submodule: allow only certain protocols for submodule fetches

Some protocols (like git-remote-ext) can execute arbitrary
code found in the URL. The URLs that submodules use may come
from arbitrary sources (e.g., .gitmodules files in a remote
repository). Let's restrict submodules to fetching from a
known-good subset of protocols.

Note that we apply this restriction to all submodule
commands, whether the URL comes from .gitmodules or not.
This is more restrictive than we need to be; for example, in
the tests we run:

  git submodule add ext::...

which should be trusted, as the URL comes directly from the
command line provided by the user. But doing it this way is
simpler, and makes it much less likely that we would miss a
case. And since such protocols should be an exception
(especially because nobody who clones from them will be able
to update the submodules!), it's not likely to inconvenience
anyone in practice.

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

git-submodule.sh

@@ -22,6 +22,15 @@ require_work_tree
 wt_prefix=$(git rev-parse --show-prefix)
 cd_to_toplevel
 
+# Restrict ourselves to a vanilla subset of protocols; the URLs
+# we get are under control of a remote repository, and we do not
+# want them kicking off arbitrary git-remote-* programs.
+#
+# If the user has already specified a set of allowed protocols,
+# we assume they know what they're doing and use that instead.
+: ${GIT_ALLOW_PROTOCOL=file:git:http:https:ssh}
+export GIT_ALLOW_PROTOCOL
+
 command=
 branch=
 force=

t/t5815-submodule-protos.sh

@@ -0,0 +1,43 @@
+#!/bin/sh
+
+test_description='test protocol whitelisting with submodules'
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-proto-disable.sh
+
+setup_ext_wrapper
+setup_ssh_wrapper
+
+test_expect_success 'setup repository with submodules' '
+	mkdir remote &&
+	git init remote/repo.git &&
+	(cd remote/repo.git && test_commit one) &&
+	# submodule-add should probably trust what we feed it on the cmdline,
+	# but its implementation is overly conservative.
+	GIT_ALLOW_PROTOCOL=ssh git submodule add remote:repo.git ssh-module &&
+	GIT_ALLOW_PROTOCOL=ext git submodule add "ext::fake-remote %S repo.git" ext-module &&
+	git commit -m "add submodules"
+'
+
+test_expect_success 'clone with recurse-submodules fails' '
+	test_must_fail git clone --recurse-submodules . dst
+'
+
+test_expect_success 'setup individual updates' '
+	rm -rf dst &&
+	git clone . dst &&
+	git -C dst submodule init
+'
+
+test_expect_success 'update of ssh allowed' '
+	git -C dst submodule update ssh-module
+'
+
+test_expect_success 'update of ext not allowed' '
+	test_must_fail git -C dst submodule update ext-module
+'
+
+test_expect_success 'user can override whitelist' '
+	GIT_ALLOW_PROTOCOL=ext git -C dst submodule update ext-module
+'
+
+test_done