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

acm_port_shutdown() is called from the tty-port machinery whenever the
last user closes the port.  acm_disconnect() sets acm->disconnected and
calls tty_port_tty_vhangup(), which eventually triggers port_shutdown().
At that point acm_disconnect() has already called acm_poison_urbs() and
is about to free the control and read URBs.

However, acm_port_shutdown() unconditionally calls acm_unpoison_urbs()
and, when the ALWAYS_POLL_CTRL quirk is set, immediately re-submits the
ctrlurb and all read URBs.  If the concurrent acm_disconnect() then
frees those URBs and their DMA buffers while they are in-flight, the
result is a use-after-free in acm_ctrl_irq() or acm_read_bulk_callback()
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.

Fixes: f58752ebcb ("USB: cdc-acm: Add quirks for Yoga Book 9 14IAH10 INGENIC touchscreen")
Cc: stable <stable@kernel.org>
Signed-off-by: Yuchao Zhang <ndaugoing@gmail.com>
Link: https://patch.msgid.link/20260918114309.3659635-2-ndaugoing@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -787,6 +787,9 @@ static void acm_port_shutdown(struct tty_port *port)
 		usb_autopm_put_interface_async(acm->control);
 	}
 
+	if (acm->disconnected)
+		return;
+
 	acm_unpoison_urbs(acm);
 
 	if (acm->quirks & ALWAYS_POLL_CTRL) {