hw/usb/hcd-dwc2: fix divide-by-zero in dwc2_handle_packet()

Check the value of mps to avoid potential divide-by-zero later in the function.
Since HCCHAR_MPS is guest controllable, this prevents a malicious/buggy guest
from crashing the QEMU process on the host.

Signed-off-by: Mauro Matteo Cascella <mcascell@redhat.com>
Reviewed-by: Paul Zimmerman <pauldzim@gmail.com>
Reported-by: Gaoning Pan <gaoning.pgn@antgroup.com>
Reported-by: Xingwei Lin <linyi.lxw@antfin.com>
Message-id: 20201015075957.268823-1-mcascell@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

hw/usb/hcd-dwc2.c

@@ -250,6 +250,12 @@ static void dwc2_handle_packet(DWC2State *s, uint32_t devadr, USBDevice *dev,
     trace_usb_dwc2_handle_packet(chan, dev, &p->packet, epnum, types[eptype],
                                  dirs[epdir], mps, len, pcnt);
 
+    if (mps == 0) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                "%s: Bad HCCHAR_MPS set to zero\n", __func__);
+        return;
+    }
+
     if (eptype == USB_ENDPOINT_XFER_CONTROL && pid == TSIZ_SC_MC_PID_SETUP) {
         pid = USB_TOKEN_SETUP;
     } else {