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

[Markus Armbruster <armbru@redhat.com>]

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.

> 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?