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, 56 insertions, 1 deletions

diff --git a/setup.c b/setup.c
index ec5b913..8c683e9 100644
--- a/setup.c
+++ b/setup.c
@@ -10,6 +10,10 @@
 static int inside_git_dir = -1;
 static int inside_work_tree = -1;
 static int work_tree_config_is_bogus;
+enum allowed_bare_repo {
+	ALLOWED_BARE_REPO_EXPLICIT = 0,
+	ALLOWED_BARE_REPO_ALL,
+};
 
 static struct startup_info the_startup_info;
 struct startup_info *startup_info = &the_startup_info;
@@ -1160,6 +1164,46 @@ static int ensure_valid_ownership(const char *gitfile,
 	return data.is_safe;
 }
 
+static int allowed_bare_repo_cb(const char *key, const char *value, void *d)
+{
+	enum allowed_bare_repo *allowed_bare_repo = d;
+
+	if (strcasecmp(key, "safe.bareRepository"))
+		return 0;
+
+	if (!strcmp(value, "explicit")) {
+		*allowed_bare_repo = ALLOWED_BARE_REPO_EXPLICIT;
+		return 0;
+	}
+	if (!strcmp(value, "all")) {
+		*allowed_bare_repo = ALLOWED_BARE_REPO_ALL;
+		return 0;
+	}
+	return -1;
+}
+
+static enum allowed_bare_repo get_allowed_bare_repo(void)
+{
+	enum allowed_bare_repo result = ALLOWED_BARE_REPO_ALL;
+	git_protected_config(allowed_bare_repo_cb, &result);
+	return result;
+}
+
+static const char *allowed_bare_repo_to_string(
+	enum allowed_bare_repo allowed_bare_repo)
+{
+	switch (allowed_bare_repo) {
+	case ALLOWED_BARE_REPO_EXPLICIT:
+		return "explicit";
+	case ALLOWED_BARE_REPO_ALL:
+		return "all";
+	default:
+		BUG("invalid allowed_bare_repo %d",
+		    allowed_bare_repo);
+	}
+	return NULL;
+}
+
 enum discovery_result {
 	GIT_DIR_NONE = 0,
 	GIT_DIR_EXPLICIT,
@@ -1169,7 +1213,8 @@ enum discovery_result {
 	GIT_DIR_HIT_CEILING = -1,
 	GIT_DIR_HIT_MOUNT_POINT = -2,
 	GIT_DIR_INVALID_GITFILE = -3,
-	GIT_DIR_INVALID_OWNERSHIP = -4
+	GIT_DIR_INVALID_OWNERSHIP = -4,
+	GIT_DIR_DISALLOWED_BARE = -5,
 };
 
 /*
@@ -1297,6 +1342,8 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir,
 		}
 
 		if (is_git_directory(dir->buf)) {
+			if (get_allowed_bare_repo() == ALLOWED_BARE_REPO_EXPLICIT)
+				return GIT_DIR_DISALLOWED_BARE;
 			if (!ensure_valid_ownership(NULL, NULL, dir->buf))
 				return GIT_DIR_INVALID_OWNERSHIP;
 			strbuf_addstr(gitdir, ".");
@@ -1443,6 +1490,14 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		}
 		*nongit_ok = 1;
 		break;
+	case GIT_DIR_DISALLOWED_BARE:
+		if (!nongit_ok) {
+			die(_("cannot use bare repository '%s' (safe.bareRepository is '%s')"),
+			    dir.buf,
+			    allowed_bare_repo_to_string(get_allowed_bare_repo()));
+		}
+		*nongit_ok = 1;
+		break;
 	case GIT_DIR_NONE:
 		/*
 		 * As a safeguard against setup_git_directory_gently_1 returning