RE: [PATCH 2/5] vmbus: add driver_override support

["Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>]

From: Stephen Hemminger <stephen@networkplumber.org>  Sent: Tuesday, August 14, 2018 9:35 AM

> On Mon, 13 Aug 2018 19:30:50 +0000
> "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com> wrote:
> 
> > > +/*
> > > + * Return a matching hv_vmbus_device_id pointer.
> > > + * If there is no match, return NULL.
> > > + */
> > > +static const struct hv_vmbus_device_id *hv_vmbus_get_id(struct hv_driver *drv,
> > > +							struct hv_device *dev)
> > > +{
> > > +	const uuid_le *guid = &dev->dev_type;
> > > +	const struct hv_vmbus_device_id *id;
> > >
> > > -	return NULL;
> > > +	/* When driver_override is set, only bind to the matching driver */
> > > +	if (dev->driver_override && strcmp(dev->driver_override, drv->name))
> > > +		return NULL;
> >
> > This function needs to be covered by the device lock, so that
> > dev->driver_override can't be set to NULL and the memory freed
> > during the above 'if' statement.  When called from vmbus_probe(),
> > the device lock is held, so it's good. But when called from
> > vmbus_match(), the device lock may not be held: consider the path
> > __driver_attach() -> driver_match_device() -> vmbus_match().
> 
> The function hv_vmbus_get_id is called from that path.
> i.e. __device_attach -> driver-match_device -> vmbus_match.
> and __device_attach always does:
> 	device_lock(dev);

Agreed.  The __device_attach() path holds the device lock and all is good.
But the __driver_attach() path does not hold the device lock when the
match function is called, leaving the code open to a potential race.  Same
problem could happen in the pci subsystem, so the issue is more generic
and probably should be evaluated and dealt with separately.

Michael

> 
> The code in driver _override_store uses the same device_lock
> when storing the new value.
> 
> This is same locking as is done in pci-sysfs.c