CVE-2025-21704

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

usb: cdc-acm: Check control transfer buffer size before access

If the first fragment is shorter than struct usb_cdc_notification, we can't

calculate an expected_size. Log an error and discard the notification

instead of reading lengths from memory outside the received data, which can

lead to memory corruption when the expected_size decreases between

fragments, causing `expected_size - acm->nb_index` to wrap.

This issue has been present since the beginning of git history; however,

it only leads to memory corruption since commit ea2583529cd1

("cdc-acm: reassemble fragmented notifications").

A mitigating factor is that acm_ctrl_irq() can only execute after userspace

has opened /dev/ttyACM*; but if ModemManager is running, ModemManager will

do that automatically depending on the USB device's vendor/product IDs and

its other interfaces.