Re: [PATCH] hw/acpi/pcihp: validate bsel property of the bus before unplugging device

[Ani Sinha]

On Tue, 24 Aug 2021, Igor Mammedov wrote:

> On Mon, 23 Aug 2021 19:06:47 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>
> > On Sat, Aug 21, 2021 at 08:35:35PM +0530, Ani Sinha wrote:
> > > Bsel property of the pci bus indicates whether the bus supports acpi 
> > > hotplug.
> > > We need to validate the presence of this property before performing any 
> > > hotplug
> > > related callback operations. Currently validation of the existence of this
> > > property was absent from acpi_pcihp_device_unplug_cb() function but is 
> > > present
> > > in other hotplug/unplug callback functions. Hence, this change adds the 
> > > missing
> > > check for the above function.
> > >
> > > Signed-off-by: Ani Sinha <ani@anisinha.ca>
> >
> > I queued this but I have a general question:
> I convinced myself that this patch is wrong, pls drop it.
>
> > are all these errors logged with LOG_GUEST_ERROR?
> > Because if not we have a security problem.
> > I also note that bsel is an internal property,
> > I am not sure we should be printing this to users,
> > it might just confuse them.
> >
> > Same question for all the other places validating bsel.
>
> Commit message misses reproducer/explanation about
> how it could be triggered?
>
> If it's actually reachable, from my point of view
> putting checks all through out call chain is not robust
> and it's easy to miss issues caused by invalid bsel.
> Instead of putting check all over the code, I'd
> check value on entry points (pci_read/pci_write)
> if code there is broken.
>
> >
> > > ---
> > >  hw/acpi/pcihp.c | 10 ++++++++--
> > >  1 file changed, 8 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/hw/acpi/pcihp.c b/hw/acpi/pcihp.c
> > > index 0fd0c1d811..9982815a87 100644
> > > --- a/hw/acpi/pcihp.c
> > > +++ b/hw/acpi/pcihp.c
> > > @@ -372,9 +372,15 @@ void acpi_pcihp_device_unplug_cb(HotplugHandler 
> > > *hotplug_dev, AcpiPciHpState *s,
> > >                                   DeviceState *dev, Error **errp)
> > >  {
> > >      PCIDevice *pdev = PCI_DEVICE(dev);
> > > +    int bsel = acpi_pcihp_get_bsel(pci_get_bus(pdev));
> > > +
> > > +    trace_acpi_pci_unplug(PCI_SLOT(pdev->devfn), bsel);
> > >
> > > -    trace_acpi_pci_unplug(PCI_SLOT(pdev->devfn),
> > > -                          acpi_pcihp_get_bsel(pci_get_bus(pdev)));
> > > +    if (bsel < 0) {
> > > +        error_setg(errp, "Unsupported bus. Bus doesn't have property '"
> > > +                   ACPI_PCIHP_PROP_BSEL "' set");
> > > +        return;
> > > +    }
>
> 1st:
>  Error here is useless. this path is triggered on guest
>  MMIO write and there is no consumer for error whatsoever.
>  If I recall correctly, in such cases we in PCIHP code we make
>  such access a silent NOP. And tracing is there for a us
>  to help figure out what's going on.
>
> 2nd:
>  if it got this far, it means a device on a bus with bsel
>  was found and we are completing cleanup. Error-ing out at
>  this point will leak acpi_index.

The above two points seems to apply in this case as well and so should we
do this?

diff --git a/hw/acpi/pcihp.c b/hw/acpi/pcihp.c
index 0fd0c1d811..c7692f5d5f 100644
--- a/hw/acpi/pcihp.c
+++ b/hw/acpi/pcihp.c
@@ -400,12 +400,6 @@ void acpi_pcihp_device_unplug_request_cb(HotplugHandler 
*hotplug_dev,

     trace_acpi_pci_unplug_request(bsel, slot);

-    if (bsel < 0) {
-        error_setg(errp, "Unsupported bus. Bus doesn't have property '"
-                   ACPI_PCIHP_PROP_BSEL "' set");
-        return;
-    }
-
     s->acpi_pcihp_pci_status[bsel].down |= (1U << slot);
     acpi_send_event(DEVICE(hotplug_dev), ACPI_PCI_HOTPLUG_STATUS);
 }


I wanted to check before sending out a formal patch. I like symmetric
code.