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.

Cc: stable <[e-mail]>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Jann Horn <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/class/cdc-acm.c

···
 static void acm_ctrl_irq(struct urb *urb)
 {
 	struct acm *acm = urb->context;
-	struct usb_cdc_notification *dr = urb->transfer_buffer;
+	struct usb_cdc_notification *dr;
 	unsigned int current_size = urb->actual_length;
 	unsigned int expected_size, copy_size, alloc_size;
 	int retval;
···
 
 	usb_mark_last_busy(acm->dev);
 
-	if (acm->nb_index)
+	if (acm->nb_index == 0) {
+		/*
+		 * The first chunk of a message must contain at least the
+		 * notification header with the length field, otherwise we
+		 * can't get an expected_size.
+		 */
+		if (current_size < sizeof(struct usb_cdc_notification)) {
+			dev_dbg(&acm->control->dev, "urb too short\n");
+			goto exit;
+		}
+		dr = urb->transfer_buffer;
+	} else {
 		dr = (struct usb_cdc_notification *)acm->notification_buffer;
-
+	}
 	/* size = notification-header + (optional) data */
 	expected_size = sizeof(struct usb_cdc_notification) +
 					le16_to_cpu(dr->wLength);