NFC: reorder the logic in nfc_{un,}register_device

There is a potential UAF between the unregistration routine and the NFC
netlink operations.

The race that cause that UAF can be shown as below:

 (FREE)                      |  (USE)
nfcmrvl_nci_unregister_dev   |  nfc_genl_dev_up
  nci_close_device           |
  nci_unregister_device      |    nfc_get_device
    nfc_unregister_device    |    nfc_dev_up
      rfkill_destory         |
      device_del             |      rfkill_blocked
  ...                        |    ...

The root cause for this race is concluded below:
1. The rfkill_blocked (USE) in nfc_dev_up is supposed to be placed after
the device_is_registered check.
2. Since the netlink operations are possible just after the device_add
in nfc_register_device, the nfc_dev_up() can happen anywhere during the
rfkill creation process, which leads to data race.

This patch reorder these actions to permit
1. Once device_del is finished, the nfc_dev_up cannot dereference the
rfkill object.
2. The rfkill_register need to be placed after the device_add of nfc_dev
because the parent device need to be created first. So this patch keeps
the order but inject device_lock to prevent the data race.
Signed-off-by: Lin Ma <linma@zju.edu.cn>
Fixes: be055b2f ("NFC: RFKILL support")
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20211116152652.19217-1-linma@zju.edu.cnSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/nfc/core.c

@@ -94,13 +94,13 @@ int nfc_dev_up(struct nfc_dev *dev)
 
 	device_lock(&dev->dev);
 
-	if (dev->rfkill && rfkill_blocked(dev->rfkill)) {
-		rc = -ERFKILL;
+	if (!device_is_registered(&dev->dev)) {
+		rc = -ENODEV;
 		goto error;
 	}
 
-	if (!device_is_registered(&dev->dev)) {
-		rc = -ENODEV;
+	if (dev->rfkill && rfkill_blocked(dev->rfkill)) {
+		rc = -ERFKILL;
 		goto error;
 	}
 
@@ -1125,11 +1125,7 @@ int nfc_register_device(struct nfc_dev *dev)
 	if (rc)
 		pr_err("Could not register llcp device\n");
 
-	rc = nfc_genl_device_added(dev);
-	if (rc)
-		pr_debug("The userspace won't be notified that the device %s was added\n",
-			 dev_name(&dev->dev));
-
+	device_lock(&dev->dev);
 	dev->rfkill = rfkill_alloc(dev_name(&dev->dev), &dev->dev,
 				   RFKILL_TYPE_NFC, &nfc_rfkill_ops, dev);
 	if (dev->rfkill) {
@@ -1138,6 +1134,12 @@ int nfc_register_device(struct nfc_dev *dev)
 			dev->rfkill = NULL;
 		}
 	}
+	device_unlock(&dev->dev);
+
+	rc = nfc_genl_device_added(dev);
+	if (rc)
+		pr_debug("The userspace won't be notified that the device %s was added\n",
+			 dev_name(&dev->dev));
 
 	return 0;
 }
@@ -1154,10 +1156,17 @@ void nfc_unregister_device(struct nfc_dev *dev)
 
 	pr_debug("dev_name=%s\n", dev_name(&dev->dev));
 
+	rc = nfc_genl_device_removed(dev);
+	if (rc)
+		pr_debug("The userspace won't be notified that the device %s "
+			 "was removed\n", dev_name(&dev->dev));
+
+	device_lock(&dev->dev);
 	if (dev->rfkill) {
 		rfkill_unregister(dev->rfkill);
 		rfkill_destroy(dev->rfkill);
 	}
+	device_unlock(&dev->dev);
 
 	if (dev->ops->check_presence) {
 		device_lock(&dev->dev);
@@ -1167,11 +1176,6 @@ void nfc_unregister_device(struct nfc_dev *dev)
 		cancel_work_sync(&dev->check_pres_work);
 	}
 
-	rc = nfc_genl_device_removed(dev);
-	if (rc)
-		pr_debug("The userspace won't be notified that the device %s "
-			 "was removed\n", dev_name(&dev->dev));
-
 	nfc_llcp_unregister_device(dev);
 
 	mutex_lock(&nfc_devlist_mutex);