new file mode 100755
index 0000000000..ecbdc8238d
--- /dev/null
+++ b/t/t0035-safe-bare-repository.sh
@@ -0,0 +1,54 @@
+#!/bin/sh
+
+test_description='verify safe.bareRepository checks'
+
+TEST_PASSES_SANITIZE_LEAK=true
+. ./test-lib.sh
+
+pwd="$(pwd)"
+
+expect_accepted () {
+	git "$@" rev-parse --git-dir
+}
+
+expect_rejected () {
+	test_must_fail git "$@" rev-parse --git-dir 2>err &&
+	grep -F "cannot use bare repository" err
+}
+
+test_expect_success 'setup bare repo in worktree' '
+	git init outer-repo &&
+	git init --bare outer-repo/bare-repo
+'
+
+test_expect_success 'safe.bareRepository unset' '
+	expect_accepted -C outer-repo/bare-repo
+'
+
+test_expect_success 'safe.bareRepository=all' '
+	test_config_global safe.bareRepository all &&
+	expect_accepted -C outer-repo/bare-repo
+'
+
+test_expect_success 'safe.bareRepository=explicit' '
+	test_config_global safe.bareRepository explicit &&
+	expect_rejected -C outer-repo/bare-repo
+'
+
+test_expect_success 'safe.bareRepository in the repository' '
+	# safe.bareRepository must not be "explicit", otherwise
+	# git config fails with "fatal: not in a git directory" (like
+	# safe.directory)
+	test_config -C outer-repo/bare-repo safe.bareRepository \
+		all &&
+	test_config_global safe.bareRepository explicit &&
+	expect_rejected -C outer-repo/bare-repo
+'
+
+test_expect_success 'safe.bareRepository on the command line' '
+	test_config_global safe.bareRepository explicit &&
+	expect_accepted -C outer-repo/bare-repo \
+		-c safe.bareRepository=all
+'
+
+test_done