From a9e285856112e5e721b6a341d15437a164128b30 Mon Sep 17 00:00:00 2001 From: Laurent Pinchart Date: Fri, 11 Jul 2008 19:32:15 -0300 Subject: [PATCH] V4L/DVB (8257): uvcvideo: Fix possible AB-BA deadlock with videodev_lock and open_mutex The uvcvideo driver's uvc_v4l2_open() method is called from videodev's video_open() function, which means it is called with the videodev_lock mutex held. uvc_v4l2_open() then takes uvc_driver.open_mutex to check dev->state and avoid racing against a device disconnect, which means that open_mutex must nest inside videodev_lock. However uvc_disconnect() takes the open_mutex around setting dev->state and also around putting its device reference. However, if uvc_disconnect() ends up dropping the last reference, it will call uvc_delete(), which calls into the videodev code to unregister its device, and this will end up taking videodev_lock. This opens a (unlikely in practice) window for an AB-BA deadlock and also causes a lockdep warning because of the lock misordering. Fortunately there is no apparent reason to hold open_mutex when doing kref_put() in uvc_disconnect(): if uvc_v4l2_open() runs before the state is set to UVC_DEV_DISCONNECTED, then it will take another reference to the device and kref_put() won't call uvc_delete; if uvc_v4l2_open() runs after the state is set, it will run before uvc_delete(), see the state, and return immediately -- uvc_delete() does uvc_unregister_video() (and hence video_unregister_device(), which is synchronized with videodev_lock) as its first thing, so there is no risk of use-after-free in uvc_v4l2_open(). Bug diagnosed based on a lockdep warning reported by Romano Giannetti . Signed-off-by: Roland Dreier Signed-off-by: Laurent Pinchart Signed-off-by: Mauro Carvalho Chehab --- drivers/media/video/uvc/uvc_driver.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/drivers/media/video/uvc/uvc_driver.c b/drivers/media/video/uvc/uvc_driver.c index 4deb04b..f2b2983 100644 --- a/drivers/media/video/uvc/uvc_driver.c +++ b/drivers/media/video/uvc/uvc_driver.c @@ -1633,13 +1633,16 @@ static void uvc_disconnect(struct usb_interface *intf) * reference to the uvc_device instance after uvc_v4l2_open() received * the pointer to the device (video_devdata) but before it got the * chance to increase the reference count (kref_get). + * + * Note that the reference can't be released with the lock held, + * otherwise a AB-BA deadlock can occur with videodev_lock that + * videodev acquires in videodev_open() and video_unregister_device(). */ mutex_lock(&uvc_driver.open_mutex); - dev->state |= UVC_DEV_DISCONNECTED; - kref_put(&dev->kref, uvc_delete); - mutex_unlock(&uvc_driver.open_mutex); + + kref_put(&dev->kref, uvc_delete); } static int uvc_suspend(struct usb_interface *intf, pm_message_t message) -- 2.7.4