fsck: reject submodule.update = !command in .gitmodules

This allows hosting providers to detect whether they are being used
to attack users using malicious 'update = !command' settings in
.gitmodules.

Since ac1fbbda20 (submodule: do not copy unknown update mode from
.gitmodules, 2013-12-02), in normal cases such settings have been
treated as 'update = none', so forbidding them should not produce any
collateral damage to legitimate uses.  A quick search does not reveal
any repositories making use of this construct, either.

Reported-by: Joern Schneeweisz <jschneeweisz@gitlab.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

fsck.c

@@ -66,6 +66,7 @@ static struct oidset gitmodules_done = OIDSET_INIT;
 	FUNC(GITMODULES_SYMLINK, ERROR) \
 	FUNC(GITMODULES_URL, ERROR) \
 	FUNC(GITMODULES_PATH, ERROR) \
+	FUNC(GITMODULES_UPDATE, ERROR) \
 	/* warnings */ \
 	FUNC(BAD_FILEMODE, WARN) \
 	FUNC(EMPTY_NAME, WARN) \
@@ -975,6 +976,12 @@ static int fsck_gitmodules_fn(const char *var, const char *value, void *vdata)
 				    FSCK_MSG_GITMODULES_PATH,
 				    "disallowed submodule path: %s",
 				    value);
+	if (!strcmp(key, "update") && value &&
+	    parse_submodule_update_type(value) == SM_UPDATE_COMMAND)
+		data->ret |= report(data->options, data->obj,
+				    FSCK_MSG_GITMODULES_UPDATE,
+				    "disallowed submodule update setting: %s",
+				    value);
 	free(name);
 
 	return 0;

t/t7406-submodule-update.sh

@@ -414,6 +414,20 @@ test_expect_success 'submodule update - command in .gitmodules is rejected' '
 	test_must_fail git -C super submodule update submodule
 '
 
+test_expect_success 'fsck detects command in .gitmodules' '
+	git init command-in-gitmodules &&
+	(
+		cd command-in-gitmodules &&
+		git submodule add ../submodule submodule &&
+		test_commit adding-submodule &&
+
+		git config -f .gitmodules submodule.submodule.update "!false" &&
+		git add .gitmodules &&
+		test_commit configuring-update &&
+		test_must_fail git fsck
+	)
+'
+
 cat << EOF >expect
 Execution of 'false $submodulesha1' failed in submodule path 'submodule'
 EOF