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

On 12/9/19 1:16 PM, Nuernberger, Stefan wrote:
> On Fri, 2019-12-06 at 15:15 -0500, Boris Ostrovsky wrote:
>> 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 <>
>>>>>   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).
> Well, the NULL ptr access causes an instant kernel panic whereas we
> have not attributed crashes to the possible UAF read until now.
>>>  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).
> Indeed. The xen_pcibk_quirks list does not have a separate lock to
> protect it. It's either modified under 'pcistub_devices_lock', from
> pcistub_remove(), or iterated over with the 'device_ids_lock' held in
> quirks_show(). Also the quirks list is amended from
>   pcistub_init_device()
>     -> xen_pcibk_config_init_dev()
>       -> xen_pcibk_config_quirks_init()
> without holding any lock at all. In fact the
> pcistub_init_devices_late() and pcistub_seize() functions deliberately
> release the pcistub_devices_lock before calling pcistub_init_device().
> This looks broken.


> The race is between
>   pcistub_remove()
>     -> pcistub_device_put()
>       -> pcistub_device_release()
> on one side and the quirks_show() on the other side. The problematic
> quirk is freed from the xen_pcibk_quirks list in pcistub_remove() early
> on under pcistub_devices_lock before the associated dev_data is freed
> eventually. So switching from device_ids_lock to pcistub_devices_lock
> in quirks_show() could be sufficient to always have valid dev_data for
> all quirks in the list.

Yes, that should do it. (I missed xen_pcibk_config_quirk_release() call,
which is why I wasn't sure pcistub_devices_lock is held where necessary).

> There is also pcistub_put_pci_dev() possibly in the race, called from
> xen_pcibk_remove_device(), or xen_pcibk_xenbus_remove(), or
> pcistub_remove(). The pcistub_remove() call site is safe when we switch
> to pcistub_devices_lock (same reasoning as above). For the others I
> currently do not see when the quirks are ever freed?

I wonder whether we should call xen_pcibk_config_quirk_release() from
pcistub_device_release() under pcistub_devices_lock.


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