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

[Greg Kurz <groug@kaod.org>]

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().
========================================================================

> 
> 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;
>  
> 
>