Re: [PATCH] acpi: pci_root: fix NULL pointer deref after resume fromsuspend

[Danny Feng]

On 09/29/2009 06:50 AM, Rafael J. Wysocki wrote:
> On Tuesday 29 September 2009, Alex Chiang wrote:
> > * Rafael J. Wysocki<rjw@xxxxxxx>:
> > > On Monday 28 September 2009, Rafael J. Wysocki wrote:
> > > > On Monday 28 September 2009, Alex Chiang wrote:
> > > > > * Xiaotian Feng<dfeng@xxxxxxxxxx>:
> > > > > > --- a/drivers/acpi/pci_root.c
> > > > > > +++ b/drivers/acpi/pci_root.c
> > > > > > @@ -387,7 +387,11 @@ struct pci_dev *acpi_get_pci_dev(acpi_handle handle)
> > > > > >   		if (!pdev || hnd == handle)
> > > > > >   			break;
> > > > > >
> > > > > > -		pbus = pdev->subordinate;
> > > > > > +		if (pdev->subordinate)
> > > > > > +			pbus = pdev->subordinate;
> > > > > > +		else
> > > > > > +			pbus = pdev->bus;
> > > > > > +
> > > > >
> > > > > I'm a little confused by this. If we start from the PCI root
> > > > > bridge and walk back down the hierarchy, shouldn't everything
> > > > > between the root and the device be a P2P bridge?
> > > >
> > > > Well, if my reading of the code is correct, there's no guarantee that
> > > > pci_get_slot() will always return either the right device or a bridge.
> > >
> > > I should have been more precise.
> > >
> > > If devfn of node happens to be the same as devfn of a non-bridge device on
> > > pbus, the pci_get_slot() will return a valid pointer to it, but
> > > pdev->subordinate will be NULL.  Is it impossible for some reason?
> >
> > Hm, that's a good thought, but I'm still confused. Here's the
> > first part of the full function (acpi_get_pci_dev):
> >
> >          phandle = handle;
> >          while (!acpi_is_root_bridge(phandle)) {
> >                  node = kzalloc(sizeof(struct acpi_handle_node), GFP_KERNEL);
> >                  if (!node)
> >                          goto out;
> >
> >                  INIT_LIST_HEAD(&node->node);
> >                  node->handle = phandle;
> >                  list_add(&node->node,&device_list);
> >
> >                  status = acpi_get_parent(phandle,&phandle);
> >                  if (ACPI_FAILURE(status))
> >                          goto out;
> >          }
> >
> > phandle starts off as the input parameter, and we make successive
> > calls to acpi_get_parent() to walk up the ACPI device tree until
> > we get to a root bridge.
> >
> > My assumption here is that all those ACPI devices must be P2P
> > bridges.
> >
> >          root = acpi_pci_find_root(phandle);
> >          if (!root)
> >                  goto out;
> >
> >          pbus = root->bus;
> >
> > Now we've got an acpi_pci_root() which has a struct pci_bus, and
> > we can start walking back down the PCI tree. Except what we're
> > really doing is iterating across the device_list which we created
> > above.
> >
> > device_list should only contain P2P bridges, based on my
> > assumption above.
> >
> >          list_for_each_entry(node,&device_list, node) {
> >                  acpi_handle hnd = node->handle;
> >                  status = acpi_evaluate_integer(hnd, "_ADR", NULL,&adr);
> >                  if (ACPI_FAILURE(status))
> >                          goto out;
> >                  dev = (adr>>  16)&  0xffff;
> >                  fn  = adr&  0xffff;
> >
> >                  pdev = pci_get_slot(pbus, PCI_DEVFN(dev, fn));
> >                  if (!pdev || hnd == handle)
> >                          break;
> >
> >                  pbus = pdev->subordinate;
> >                  pci_dev_put(pdev);
> >          }
> >
> > The point you raise about collision between the devfn of 'node'
> > and some non-bridge device returned by pci_get_slot() seems like
> > it really shouldn't happen, because we evaluate _ADR for each
> > node on device_list, in the reverse order that we found them, and
> > based on my assumption, all those nodes should be bridges.
>
> You seem to be right, but if the Xiaotian's patch actually fixes the NULL
> pointer deref, one of the assumptions is clearly wrong.
>
> > I'm not saying that Xiaotian's patch is wrong. I'm saying I'd
> > like to be educated as to why my basic assumption was wrong,
> > because now you're making me think that this code is pretty
> > fragile. :-/
>
> Perhaps Xiaotian can add some printk()s on top of his patch that will show us
> exactly in what conditions pbus becomes NULL.
>
> Thanks,
> Rafael
Is there any cases that pdev->subordinate is NULL while pdev is bridge 
device?
From pci_slot.c::walk_p2p_bridge, there's code like following:

        dev = pci_get_slot(pci_bus, PCI_DEVFN(device, function));
        if (!dev || !dev->subordinate)
                goto out;

It looks like dev->subordinate can be NULL even if in p2p bridge, right?

Thanks
Xiaotian

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