transport: add a protocol-whitelist environment variable

If we are cloning an untrusted remote repository into a
sandbox, we may also want to fetch remote submodules in
order to get the complete view as intended by the other
side. However, that opens us up to attacks where a malicious
user gets us to clone something they would not otherwise
have access to (this is not necessarily a problem by itself,
but we may then act on the cloned contents in a way that
exposes them to the attacker).

Ideally such a setup would sandbox git entirely away from
high-value items, but this is not always practical or easy
to set up (e.g., OS network controls may block multiple
protocols, and we would want to enable some but not others).

We can help this case by providing a way to restrict
particular protocols. We use a whitelist in the environment.
This is more annoying to set up than a blacklist, but
defaults to safety if the set of protocols git supports
grows). If no whitelist is specified, we continue to default
to allowing all protocols (this is an "unsafe" default, but
since the minority of users will want this sandboxing
effect, it is the only sensible one).

A note on the tests: ideally these would all be in a single
test file, but the git-daemon and httpd test infrastructure
is an all-or-nothing proposition rather than a test-by-test
prerequisite. By putting them all together, we would be
unable to test the file-local code on machines without
apache.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 20 insertions, 1 deletions

diff --git a/transport.c b/transport.c
index 88bde1d85e..94fe8658f2 100644
--- a/transport.c
+++ b/transport.c
@@ -909,6 +909,20 @@ static int external_specification_len(const char *url)
 	return strchr(url, ':') - url;
 }
 
+void transport_check_allowed(const char *type)
+{
+	struct string_list allowed = STRING_LIST_INIT_DUP;
+	const char *v = getenv("GIT_ALLOW_PROTOCOL");
+
+	if (!v)
+		return;
+
+	string_list_split(&allowed, v, ':', -1);
+	if (!unsorted_string_list_has_string(&allowed, type))
+		die("transport '%s' not allowed", type);
+	string_list_clear(&allowed, 0);
+}
+
 struct transport *transport_get(struct remote *remote, const char *url)
 {
 	const char *helper;
@@ -940,12 +954,14 @@ struct transport *transport_get(struct remote *remote, const char *url)
 	if (helper) {
 		transport_helper_init(ret, helper);
 	} else if (starts_with(url, "rsync:")) {
+		transport_check_allowed("rsync");
 		ret->get_refs_list = get_refs_via_rsync;
 		ret->fetch = fetch_objs_via_rsync;
 		ret->push = rsync_transport_push;
 		ret->smart_options = NULL;
 	} else if (url_is_local_not_ssh(url) && is_file(url) && is_bundle(url, 1)) {
 		struct bundle_transport_data *data = xcalloc(1, sizeof(*data));
+		transport_check_allowed("file");
 		ret->data = data;
 		ret->get_refs_list = get_refs_from_bundle;
 		ret->fetch = fetch_refs_from_bundle;
@@ -957,7 +973,10 @@ struct transport *transport_get(struct remote *remote, const char *url)
 		|| starts_with(url, "ssh://")
 		|| starts_with(url, "git+ssh://")
 		|| starts_with(url, "ssh+git://")) {
-		/* These are builtin smart transports. */
+		/*
+		 * These are builtin smart transports; "allowed" transports
+		 * will be checked individually in git_connect.
+		 */
 		struct git_transport_data *data = xcalloc(1, sizeof(*data));
 		ret->data = data;
 		ret->set_option = NULL;