[PATCH 3.16.y-ckt 127/130] xen/gntdevt: Fix race condition in gntdev_release()

[Luis Henriques]

3.16.7-ckt17 -stable review patch.  If anyone has any objections, please let me know.

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

From: =?UTF-8?q?Marek=20Marczykowski-G=C3=B3recki?=

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>
Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
 drivers/xen/gntdev.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index b7f028a8f542..91cc44611062 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -529,12 +529,14 @@ static int gntdev_release(struct inode *inode, struct file *flip)
 
 	pr_debug("priv %p\n", priv);
 
+	mutex_lock(&priv->lock);
 	while (!list_empty(&priv->maps)) {
 		map = list_entry(priv->maps.next, struct grant_map, next);
 		list_del(&map->next);
 		gntdev_put_map(NULL /* already removed */, map);
 	}
 	WARN_ON(!list_empty(&priv->freeable_maps));
+	mutex_unlock(&priv->lock);
 
 	if (use_ptemod)
 		mmu_notifier_unregister(&priv->mn, priv->mm);
--
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