[PATCH Next V2] media: mc: Clear minor number before put device

[Edward Adam Davis <eadavis@qq.com>]

syzbot report a slab-use-after-free in media_devnode_unregister.

The following calltrace shows the entire process of UAF generation:

hub_event()->
  port_event()->
    hub_port_connect_change()->
      hub_port_connect()->
        usb_disconnect()->
	  usb_disable_device()->
	    device_del()->
	      bus_remove_device()->
	        device_release_driver_internal()->
		  __device_release_driver()->
		    device_remove()->
		      usb_unbind_interface()->
		        em28xx_usb_disconnect()->
			  em28xx_release_resources()->
			    em28xx_unregister_media_device()->
			      media_device_unregister()->
			        media_devnode_unregister()->
				  put_device()->
				    media_devnode_release()->
				      kfree(devnode)
				  clear_bit(devnode->minor, media_devnode_nums) 

[1] kfree(devnode), after this code is executed, devnode is released.
[2] clear_bit(devnode->minor, media_devnode_nums), this accesses the
freed devnode, trigger uaf

We clear the device's minor num before freeing devnode to avoid a UAF.

Fixes: 9e14868dc952 ("media: mc: Clear minor number reservation at unregistration time")
Reported-by: syzbot+031d0cfd7c362817963f@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=031d0cfd7c362817963f
Tested-by: syzbot+031d0cfd7c362817963f@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
V1 -> V2: update comments for rootcause

 drivers/media/mc/mc-devnode.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/drivers/media/mc/mc-devnode.c b/drivers/media/mc/mc-devnode.c
index 0d01cbae98f2..6daa7aa99442 100644
--- a/drivers/media/mc/mc-devnode.c
+++ b/drivers/media/mc/mc-devnode.c
@@ -276,13 +276,10 @@ void media_devnode_unregister(struct media_devnode *devnode)
 	/* Delete the cdev on this minor as well */
 	cdev_device_del(&devnode->cdev, &devnode->dev);
 	devnode->media_dev = NULL;
+	clear_bit(devnode->minor, media_devnode_nums);
 	mutex_unlock(&media_devnode_lock);
 
 	put_device(&devnode->dev);
-
-	mutex_lock(&media_devnode_lock);
-	clear_bit(devnode->minor, media_devnode_nums);
-	mutex_unlock(&media_devnode_lock);
 }
 
 /*
-- 
2.43.0