CVE-2026-98773: USB: cdc-acm: skip URB restart in port_shutdown if disconnected

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

USB: cdc-acm: skip URB restart in portshutdown if disconnected

acmportshutdown() is called from the tty-port machinery whenever the last user closes the port. acmdisconnect() sets acm->disconnected and calls ttyportttyvhangup(), which eventually triggers portshutdown(). At that point acmdisconnect() has already called acmpoisonurbs() and is about to free the control and read URBs.

However, acmportshutdown() unconditionally calls acmunpoisonurbs() and, when the ALWAYSPOLLCTRL quirk is set, immediately re-submits the ctrlurb and all read URBs. If the concurrent acmdisconnect() then frees those URBs and their DMA buffers while they are in-flight, the result is a use-after-free in acmctrlirq() or acmreadbulkcallback() accessing the freed DMA memory.

Fix by checking acm->disconnected after draining the delayed anchor and returning early: if we are in a disconnect path the URBs must remain poisoned and must not be resubmitted.

The early return is placed after the delayed-anchor drain loop because that loop only cancels write-side accounting (wb->use = false and autopm counter) and does not touch the hardware path.