On 06/17/2016 12:08 AM, Sakari Ailus wrote: > Hi Shuah, > > On Fri, Jun 10, 2016 at 11:37:23AM -0600, Shuah Khan wrote: >> Media devnode open/ioctl could be in progress when media device unregister >> is initiated. System calls and ioctls check media device registered status >> at the beginning, however, there is a window where unregister could be in >> progress without changing the media devnode status to unregistered. >> >> process 1 process 2 >> fd = open(/dev/media0) >> media_devnode_is_registered() >> (returns true here) >> >> media_device_unregister() >> (unregister is in progress >> and devnode isn't >> unregistered yet) >> ... >> ioctl(fd, ...) >> __media_ioctl() >> media_devnode_is_registered() >> (returns true here) >> ... >> media_devnode_unregister() >> ... >> (driver releases the media device >> memory) >> >> media_device_ioctl() >> (By this point >> devnode->media_dev does not >> point to allocated memory. >> use-after free in in mutex_lock_nested) >> >> BUG: KASAN: use-after-free in mutex_lock_nested+0x79c/0x800 at addr >> ffff8801ebe914f0 >> >> Fix it by clearing register bit when unregister starts to avoid the race. > > Does this patch solve the problem? You'd have to take the mutex for the > duration of the IOCTL which I don't see the patch doing. > > Instead of serialising operations using mutexes, I believe a proper fix for > this is to take a reference to the data structures required. >
It fixes the problem Mauro and I have seen. It closes the window enough to avoid problem. We have disconnected data structure issue as you pointed out. media devnode register/unregister are protected by media_devnode_lock and the graph is protected by graph_mutex. I avoided taking the mutex for the entire duration of the ioctl. I think what you are suggesting is that the ioctl take a reference to media_device? One reason I avoided that is by doing that we will end up with 3 different objects with varied lifetimes dependent on each other. We have media_devnode and cdev dependency which handled by using devnode struct dev as cdev parent. It is possible to link media_device to devnode lifetime, by having media_ioctl or media_opne take reference to media_device. Is that what you have in mind? thanks, -- Shuah
