CVE-2024-36894: usb: gadget: f_fs: Fix race between aio_cancel() and AIO request complete

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

usb: gadget: ffs: Fix race between aiocancel() and AIO request complete

FFS based applications can utilize the aiocancel() callback to dequeue pending USB requests submitted to the UDC. There is a scenario where the FFS application issues an AIO cancel call, while the UDC is handling a soft disconnect. For a DWC3 based implementation, the callstack looks like the following:

DWC3 Gadget FFS Application dwc3gadgetsoftdisconnect() ... --> dwc3stopactivetransfers() --> dwc3gadgetgiveback(-ESHUTDOWN) --> ffsepfileasynciocomplete() ffsaiocancel() --> usbepfreerequest() --> usbepdequeue()

There is currently no locking implemented between the AIO completion handler and AIO cancel, so the issue occurs if the completion routine is running in parallel to an AIO cancel call coming from the FFS application. As the completion call frees the USB request (iodata->req) the FFS application is also referencing it for the usbepdequeue() call. This can lead to accessing a stale/hanging pointer.

commit b566d38857fc ("usb: gadget: ffs: use iodata->status consistently") relocated the usbepfreerequest() into ffsepfileasynciocomplete(). However, in order to properly implement locking to mitigate this issue, the spinlock can't be added to ffsepfileasynciocomplete(), as usbepdequeue() (if successfully dequeuing a USB request) will call the function driver's completion handler in the same context. Hence, leading into a deadlock.

Fix this issue by moving the usbepfreerequest() back to ffsusercopyworker(), and ensuring that it explicitly sets iodata->req to NULL after freeing it within the ffs->epslock. This resolves the race condition above, as the ffsaiocancel() routine will not continue attempting to dequeue a request that has already been freed, or the ffsusercopywork() not freeing the USB request until the AIO cancel is done referencing it.

This fix depends on commit b566d38857fc ("usb: gadget: ffs: use iodata->status consistently")