submodule: add a failing test for a submodule escaping .git/modules

We should pretend such submdules do not exist as it can lead to RCE.

tests/submodule/escape.c

+#include "clar_libgit2.h"
+#include "posix.h"
+#include "path.h"
+#include "submodule_helpers.h"
+#include "fileops.h"
+#include "repository.h"
+
+static git_repository *g_repo = NULL;
+
+void test_submodule_escape__cleanup(void)
+{
+	cl_git_sandbox_cleanup();
+}
+
+#define EVIL_SM_NAME "../../modules/evil"
+
+static int find_evil(git_submodule *sm, const char *name, void *payload)
+{
+	int *foundit = (int *) payload;
+
+	GIT_UNUSED(sm);
+
+	if (!git__strcmp(EVIL_SM_NAME, name))
+		*foundit = true;
+
+	return 0;
+}
+
+void test_submodule_escape__from_gitdir(void)
+{
+	int foundit;
+	git_config *cfg;
+	git_submodule *sm;
+	git_buf buf = GIT_BUF_INIT;
+
+	g_repo = setup_fixture_submodule_simple();
+
+	cl_git_pass(git_buf_joinpath(&buf, git_repository_workdir(g_repo), ".gitmodules"));
+	cl_git_pass(git_config_open_ondisk(&cfg, git_buf_cstr(&buf)));
+
+	/* We don't have a function to rename a subsection so we do it manually */
+	cl_git_pass(git_submodule_lookup(&sm, g_repo, "testrepo"));
+	cl_git_pass(git_config_set_string(cfg, "submodule." EVIL_SM_NAME ".path", git_submodule_path(sm)));
+	cl_git_pass(git_config_set_string(cfg, "submodule." EVIL_SM_NAME ".url", git_submodule_url(sm)));
+	cl_git_pass(git_config_delete_entry(cfg, "submodule.testrepo.path"));
+	cl_git_pass(git_config_delete_entry(cfg, "submodule.testrepo.url"));
+	git_config_free(cfg);
+
+	/* We also need to update the value in the config */
+	cl_git_pass(git_repository_config__weakptr(&cfg, g_repo));
+	cl_git_pass(git_config_set_string(cfg, "submodule." EVIL_SM_NAME ".url", git_submodule_url(sm)));
+	cfg = NULL;
+
+	/* Find it all the different ways we know about it */
+	cl_git_fail_with(GIT_ENOTFOUND, git_submodule_lookup(&sm, g_repo, EVIL_SM_NAME));
+	cl_git_fail_with(GIT_ENOTFOUND, git_submodule_lookup(&sm, g_repo, "testrepo"));
+	foundit = 0;
+	cl_git_pass(git_submodule_foreach(g_repo, find_evil, &foundit));
+	cl_assert_equal_i(0, foundit);
+}