xen-netfront: Fix race between device setup and open

Web:        https://git.kernel.org/torvalds/c/f599c64fdf7d9c108e8717fb04bc41c680120da4
Commit:     f599c64fdf7d9c108e8717fb04bc41c680120da4
Parent:     3ac7292a25db1c607a50752055a18aba32ac2176
Refname:    refs/heads/master
Author:     Ross Lagerwall <[email protected]>
AuthorDate: Thu Jan 11 09:36:38 2018 +0000
Committer:  Juergen Gross <[email protected]>
CommitDate: Tue Feb 6 09:55:40 2018 +0100

    xen-netfront: Fix race between device setup and open
    
    When a netfront device is set up it registers a netdev fairly early on,
    before it has set up the queues and is actually usable. A userspace tool
    like NetworkManager will immediately try to open it and access its state
    as soon as it appears. The bug can be reproduced by hotplugging VIFs
    until the VM runs out of grant refs. It registers the netdev but fails
    to set up any queues (since there are no more grant refs). In the
    meantime, NetworkManager opens the device and the kernel crashes trying
    to access the queues (of which there are none).
    
    Fix this in two ways:
    * For initial setup, register the netdev much later, after the queues
    are setup. This avoids the race entirely.
    * During a suspend/resume cycle, the frontend reconnects to the backend
    and the queues are recreated. It is possible (though highly unlikely) to
    race with something opening the device and accessing the queues after
    they have been destroyed but before they have been recreated. Extend the
    region covered by the rtnl semaphore to protect against this race. There
    is a possibility that we fail to recreate the queues so check for this
    in the open function.
    
    Signed-off-by: Ross Lagerwall <[email protected]>
    Reviewed-by: Boris Ostrovsky <[email protected]>
    Signed-off-by: Juergen Gross <[email protected]>
---
 drivers/net/xen-netfront.c | 46 ++++++++++++++++++++++++----------------------
 1 file changed, 24 insertions(+), 22 deletions(-)

diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
index 9bd7ddeeb6a5..8328d395e332 100644
--- a/drivers/net/xen-netfront.c
+++ b/drivers/net/xen-netfront.c
@@ -351,6 +351,9 @@ static int xennet_open(struct net_device *dev)
 	unsigned int i = 0;
 	struct netfront_queue *queue = NULL;
 
+	if (!np->queues)
+		return -ENODEV;
+
 	for (i = 0; i < num_queues; ++i) {
 		queue = &np->queues[i];
 		napi_enable(&queue->napi);
@@ -1358,18 +1361,8 @@ static int netfront_probe(struct xenbus_device *dev,
 #ifdef CONFIG_SYSFS
 	info->netdev->sysfs_groups[0] = &xennet_dev_group;
 #endif
-	err = register_netdev(info->netdev);
-	if (err) {
-		pr_warn("%s: register_netdev err=%d\n", __func__, err);
-		goto fail;
-	}
 
 	return 0;
-
- fail:
-	xennet_free_netdev(netdev);
-	dev_set_drvdata(&dev->dev, NULL);
-	return err;
 }
 
 static void xennet_end_access(int ref, void *page)
@@ -1737,8 +1730,6 @@ static void xennet_destroy_queues(struct netfront_info *info)
 {
 	unsigned int i;
 
-	rtnl_lock();
-
 	for (i = 0; i < info->netdev->real_num_tx_queues; i++) {
 		struct netfront_queue *queue = &info->queues[i];
 
@@ -1747,8 +1738,6 @@ static void xennet_destroy_queues(struct netfront_info *info)
 		netif_napi_del(&queue->napi);
 	}
 
-	rtnl_unlock();
-
 	kfree(info->queues);
 	info->queues = NULL;
 }
@@ -1764,8 +1753,6 @@ static int xennet_create_queues(struct netfront_info *info,
 	if (!info->queues)
 		return -ENOMEM;
 
-	rtnl_lock();
-
 	for (i = 0; i < *num_queues; i++) {
 		struct netfront_queue *queue = &info->queues[i];
 
@@ -1774,7 +1761,7 @@ static int xennet_create_queues(struct netfront_info *info,
 
 		ret = xennet_init_queue(queue);
 		if (ret < 0) {
-			dev_warn(&info->netdev->dev,
+			dev_warn(&info->xbdev->dev,
 				 "only created %d queues\n", i);
 			*num_queues = i;
 			break;
@@ -1788,10 +1775,8 @@ static int xennet_create_queues(struct netfront_info *info,
 
 	netif_set_real_num_tx_queues(info->netdev, *num_queues);
 
-	rtnl_unlock();
-
 	if (*num_queues == 0) {
-		dev_err(&info->netdev->dev, "no queues\n");
+		dev_err(&info->xbdev->dev, "no queues\n");
 		return -EINVAL;
 	}
 	return 0;
@@ -1828,6 +1813,7 @@ static int talk_to_netback(struct xenbus_device *dev,
 		goto out;
 	}
 
+	rtnl_lock();
 	if (info->queues)
 		xennet_destroy_queues(info);
 
@@ -1838,6 +1824,7 @@ static int talk_to_netback(struct xenbus_device *dev,
 		info->queues = NULL;
 		goto out;
 	}
+	rtnl_unlock();
 
 	/* Create shared ring, alloc event channel -- for each queue */
 	for (i = 0; i < num_queues; ++i) {
@@ -1934,8 +1921,10 @@ abort_transaction_no_dev_fatal:
 	xenbus_transaction_end(xbt, 1);
  destroy_ring:
 	xennet_disconnect_backend(info);
+	rtnl_lock();
 	xennet_destroy_queues(info);
  out:
+	rtnl_unlock();
 	device_unregister(&dev->dev);
 	return err;
 }
@@ -1965,6 +1954,15 @@ static int xennet_connect(struct net_device *dev)
 	netdev_update_features(dev);
 	rtnl_unlock();
 
+	if (dev->reg_state == NETREG_UNINITIALIZED) {
+		err = register_netdev(dev);
+		if (err) {
+			pr_warn("%s: register_netdev err=%d\n", __func__, err);
+			device_unregister(&np->xbdev->dev);
+			return err;
+		}
+	}
+
 	/*
 	 * All public and private state should now be sane.  Get
 	 * ready to start sending and receiving packets and give the driver
@@ -2150,10 +2148,14 @@ static int xennet_remove(struct xenbus_device *dev)
 
 	xennet_disconnect_backend(info);
 
-	unregister_netdev(info->netdev);
+	if (info->netdev->reg_state == NETREG_REGISTERED)
+		unregister_netdev(info->netdev);
 
-	if (info->queues)
+	if (info->queues) {
+		rtnl_lock();
 		xennet_destroy_queues(info);
+		rtnl_unlock();
+	}
 	xennet_free_netdev(info->netdev);
 
 	return 0;
--
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