setup: allow cwd=.git w/ bareRepository=explicit

The safe.bareRepository setting can be set to 'explicit' to disallow
implicit uses of bare repositories, preventing an attack [1] where an
artificial and malicious bare repository is embedded in another git
repository. Unfortunately, some tooling uses myrepo/.git/ as the cwd
when executing commands, and this is blocked when
safe.bareRepository=explicit. Blocking is unnecessary, as git already
prevents nested .git directories.

Teach git to not reject uses of git inside of the .git directory: check
if cwd is .git (or a subdirectory of it) and allow it even if
safe.bareRepository=explicit.

[1] https://github.com/justinsteven/advisories/blob/main/2022_git_buried_bare_repos_and_fsmonitor_various_abuses.md

Signed-off-by: Kyle Lippincott <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: spectral54

setup.c

@@ -1359,7 +1359,8 @@ static enum discovery_result setup_git_directory_gently_1(struct strbuf *dir,
 
 		if (is_git_directory(dir->buf)) {
 			trace2_data_string("setup", NULL, "implicit-bare-repository", dir->buf);
-			if (get_allowed_bare_repo() == ALLOWED_BARE_REPO_EXPLICIT)
+			if (get_allowed_bare_repo() == ALLOWED_BARE_REPO_EXPLICIT &&
+			    !ends_with_path_components(dir->buf, ".git"))
 				return GIT_DIR_DISALLOWED_BARE;
 			if (!ensure_valid_ownership(NULL, NULL, dir->buf, report))
 				return GIT_DIR_INVALID_OWNERSHIP;

t/t0035-safe-bare-repository.sh

@@ -78,4 +78,12 @@ test_expect_success 'no trace when GIT_DIR is explicitly provided' '
 	expect_accepted_explicit "$pwd/outer-repo/bare-repo"
 '
 
+test_expect_success 'no trace when "bare repository" is .git' '
+	expect_accepted_implicit -C outer-repo/.git
+'
+
+test_expect_success 'no trace when "bare repository" is a subdir of .git' '
+	expect_accepted_implicit -C outer-repo/.git/objects
+'
+
 test_done