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

["Nuernberger, Stefan" <snu@amazon.de>]

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 <uwed@amazon.de>
> > > >  
> > > >  		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.

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?

- Stefan



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