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

commit e563b01208f4d1f609bcab13333b6c0e24ce6a01 upstream.

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 ea258352
("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 <stable@kernel.org>
Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-acm.c

@@ -360,7 +360,7 @@ static void acm_process_notification(struct acm *acm, unsigned char *buf)
 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;
@@ -387,9 +387,20 @@ static void acm_ctrl_irq(struct urb *urb)
 
 	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);