spapr: sanity check size of the CAS buffer

The CAS buffer is provided by SLOF. A broken SLOF could pass a silly
size: either smaller than the diff header, in which case the current
code will try to allocate 16 Exabytes of memory and g_malloc0() will
abort, or bigger than the maximum memory provisioned for SLOF (ie,
40 Megabytes), which doesn't make sense. Both cases indicate that
SLOF has a bug.

Let's print out an explicit error message and exit since rebooting as
we do with other errors would only result in a reset loop.

Signed-off-by: Greg Kurz <groug@kaod.org>
[dwg: Fix format specifier that broke 32-bit builds]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

hw/ppc/spapr.c

@@ -819,6 +819,13 @@ int spapr_h_cas_compose_response(sPAPRMachineState *spapr,
         return 1;
     }
 
+    if (size < sizeof(hdr) || size > FW_MAX_SIZE) {
+        error_report("SLOF provided an unexpected CAS buffer size "
+                     TARGET_FMT_lu " (min: %zu, max: %u)",
+                     size, sizeof(hdr), FW_MAX_SIZE);
+        exit(EXIT_FAILURE);
+    }
+
     size -= sizeof(hdr);
 
     /* Create skeleton */