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

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 18 Jul 2018 10:40:18 +0200
David Hildenbrand <david@redhat.com> wrote:

> On 18.07.2018 10:39, Christian Borntraeger wrote:
> > 
> > 
> > On 07/18/2018 10:24 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 make something like this:
> >>
> >> { "execute": "query-cpu-model-baseline",
> >>   "arguments" : { "modela": { "name": "z14-base", "props": {"esan3" : false}},
> >>                   "modelb": { "name": "z14"}} }
> >>
> >> Produce:
> >>
> >> {"return": {"model": {"name": "z900-base", "props": {"esan3": false}}}}
> >>
> >> Instead of segfaulting.
> >>
> >> This could of course be improved (e.g. to z14-base,esan3=false), however
> >> as this ususally won't happen, let's just avoid crashes.
> >>
> >> Signed-off-by: David Hildenbrand <david@redhat.com>
> >> ---
> >>  target/s390x/cpu_models.c | 6 ++++++
> >>  1 file changed, 6 insertions(+)
> >>
> >> diff --git a/target/s390x/cpu_models.c b/target/s390x/cpu_models.c
> >> index cfdbccf46d..13a5d4f095 100644
> >> --- a/target/s390x/cpu_models.c
> >> +++ b/target/s390x/cpu_models.c
> >> @@ -716,6 +716,12 @@ 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 - fallback to z900 */
> >> +    if (!model.def) {
> >> +        model.def = s390_find_cpu_def(0x2064, 7, 1, NULL);
> >> +    }
> >> +  
> > 
> > Is there a way to not even return z900 but retuning an empty model (e.g. no model that 
> > matches) ?  
> 
> An error would be an alternative.
> 
> 

An error looks a bit saner to me. As long as we avoid unexpected
segfaults :)