Re: [Qemu-ppc] [PATCH v3] ppc: add host-serial and host-model machine attributes

[Daniel P . Berrangé]

On Mon, Feb 18, 2019 at 12:38:11PM +0100, Greg Kurz wrote:
> On Mon, 18 Feb 2019 15:42:18 +0530
> P J P <address@hidden> wrote:
> 
> > From: Prasad J Pandit <address@hidden>
> > 
> > On ppc hosts, hypervisor shares following system attributes
> > 
> >   - /proc/device-tree/system-id
> >   - /proc/device-tree/model
> > 
> > with a guest. This could lead to information leakage and misuse.[*]
> > Add machine attributes to control such system information exposure
> > to a guest.
> > 
> > [*] https://wiki.openstack.org/wiki/OSSN/OSSN-0028
> > 
> > Reported-by: Daniel P. Berrangé <address@hidden>
> > Fix-suggested-by: Daniel P. Berrangé <address@hidden>
> > Signed-off-by: Prasad J Pandit <address@hidden>
> > ---
> >  hw/ppc/spapr.c         | 79 ++++++++++++++++++++++++++++++++++++++----
> >  include/hw/ppc/spapr.h |  2 ++
> >  2 files changed, 75 insertions(+), 6 deletions(-)
> > 
> > Update v3: move host-serial,host-model options to ppc sPAPR machine
> >   -> https://lists.gnu.org/archive/html/qemu-devel/2019-02/msg03182.html  
> > 
> > diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> > index 0942f35bf8..666e500376 100644
> > --- a/hw/ppc/spapr.c
> > +++ b/hw/ppc/spapr.c
> > @@ -1249,13 +1249,30 @@ static void *spapr_build_fdt(sPAPRMachineState 
> > *spapr,
> >       * Add info to guest to indentify which host is it being run on
> >       * and what is the uuid of the guest
> >       */
> > -    if (kvmppc_get_host_model(&buf)) {
> > -        _FDT(fdt_setprop_string(fdt, 0, "host-model", buf));
> > -        g_free(buf);
> > +    if (spapr->host_model && !g_str_equal(spapr->host_model, "none")) {
> > +        if (g_str_equal(spapr->host_model, "passthrough")) {
> > +            /* -M host-model=passthrough */
> > +            if (kvmppc_get_host_model(&buf)) {
> > +                _FDT(fdt_setprop_string(fdt, 0, "host-model", buf));
> > +                g_free(buf);
> > +            }
> > +        } else {
> > +            /* -M host-model=<user-string> */
> > +            _FDT(fdt_setprop_string(fdt, 0, "host-model", 
> > spapr->host_model));
> > +        }
> >      }
> > -    if (kvmppc_get_host_serial(&buf)) {
> > -        _FDT(fdt_setprop_string(fdt, 0, "host-serial", buf));
> > -        g_free(buf);
> > +
> > +    if (spapr->host_serial && !g_str_equal(spapr->host_serial, "none")) {
> > +        if (g_str_equal(spapr->host_serial, "passthrough")) {
> > +            /* -M host-serial=passthrough */
> > +            if (kvmppc_get_host_serial(&buf)) {
> > +                _FDT(fdt_setprop_string(fdt, 0, "host-serial", buf));
> > +                g_free(buf);
> > +            }
> > +        } else {
> > +            /* -M host-serial=<user-string> */
> > +            _FDT(fdt_setprop_string(fdt, 0, "host-serial", 
> > spapr->host_serial));
> > +        }
> >      }
> >  
> >      buf = qemu_uuid_unparse_strdup(&qemu_uuid);
> > @@ -3138,6 +3155,36 @@ static void spapr_set_ic_mode(Object *obj, const 
> > char *value, Error **errp)
> >      }
> >  }
> >  
> > +static char *spapr_get_host_model(Object *obj, Error **errp)
> > +{
> > +    sPAPRMachineState *spapr = SPAPR_MACHINE(obj);
> > +
> > +    return g_strdup(spapr->host_model);
> > +}
> > +
> > +static void spapr_set_host_model(Object *obj, const char *value, Error 
> > **errp)
> > +{
> > +    sPAPRMachineState *spapr = SPAPR_MACHINE(obj);
> > +
> > +    g_free(spapr->host_model);
> > +    spapr->host_model = g_strdup(value);
> > +}
> > +
> > +static char *spapr_get_host_serial(Object *obj, Error **errp)
> > +{
> > +    sPAPRMachineState *spapr = SPAPR_MACHINE(obj);
> > +
> > +    return g_strdup(spapr->host_serial);
> > +}
> > +
> > +static void spapr_set_host_serial(Object *obj, const char *value, Error 
> > **errp)
> > +{
> > +    sPAPRMachineState *spapr = SPAPR_MACHINE(obj);
> > +
> > +    g_free(spapr->host_serial);
> > +    spapr->host_serial = g_strdup(value);
> > +}
> > +
> >  static void spapr_instance_init(Object *obj)
> >  {
> >      sPAPRMachineState *spapr = SPAPR_MACHINE(obj);
> > @@ -3183,6 +3230,20 @@ static void spapr_instance_init(Object *obj)
> >      object_property_set_description(obj, "ic-mode",
> >                   "Specifies the interrupt controller mode (xics, xive, 
> > dual)",
> >                   NULL);
> > +
> > +    spapr->host_model = NULL;
> 
> This isn't needed since object_initialize_with_type() already takes care
> of zeroing the instance for us.
> 
> > +    object_property_add_str(obj, "host-model",
> > +        spapr_get_host_model, spapr_set_host_model,
> > +        &error_abort);
> > +    object_property_set_description(obj, "host-model",
> > +        "Set host's model-id to use - none|passthrough|string", 
> > &error_abort);
> > +
> > +    spapr->host_serial = NULL;
> 
> Same here.
> 
> > +    object_property_add_str(obj, "host-serial",
> > +        spapr_get_host_serial, spapr_set_host_serial,
> > +        &error_abort);
> > +    object_property_set_description(obj, "host-serial",
> > +        "Set host's system-id to use - none|passthrough|string", 
> > &error_abort);
> >  }
> >  
> >  static void spapr_machine_finalizefn(Object *obj)
> > @@ -4080,9 +4141,15 @@ DEFINE_SPAPR_MACHINE(4_0, "4.0", true);
> >  static void spapr_machine_3_1_class_options(MachineClass *mc)
> >  {
> >      sPAPRMachineClass *smc = SPAPR_MACHINE_CLASS(mc);
> > +    static GlobalProperty compat[] = {
> > +        { TYPE_SPAPR_MACHINE, "host-model", "passthrough" },
> > +        { TYPE_SPAPR_MACHINE, "host-serial", "passthrough" },
> > +    };
> >  
> 
> So... we don't fix the information leak for older machines by default ? From
> previous discussions, I understand it is for the sake of compatibility, but
> leaving the burden of securing the host to downstream or to the user still
> looks unsecure to me FWIW.

Maintaining guest ABI compatibility has to take priority, even over
fixing security issues, because we must never intentionally break
guest OS/applications by silently altering guest ABI. This is one of
the two reasons why machine type versioning exists (the other reason
being live migration data stream).

This is nothing new - we've done it before for security flaws where
a fix would involve changing guest ABI. This particular security flaw
is pretty minor compared to other cases that we've left unfixed by
default eg Meltdown / Spectre and is easily addressed by the user if
needed.

Regards,
Daniel
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