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

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Message-ID: <5a967265-b527-4ca0-bf63-f5f7cf9013a3@arm.com>
Date: Fri, 19 Dec 2025 11:48:35 +0000
From: Suzuki K Poulose <suzuki.poulose@....com>
To: Leo Yan <leo.yan@....com>
Cc: Ma Ke <make24@...as.ac.cn>, jie.gan@....qualcomm.com,
 james.clark@...aro.org, akpm@...ux-foundation.org,
 alexander.shishkin@...ux.intel.com, coresight@...ts.linaro.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 mathieu.poirier@...aro.org, mike.leach@...aro.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 RESEND] coresight: etm-perf: Fix reference count leak
 in etm_setup_aux

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