Bad behaving user-space USB applicatoins may crash the kernel by issuing
USB FS related ioctl(2)'s out of their expected order. By default
the USB FS ioctl(2) interface is only available to the
administrator, root, and driver applications like webcamd(8) needs
to be hijacked in order for this to happen.
The issue is that the fast-path code does not always see updates made
by the slow-path code, and may then work on freed memory.
This is easily fixed by using an EPOCH(9) type of synchronization
mechanism. A SX(9) lock will be used as a substitute for EPOCH(9),
due to the need for sleepability. In addition most calls going into
the fast-path originate from a single user-space process and the
need for multi-thread performance is not present.
MFC after: 1 week
Sponsored by: NVIDIA Networking