Re: [Qemu-devel] [PATCH v3 1/6] pc: fix possible NULL pointer dereference in pc_machine_get_device_memory_region_size()

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 27 May 2019 18:36:25 +0200
Markus Armbruster <armbru@redhat.com> wrote:

> Igor Mammedov <imammedo@redhat.com> writes:
> 
> > QEMU will crash when device-memory-region-size property is read if ms->device_memory
> > wasn't initialized yet (ex: property being inspected during preconfig time).  
> 
> Reproduced:
> 
>     $ qemu-system-x86_64 -nodefaults -S -display none -preconfig -qmp stdio
>     {"QMP": {"version": {"qemu": {"micro": 50, "minor": 0, "major": 4}, "package": "v4.0.0-828-ga7b21f6762"}, "capabilities": ["oob"]}}
>     {"execute": "qmp_capabilities"}
>     {"return": {}}
>     {"execute": "qom-get", "arguments": {"path": "/machine", "property": "device-memory-region-size"}}
>     Segmentation fault (core dumped)
> 
> First time I started looking at this series, I went "I'll need a
> reproducer to fully understand what's up, and I don't feel like finding
> one now; next series, please".  Second time, I had to spend a few
> minutes on the reproducer.  Wasn't hard, since you provided a clue.
> Still: make review easy, include a reproducer whenever you can.

sure

> 
> > Instead of crashing return 0 if ms->device_memory hasn't been initialized.
> >
> > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > ---
> >  hw/i386/pc.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > index d98b737..de91e90 100644
> > --- a/hw/i386/pc.c
> > +++ b/hw/i386/pc.c
> > @@ -2461,7 +2461,11 @@ pc_machine_get_device_memory_region_size(Object *obj, Visitor *v,
> >                                           Error **errp)
> >  {
> >      MachineState *ms = MACHINE(obj);
> > -    int64_t value = memory_region_size(&ms->device_memory->mr);
> > +    int64_t value = 0;
> > +
> > +    if (ms->device_memory) {
> > +        memory_region_size(&ms->device_memory->mr);
> > +    }
> >  
> >      visit_type_int(v, name, &value, errp);
> >  }  
> 
> This makes qom-get return 0 for the size of memory that doesn't exist,
> yet.
> 
> A possible alternative would be setting an error.
> 
> Opinions?
We don't have a notion of property not set in QOM, so a code that
will receive a text based error will have to parse it (horrible idea)
to avoid generation of related ACPI parts.

In case of not enabled memory hotplug, PC_MACHINE_DEVMEM_REGION_SIZE == 0
is valid value and it's what's expected by other code.