usb: cdc-acm: Fix handling of oversized fragments

If we receive an initial fragment of size 8 bytes which specifies a wLength
of 1 byte (so the reassembled message is supposed to be 9 bytes long), and
we then receive a second fragment of size 9 bytes (which is not supposed to
happen), we currently wrongly bypass the fragment reassembly code but still
pass the pointer to the acm->notification_buffer to
acm_process_notification().

Make this less wrong by always going through fragment reassembly when we
expect more fragments.

Before this patch, receiving an overlong fragment could lead to `newctrl`
in acm_process_notification() being uninitialized data (instead of data
coming from the device).

Cc: stable <[e-mail]>
Fixes: ea2583529cd1 ("cdc-acm: reassemble fragmented notifications")
Signed-off-by: Jann Horn <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/class/cdc-acm.c

···
 	expected_size = sizeof(struct usb_cdc_notification) +
 					le16_to_cpu(dr->wLength);
 
-	if (current_size < expected_size) {
+	if (acm->nb_index != 0 || current_size < expected_size) {
 		/* notification is transmitted fragmented, reassemble */
 		if (acm->nb_size < expected_size) {
 			u8 *new_buffer;