AZL-66860

In the Linux kernel, the following vulnerability has been resolved:

comedi: fix race between polling and detaching

syzbot reports a use-after-free in comedi in the below link, which is due to comedi gladly removing the allocated async area even though poll requests are still active on the waitqueuehead inside of it. This can cause a use-after-free when the poll entries are later triggered or removed, as the memory for the waitqueuehead has been freed. We need to check there are no tasks queued on any of the subdevices' wait queues before allowing the device to be detached by the COMEDI_DEVCONFIG ioctl.

Tasks will read-lock dev->attach_lock before adding themselves to the subdevice wait queue, so fix the problem in the COMEDI_DEVCONFIG ioctl handler by write-locking dev->attach_lock before checking that all of the subdevices are safe to be deleted. This includes testing for any sleepers on the subdevices' wait queues. It remains locked until the device has been detached. This requires the comedi_device_detach() function to be refactored slightly, moving the bulk of it into new function comedi_device_detach_locked().

Note that the refactor of comedi_device_detach() results in comedi_device_cancel_all() now being called while dev->attach_lock is write-locked, which wasn't the case previously, but that does not matter.

Thanks to Jens Axboe for diagnosing the problem and co-developing this patch.