Re: [PATCH] xen/pciback: Prevent NULL pointer dereference in quirks_show

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 12/6/19 1:09 PM, Nuernberger, Stefan wrote:
> On Fri, 2019-12-06 at 10:11 -0500, Boris Ostrovsky wrote:
>> On 12/6/19 8:48 AM, Stefan Nuernberger wrote:
>>> From: Uwe Dannowski <uwed@amazon.de>
>>>
>>> Reading /sys/bus/pci/drivers/pciback/quirks while unbinding can
>>> result
>>> in dereferencing a NULL pointer. Instead, skip printing information
>>> about the dangling quirk.
>>>
>>> Reported-by: Conny Seidel <consei@amazon.de>
>>> Signed-off-by: Uwe Dannowski <uwed@amazon.de>
>>> Signed-off-by: Stefan Nuernberger <snu@amazon.com>
>>>
>>> Cc: xen-devel@lists.xenproject.org
>>> Cc: stable@vger.kernel.org
>>> ---
>>>  drivers/xen/xen-pciback/pci_stub.c | 2 ++
>>>  1 file changed, 2 insertions(+)
>>>
>>> diff --git a/drivers/xen/xen-pciback/pci_stub.c b/drivers/xen/xen-
>>> pciback/pci_stub.c
>>> index 097410a7cdb7..da725e474294 100644
>>> --- a/drivers/xen/xen-pciback/pci_stub.c
>>> +++ b/drivers/xen/xen-pciback/pci_stub.c
>>> @@ -1346,6 +1346,8 @@ static ssize_t quirks_show(struct
>>> device_driver *drv, char *buf)
>>>  				   quirk->devid.subdevice);
>>>  
>>>  		dev_data = pci_get_drvdata(quirk->pdev);
>>> +		if (!dev_data)
>>> +			continue;
>>>  
>>>  		list_for_each_entry(cfg_entry, &dev_data-
>>>> config_fields, list) {
>> Couldn't you have the same race here?
> Not quite the same, but it might not be entirely safe yet. The
> 'quirks_show' takes the 'device_ids_lock' and races with unbind /
> 'pcistub_device_release' "which takes device_lock mutex". So this might
> now be a UAF read access instead of a NULL pointer dereference.

Yes, that's what I meant (although I don't see much difference in this
context).

>  We have
> not observed adversarial effects in our testing (compared to the
> obvious issues with NULL pointer) but that's not a guarantee of course.
>
> So should quirks_show actually be protected by pcistub_devices_lock
> instead as are other functions that access dev_data? Does it need both
> locks in that case?

device_ids_lock protects device_ids list, which is not what you are
trying to access, so that doesn't look like right lock to hold. And
AFAICT pcistub_devices_lock is not held when device data is cleared in
pcistub_device_release() (which I think is where we are racing).

-boris