[PATCH 3.2 004/107] xen/gntdevt: Fix race condition in gntdev_release()

[Ben Hutchings]

3.2.72-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Marek Marczykowski-GÃrecki
 <marmarek@xxxxxxxxxxxxxxxxxxxxxx>

commit 30b03d05e07467b8c6ec683ea96b5bffcbcd3931 upstream.

While gntdev_release() is called the MMU notifier is still registered
and can traverse priv->maps list even if no pages are mapped (which is
the case -- gntdev_release() is called after all). But
gntdev_release() will clear that list, so make sure that only one of
those things happens at the same time.

Signed-off-by: Marek Marczykowski-GÃrecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
[bwh: Backported to 3.2:
 - Adjust context
 - gntdev_priv::lock is a spinlock rather than a mutex.  As gntdev_put_map()
   may sleep, we need to unlock inside the loop.]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -493,11 +493,15 @@ static int gntdev_release(struct inode *
 
 	pr_debug("priv %p\n", priv);
 
+	spin_lock(&priv->lock);
 	while (!list_empty(&priv->maps)) {
 		map = list_entry(priv->maps.next, struct grant_map, next);
 		list_del(&map->next);
+		spin_unlock(&priv->lock);
 		gntdev_put_map(map);
+		spin_lock(&priv->lock);
 	}
+	spin_unlock(&priv->lock);
 
 	if (use_ptemod)
 		mmu_notifier_unregister(&priv->mn, priv->mm);

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