[LTP] [PATCH v2 4/4] syscalls/add_key03: new test for forging user keyrings

[Eric Biggers <ebiggers3@gmail.com>]

From: Eric Biggers <ebiggers@google.com>

Add a test for a bug which allowed a user to create the user and user
session keyrings for another user.

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 runtest/syscalls                              |  1 +
 testcases/kernel/syscalls/.gitignore          |  1 +
 testcases/kernel/syscalls/add_key/add_key03.c | 98 +++++++++++++++++++++++++++
 3 files changed, 100 insertions(+)
 create mode 100644 testcases/kernel/syscalls/add_key/add_key03.c

diff --git a/runtest/syscalls b/runtest/syscalls
index 822608613..e345325a2 100644
--- a/runtest/syscalls
+++ b/runtest/syscalls
@@ -13,6 +13,7 @@ acct01 acct01
 
 add_key01 add_key01
 add_key02 add_key02
+add_key03 add_key03
 
 adjtimex01 adjtimex01
 adjtimex02 adjtimex02
diff --git a/testcases/kernel/syscalls/.gitignore b/testcases/kernel/syscalls/.gitignore
index 8a2a8d21e..2a770a998 100644
--- a/testcases/kernel/syscalls/.gitignore
+++ b/testcases/kernel/syscalls/.gitignore
@@ -8,6 +8,7 @@
 /acct/acct01
 /add_key/add_key01
 /add_key/add_key02
+/add_key/add_key03
 /adjtimex/adjtimex01
 /adjtimex/adjtimex02
 /alarm/alarm01
diff --git a/testcases/kernel/syscalls/add_key/add_key03.c b/testcases/kernel/syscalls/add_key/add_key03.c
new file mode 100644
index 000000000..27edcb27f
--- /dev/null
+++ b/testcases/kernel/syscalls/add_key/add_key03.c
@@ -0,0 +1,98 @@
+/*
+ * Copyright (c) 2017 Google, Inc.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program, if not, see <http://www.gnu.org/licenses/>.
+ */
+
+/*
+ * Regression test for commit 237bbd29f7a0 ("KEYS: prevent creating a different
+ * user's keyrings").  The bug allowed any random user to create a keyring named
+ * "_uid.$UID" (or "_uid_ses.$UID"), and it would become the user keyring (or
+ * user session keyring) for user $UID, provided that it hadn't already been
+ * created.
+ *
+ * This test must be run as root so that it has permission to switch to another
+ * user ID and check whether the keyrings are wrong.  However, the underlying
+ * bug is actually reachable/exploitable by a non-root user.
+ */
+
+#include <errno.h>
+#include <pwd.h>
+#include <stdio.h>
+
+#include "tst_test.h"
+#include "lapi/keyctl.h"
+
+static key_serial_t create_keyring(const char *description)
+{
+	TEST(add_key("keyring", description, NULL, 0,
+		     KEY_SPEC_PROCESS_KEYRING));
+	if (TEST_RETURN < 0) {
+		tst_brk(TBROK | TTERRNO,
+			"unable to create keyring '%s'", description);
+	}
+	return TEST_RETURN;
+}
+
+static key_serial_t get_keyring_id(key_serial_t special_id)
+{
+	TEST(keyctl(KEYCTL_GET_KEYRING_ID, special_id, 1));
+	if (TEST_RETURN < 0) {
+		tst_brk(TBROK | TTERRNO,
+			"unable to get ID of keyring %d", special_id);
+	}
+	return TEST_RETURN;
+}
+
+static void do_test(void)
+{
+	uid_t uid = 1;
+	char description[32];
+	key_serial_t fake_user_keyring;
+	key_serial_t fake_user_session_keyring;
+
+	/*
+	 * We need a user to forge the keyrings for.  But the bug is not
+	 * reproducible for a UID which already has its keyrings, so find an
+	 * unused UID.  Note that it would be better to directly check for the
+	 * presence of the UID's keyrings than to search the passwd file.
+	 * However, that's not easy to do given that even if we assumed the UID
+	 * temporarily to check, KEYCTL_GET_KEYRING_ID for the user and user
+	 * session keyrings will create them rather than failing (even if the
+	 * 'create' argument is 0).
+	 */
+	while (getpwuid(uid))
+		uid++;
+
+	sprintf(description, "_uid.%u", uid);
+	fake_user_keyring = create_keyring(description);
+	sprintf(description, "_uid_ses.%u", uid);
+	fake_user_session_keyring = create_keyring(description);
+
+	SAFE_SETUID(uid);
+
+	if (fake_user_keyring == get_keyring_id(KEY_SPEC_USER_KEYRING))
+		tst_brk(TFAIL, "created user keyring for another user");
+
+	if (fake_user_session_keyring ==
+	    get_keyring_id(KEY_SPEC_USER_SESSION_KEYRING))
+		tst_brk(TFAIL, "created user session keyring for another user");
+
+	tst_res(TPASS, "expectedly could not create another user's keyrings");
+}
+
+static struct tst_test test = {
+	.test_all = do_test,
+	.needs_root = 1,
+};
-- 
2.15.0.rc0.271.g36b669edcc-goog