References: bsc#959006 CVE-2015-8558

Make ehci_process_itd return an error in case we didn't do any actual
iso transfer because we've found no active transaction.  That'll avoid
ehci happily run in circles forever if the guest builds a loop out of
idts.

Reported-by: Qinghao Tang <address@hidden>
Tested-by: P J P <address@hidden>
Signed-off-by: Gerd Hoffmann <address@hidden>
---
 hw/usb/hcd-ehci.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

Index: xen-4.6.0-testing/tools/qemu-xen-dir-remote/hw/usb/hcd-ehci.c
===================================================================
--- xen-4.6.0-testing.orig/tools/qemu-xen-dir-remote/hw/usb/hcd-ehci.c
+++ xen-4.6.0-testing/tools/qemu-xen-dir-remote/hw/usb/hcd-ehci.c
@@ -1395,7 +1395,7 @@ static int ehci_process_itd(EHCIState *e
 {
     USBDevice *dev;
     USBEndpoint *ep;
-    uint32_t i, len, pid, dir, devaddr, endp;
+    uint32_t i, len, pid, dir, devaddr, endp, xfers = 0;
     uint32_t pg, off, ptr1, ptr2, max, mult;
 
     ehci->periodic_sched_active = PERIODIC_ACTIVE;
@@ -1485,9 +1485,10 @@ static int ehci_process_itd(EHCIState *e
                 ehci_raise_irq(ehci, USBSTS_INT);
             }
             itd->transact[i] &= ~ITD_XACT_ACTIVE;
+            xfers++;
         }
     }
-    return 0;
+    return xfers ? 0 : -1;
 }