s390x/cpumodel: fix segmentation fault when baselining models

Usually, when baselining two CPU models, whereby one of them has base
CPU features disabled (e.g. z14-base,msa=off), we fallback to an older
model that did not have these features in the base model. We always try to
create a "sane" CPU model (as far as possible), and one part of it is that
removing base features is no good and to be avoided.

Now, if we disable base features that were part of a z900, we're out of
luck. We won't find a CPU model and QEMU will segfault. This is a
scenario that should never happen in real life, but it can be used to
crash QEMU.

So let's properly report an error if we baseline e.g.:

{ "execute": "query-cpu-model-baseline",
  "arguments" : { "modela": { "name": "z14-base", "props": {"esan3" : false}},
                  "modelb": { "name": "z14"}} }

Instead of segfaulting.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180718092330.19465-1-david@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

diff --git a/target/s390x/cpu_models.c b/target/s390x/cpu_models.c
index cfdbccf46d0363efd6520cb8b84fb116edde3198..604898a882eb1f0e6db184e109db368ef56f1a64 100644 (file)
--- a/target/s390x/cpu_models.c
+++ b/target/s390x/cpu_models.c
@@ -716,6 +716,14 @@ CpuModelBaselineInfo *arch_query_cpu_model_baseline(CpuModelInfo *infoa,
 
     model.def = s390_find_cpu_def(cpu_type, max_gen, max_gen_ga,
                                   model.features);
+
+    /* models without early base features (esan3) are bad */
+    if (!model.def) {
+        error_setg(errp, "No compatible CPU model could be created as"
+                   " important base features are disabled");
+        return NULL;
+    }
+
     /* strip off features not part of the max model */
     bitmap_and(model.features, model.features, model.def->full_feat,
                S390_FEAT_MAX);