Re: [PATCH v2 RESEND] coresight: etm-perf: Fix reference count leak in etm_setup_aux

[Suzuki K Poulose]

On 19/12/2025 11:38, Leo Yan wrote:
> On Fri, Dec 19, 2025 at 09:59:54AM +0000, Suzuki K Poulose wrote:
>
> [...]
>
> > > diff --git a/drivers/hwtracing/coresight/coresight-platform.c b/drivers/hwtracing/coresight/coresight-platform.c
> > > index 0db64c5f4995..2b34f818ba88 100644
> > > --- a/drivers/hwtracing/coresight/coresight-platform.c
> > > +++ b/drivers/hwtracing/coresight/coresight-platform.c
> > > @@ -107,14 +107,16 @@ coresight_find_device_by_fwnode(struct fwnode_handle *fwnode)
> > >    	 * platform bus.
> > >    	 */
> > >    	dev = bus_find_device_by_fwnode(&platform_bus_type, fwnode);
> > > -	if (dev)
> > > -		return dev;
> > >    	/*
> > >    	 * We have a configurable component - circle through the AMBA bus
> > >    	 * looking for the device that matches the endpoint node.
> > >    	 */
> > > -	return bus_find_device_by_fwnode(&amba_bustype, fwnode);
> > > +	if (!dev)
> > > +		dev = bus_find_device_by_fwnode(&amba_bustype, fwnode);
> > > +
> > > +	put_device(dev);
> >
> > ^^ NAK, see below.
> >
> > > +	return dev;
> > >    }
> > >    /*
> > > @@ -274,7 +276,6 @@ static int of_coresight_parse_endpoint(struct device *dev,
> > >    	of_node_put(rparent);
> > >    	of_node_put(rep);
> > > -	put_device(rdev);
> >
> > This doesn't look good. We can't use the "dev" reliably without the
> > reference count. We are opening up use-after-free.
>
> My understanding is we don't grab a device from
> coresight_find_device_by_fwnode().  The callers only check whether the
> device is present on the bus; if it isn't, the driver defers probe.
>
> This is similiar to coresight_find_csdev_by_fwnode(), which calls
> put_device(dev) to release refcnt immediately.  This is why I
> suggested the change, so the two functions behave consistently.

I see, sorry. I saw some other uses of the device, but clearly I was 
wrong. May be we should simply re-structure the function to :

bool coresight_fwnode_device_present(fwnode)
{

	// find and drop the ref if required.
	return true/false;
}

The name "find_device_by_fwnode" and returning a freed reference doesn't
look good to me.

Suzuki

> Thanks,
> Leo