hpet: fix buffer overrun on invalid state load

CVE-2013-4527 hw/timer/hpet.c buffer overrun

hpet is a VARRAY with a uint8 size but static array of 32

To fix, make sure num_timers is valid using VMSTATE_VALID hook.
Reported-by: NAnthony Liguori <anthony@codemonkey.ws>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

hw/timer/hpet.c

@@ -239,6 +239,18 @@ static int hpet_pre_load(void *opaque)
     return 0;
 }
 
+static bool hpet_validate_num_timers(void *opaque, int version_id)
+{
+    HPETState *s = opaque;
+
+    if (s->num_timers < HPET_MIN_TIMERS) {
+        return false;
+    } else if (s->num_timers > HPET_MAX_TIMERS) {
+        return false;
+    }
+    return true;
+}
+
 static int hpet_post_load(void *opaque, int version_id)
 {
     HPETState *s = opaque;
@@ -307,6 +319,7 @@ static const VMStateDescription vmstate_hpet = {
         VMSTATE_UINT64(isr, HPETState),
         VMSTATE_UINT64(hpet_counter, HPETState),
         VMSTATE_UINT8_V(num_timers, HPETState, 2),
+        VMSTATE_VALIDATE("num_timers in range", hpet_validate_num_timers),
         VMSTATE_STRUCT_VARRAY_UINT8(timer, HPETState, num_timers, 0,
                                     vmstate_hpet_timer, HPETTimer),
         VMSTATE_END_OF_LIST()