NFC: add NCI_UNREG flag to eliminate the race

There are two sites that calls queue_work() after the
destroy_workqueue() and lead to possible UAF.

The first site is nci_send_cmd(), which can happen after the
nci_close_device as below

nfcmrvl_nci_unregister_dev   |  nfc_genl_dev_up
  nci_close_device           |
    flush_workqueue          |
    del_timer_sync           |
  nci_unregister_device      |    nfc_get_device
    destroy_workqueue        |    nfc_dev_up
    nfc_unregister_device    |      nci_dev_up
      device_del             |        nci_open_device
                             |          __nci_request
                             |            nci_send_cmd
                             |              queue_work !!!

Another site is nci_cmd_timer, awaked by the nci_cmd_work from the
nci_send_cmd.

  ...                        |  ...
  nci_unregister_device      |  queue_work
    destroy_workqueue        |
    nfc_unregister_device    |  ...
      device_del             |  nci_cmd_work
                             |  mod_timer
                             |  ...
                             |  nci_cmd_timer
                             |    queue_work !!!

For the above two UAF, the root cause is that the nfc_dev_up can race
between the nci_unregister_device routine. Therefore, this patch
introduce NCI_UNREG flag to easily eliminate the possible race. In
addition, the mutex_lock in nci_close_device can act as a barrier.
Signed-off-by: Lin Ma <linma@zju.edu.cn>
Fixes: 6a2968aa ("NFC: basic NCI protocol implementation")
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20211116152732.19238-1-linma@zju.edu.cnSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/nfc/nci_core.h

@@ -30,6 +30,7 @@ enum nci_flag {
 	NCI_UP,
 	NCI_DATA_EXCHANGE,
 	NCI_DATA_EXCHANGE_TO,
+	NCI_UNREG,
 };
 
 /* NCI device states */

net/nfc/nci/core.c

@@ -476,6 +476,11 @@ static int nci_open_device(struct nci_dev *ndev)
 
 	mutex_lock(&ndev->req_lock);
 
+	if (test_bit(NCI_UNREG, &ndev->flags)) {
+		rc = -ENODEV;
+		goto done;
+	}
+
 	if (test_bit(NCI_UP, &ndev->flags)) {
 		rc = -EALREADY;
 		goto done;
@@ -548,6 +553,10 @@ static int nci_open_device(struct nci_dev *ndev)
 static int nci_close_device(struct nci_dev *ndev)
 {
 	nci_req_cancel(ndev, ENODEV);
+
+	/* This mutex needs to be held as a barrier for
+	 * caller nci_unregister_device
+	 */
 	mutex_lock(&ndev->req_lock);
 
 	if (!test_and_clear_bit(NCI_UP, &ndev->flags)) {
@@ -585,8 +594,8 @@ static int nci_close_device(struct nci_dev *ndev)
 
 	del_timer_sync(&ndev->cmd_timer);
 
-	/* Clear flags */
-	ndev->flags = 0;
+	/* Clear flags except NCI_UNREG */
+	ndev->flags &= BIT(NCI_UNREG);
 
 	mutex_unlock(&ndev->req_lock);
 
@@ -1269,6 +1278,12 @@ void nci_unregister_device(struct nci_dev *ndev)
 {
 	struct nci_conn_info *conn_info, *n;
 
+	/* This set_bit is not protected with specialized barrier,
+	 * However, it is fine because the mutex_lock(&ndev->req_lock);
+	 * in nci_close_device() will help to emit one.
+	 */
+	set_bit(NCI_UNREG, &ndev->flags);
+
 	nci_close_device(ndev);
 
 	destroy_workqueue(ndev->cmd_wq);