Bug 2513449 (CVE-2026-68370)

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

usb: gadget: dummy_hcd: prevent fifo_req reuse during giveback

dummy_hcd embeds a single shared usb_request (dum->fifo_req) that the
"emulated single-request FIFO" fast-path in dummy_queue() reuses for
small IN transfers: it copies the caller's request into it
(req->req = *_req) and queues it, treating list_empty(&fifo_req.queue)
as "the slot is free".

The completion side (dummy_timer/transfer/nuke/dummy_dequeue) follows
the standard pattern: list_del_init(&req->queue) unlinks the request,
then the lock is dropped and usb_gadget_giveback_request() invokes
req->complete().  But list_del_init() makes fifo_req.queue look empty
*before* the completion callback returns, so a concurrent dummy_queue()
on another CPU sees the slot as free, reuses fifo_req and runs
req->req = *_req -- overwriting req->complete while dummy_timer is
mid-calling it.  The indirect call then jumps to a clobbered pointer,
causing a general protection fault / page fault in dummy_timer
(syzkaller extid faf3a6cf579fc65591ca).  The clobbering write is an
in-bounds memcpy on a live shared object, so KASAN cannot flag it.

Add a fifo_req_busy bit covering the shared request's whole lifetime:
set it in dummy_queue() when the FIFO fast-path takes fifo_req (making
it the fast-path guard, replacing the list_empty(&fifo_req.queue)
test), and clear it after the completion callback has returned, via a
dummy_giveback() helper used at all four gadget-request giveback
sites.  The shared slot can no longer be reused until its completion
callback has finished.