linux-kernel - Re: [PATCH] acpi: pci_root: fix NULL pointer deref after resume from suspend

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Date:	Tue, 29 Sep 2009 18:11:30 +0800
From:	Danny Feng <dfeng@...hat.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
CC:	Alex Chiang <achiang@...com>, lenb@...nel.org,
	bjorn.helgaas@...com, andrew.patterson@...com,
	jbarnes@...tuousgeek.org, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] acpi: pci_root: fix NULL pointer deref after resume from
 suspend

On 09/29/2009 06:50 AM, Rafael J. Wysocki wrote:
> On Tuesday 29 September 2009, Alex Chiang wrote:
>> * Rafael J. Wysocki<rjw@...k.pl>:
>>> On Monday 28 September 2009, Rafael J. Wysocki wrote:
>>>> On Monday 28 September 2009, Alex Chiang wrote:
>>>>> * Xiaotian Feng<dfeng@...hat.com>:
>>>>>> --- 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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