CVE-2022-49755

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

usb: gadget: ffs: Prevent race during ffsep0queuewait

While performing fast composition switch, there is a possibility that the process of ffsep0write/ffsep0read get into a race condition due to ep0req being freed up from functionfs_unbind.

Consider the scenario that the ffsep0write calls the ffsep0queuewait by taking a lock &ffs->ev.waitq.lock. However, the functionfsunbind isn't bounded so it can go ahead and mark the ep0req to NULL, and since there is no NULL check in ffsep0queue_wait we will end up in use-after-free.

Fix this by making a serialized execution between the two functions using a mutex_lock(ffs->mutex).