Re: [Qemu-devel] [Qemu-ppc] [PATCH] spapr: fix core unplug crash

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Jul 08, 2016 at 05:47:01PM +0200, Greg Kurz wrote:
> On Fri, 08 Jul 2016 15:12:07 +0200
> Greg Kurz <groug@kaod.org> wrote:
> 
> > If the host has 8 threads/core and the guest is started with:
> > 
> > -smp cores=1,threads=4,maxcpus=12
> > 
> > It is possible to crash QEMU by doing:
> > 
> > (qemu) device_add host-spapr-cpu-core,core-id=16,id=foo
> > (qemu) device_del foo
> > Segmentation fault
> > 
> > This is caused because spapr_core_unplug() assumes cpu_dt_id == core_id.
> > Even if it happens to be the case when the host and guest have the same
> > number of threads per core, it is conceptually wrong and we may pass a
> > bogus id to spapr_dr_connector_by_id() and spapr_core_release() crashes.
> > 
> > Let's use cc->core_id, which is the id that was used to create th DR
> > connector.
> 
> My bad, I got excited and pointed out the wrong culprit... it is cpu_index
> again of course ! Please find an updated explanation to be put in the
> changelog after "Segmentation fault":
> 
> ========================================================================
> This happens because spapr_core_unplug() assumes cpu_dt_id == core_id.
> As long as cpu_dt_id is derived from the non-table cpu_index, this is
> only true when you plug cores with contiguous ids.
> 
> It is safer to be consistent: the DR connector was created with an
> index that is immediately written to cc->core_id, and spapr_core_plug()
> also relies on cc->core_id.
> 
> Let's use it also in spapr_core_unplug().
> ========================================================================

Reworded in place, thanks.

> 
> > 
> > Signed-off-by: Greg Kurz <groug@kaod.org>
> > ---
> >  hw/ppc/spapr_cpu_core.c |    6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > diff --git a/hw/ppc/spapr_cpu_core.c b/hw/ppc/spapr_cpu_core.c
> > index 70b6b0b5ee17..106eaf45b399 100644
> > --- a/hw/ppc/spapr_cpu_core.c
> > +++ b/hw/ppc/spapr_cpu_core.c
> > @@ -126,11 +126,9 @@ static void spapr_core_release(DeviceState *dev, void *opaque)
> >  void spapr_core_unplug(HotplugHandler *hotplug_dev, DeviceState *dev,
> >                         Error **errp)
> >  {
> > -    sPAPRCPUCore *core = SPAPR_CPU_CORE(OBJECT(dev));
> > -    PowerPCCPU *cpu = POWERPC_CPU(core->threads);
> > -    int id = ppc_get_vcpu_dt_id(cpu);
> > +    CPUCore *cc = CPU_CORE(dev);
> >      sPAPRDRConnector *drc =
> > -        spapr_dr_connector_by_id(SPAPR_DR_CONNECTOR_TYPE_CPU, id);
> > +        spapr_dr_connector_by_id(SPAPR_DR_CONNECTOR_TYPE_CPU, cc->core_id);
> >      sPAPRDRConnectorClass *drck;
> >      Error *local_err = NULL;
> >  
> > 
> > 
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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