Re: [Qemu-devel] [PATCH v2] s390x/cpumodel: fix segmentation fault when baselining models

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 07/18/2018 11:23 AM, David Hildenbrand wrote:
> 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>

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>

> ---
>  target/s390x/cpu_models.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/target/s390x/cpu_models.c b/target/s390x/cpu_models.c
> index cfdbccf46d..604898a882 100644
> --- 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);
>