[Bluetooth] Reject L2CAP connections on an insecure ACL link

The Security Mode 4 of the Bluetooth 2.1 specification has strict
authentication and encryption requirements. It is the initiators job
to create a secure ACL link. However in case of malicious devices, the
acceptor has to make sure that the ACL is encrypted before allowing
any kind of L2CAP connection. The only exception here is the PSM 1 for
the service discovery protocol, because that is allowed to run on an
insecure ACL link.

Previously it was enough to reject a L2CAP connection during the
connection setup phase, but with Bluetooth 2.1 it is forbidden to
do any L2CAP protocol exchange on an insecure link (except SDP).

The new hci_conn_check_link_mode() function can be used to check the
integrity of an ACL link. This functions also takes care of the cases
where Security Mode 4 is disabled or one of the devices is based on
an older specification.

Signed-off-by: Marcel Holtmann <[e-mail]>

include/net/bluetooth/hci_core.h

···
 void hci_conn_check_pending(struct hci_dev *hdev);
 
 struct hci_conn *hci_connect(struct hci_dev *hdev, int type, bdaddr_t *dst, __u8 auth_type);
+int hci_conn_check_link_mode(struct hci_conn *conn);
 int hci_conn_auth(struct hci_conn *conn);
 int hci_conn_encrypt(struct hci_conn *conn);
 int hci_conn_change_link_key(struct hci_conn *conn);

net/bluetooth/af_bluetooth.c

···
 #define BT_DBG(D...)
 #endif
 
-#define VERSION "2.12"
+#define VERSION "2.13"
 
 /* Bluetooth sockets */
 #define BT_MAX_PROTO	8

net/bluetooth/hci_conn.c

···
 }
 EXPORT_SYMBOL(hci_connect);
 
+/* Check link security requirement */
+int hci_conn_check_link_mode(struct hci_conn *conn)
+{
+	BT_DBG("conn %p", conn);
+
+	if (conn->ssp_mode > 0 && conn->hdev->ssp_mode > 0 &&
+					!(conn->link_mode & HCI_LM_ENCRYPT))
+		return 0;
+
+	return 1;
+}
+EXPORT_SYMBOL(hci_conn_check_link_mode);
+
 /* Authenticate remote device */
 int hci_conn_auth(struct hci_conn *conn)
 {

net/bluetooth/l2cap.c

···
 	struct l2cap_conn_req *req = (struct l2cap_conn_req *) data;
 	struct l2cap_conn_rsp rsp;
 	struct sock *sk, *parent;
-	int result, status = 0;
+	int result, status = L2CAP_CS_NO_INFO;
 
 	u16 dcid = 0, scid = __le16_to_cpu(req->scid);
-	__le16 psm  = req->psm;
+	__le16 psm = req->psm;
 
 	BT_DBG("psm 0x%2.2x scid 0x%4.4x", psm, scid);
 
···
 	if (!parent) {
 		result = L2CAP_CR_BAD_PSM;
 		goto sendresp;
+	}
+
+	/* Check if the ACL is secure enough (if not SDP) */
+	if (psm != cpu_to_le16(0x0001) &&
+				!hci_conn_check_link_mode(conn->hcon)) {
+		result = L2CAP_CR_SEC_BLOCK;
+		goto response;
 	}
 
 	result = L2CAP_CR_NO_MEM;
···
 			rsp.scid   = cpu_to_le16(l2cap_pi(sk)->dcid);
 			rsp.dcid   = cpu_to_le16(l2cap_pi(sk)->scid);
 			rsp.result = cpu_to_le16(result);
-			rsp.status = cpu_to_le16(0);
+			rsp.status = cpu_to_le16(L2CAP_CS_NO_INFO);
 			l2cap_send_cmd(conn, l2cap_pi(sk)->ident,
 					L2CAP_CONN_RSP, sizeof(rsp), &rsp);
 		}
···
 			rsp.scid   = cpu_to_le16(l2cap_pi(sk)->dcid);
 			rsp.dcid   = cpu_to_le16(l2cap_pi(sk)->scid);
 			rsp.result = cpu_to_le16(result);
-			rsp.status = cpu_to_le16(0);
+			rsp.status = cpu_to_le16(L2CAP_CS_NO_INFO);
 			l2cap_send_cmd(conn, l2cap_pi(sk)->ident,
 					L2CAP_CONN_RSP, sizeof(rsp), &rsp);
 		}