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

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

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. 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?

-Stefan

> 
> -boris
> 
> > 
> >  			field = cfg_entry->field;



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