linux-kernel - Re: [PATCH] xen/pciback: Prevent NULL pointer dereference in quirks

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Message-ID: <1575655787.7257.42.camel@amazon.de>
Date:   Fri, 6 Dec 2019 18:09:48 +0000
From:   "Nuernberger, Stefan" <snu@...zon.de>
To:     "boris.ostrovsky@...cle.com" <boris.ostrovsky@...cle.com>,
        "Nuernberger, Stefan" <snu@...zon.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "Seidel, Conny" <consei@...zon.de>,
        "jgross@...e.com" <jgross@...e.com>,
        "ross.lagerwall@...rix.com" <ross.lagerwall@...rix.com>,
        "Dannowski, Uwe" <uwed@...zon.de>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] xen/pciback: Prevent NULL pointer dereference in
 quirks_show

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@...zon.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@...zon.de>
> > Signed-off-by: Uwe Dannowski <uwed@...zon.de>
> > Signed-off-by: Stefan Nuernberger <snu@...zon.com>
> > 
> > Cc: xen-devel@...ts.xenproject.org
> > Cc: stable@...r.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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