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

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

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().
> ========================================================================
> 
> > 
> > Signed-off-by: Greg Kurz <groug@kaod.org>

Reviewed-by: Bharata B Rao <bharata@linux.vnet.ibm.com>

This prevents the crash, but unplug still fails and that will be fixed
only by having your patchset where device tree id is derived from
core index.

Regards,
Bharata.