[PATCH] uio: Fix use-after-free in uio_open

[Guanghui Feng]

core-1					core-2
-------------------------------------------------------------
uio_unregister_device			uio_open
					idev = idr_find()
device_unregister(&idev->dev)
put_device(&idev->dev)
uio_device_release
					get_device(&idev->dev)
kfree(idev)
uio_free_minor(minor)
					uio_release
					put_device(&idev->dev)
					kfree(idev)
-------------------------------------------------------------

In the core-1 uio_unregister_device(), the device_unregister will kfree idev
when the idev->dev kobject ref is one. But after core-1 device_unregister,
put_device and before doing kfree, the core-2 may get_device. Then:
1. After core-1 kfree idev, the core-2 will do use-after-free for idev.
2. When core-2 do uio_release and put_device, the idev will be double freed.

To address this issue, we can get idev atomic & inc idev reference with minor_lock.

Signed-off-by: Guanghui Feng <guanghuifeng@xxxxxxxxxxxxxxxxx>
---
 drivers/uio/uio.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/uio/uio.c b/drivers/uio/uio.c
index 62082d6..2d572f6 100644
--- a/drivers/uio/uio.c
+++ b/drivers/uio/uio.c
@@ -466,13 +466,13 @@ static int uio_open(struct inode *inode, struct file *filep)
 
 	mutex_lock(&minor_lock);
 	idev = idr_find(&uio_idr, iminor(inode));
-	mutex_unlock(&minor_lock);
 	if (!idev) {
 		ret = -ENODEV;
+		mutex_unlock(&minor_lock);
 		goto out;
 	}
-
 	get_device(&idev->dev);
+	mutex_unlock(&minor_lock);
 
 	if (!try_module_get(idev->owner)) {
 		ret = -ENODEV;
@@ -1064,9 +1064,8 @@ void uio_unregister_device(struct uio_info *info)
 	wake_up_interruptible(&idev->wait);
 	kill_fasync(&idev->async_queue, SIGIO, POLL_HUP);
 
-	device_unregister(&idev->dev);
-
 	uio_free_minor(minor);
+	device_unregister(&idev->dev);
 
 	return;
 }
-- 
1.8.3.1