linux-kernel - Re: [PATCHv2 2/4] coresight: tmc-etf: Fix NULL ptr dereference in tmc_enable_etf_sink

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Date:   Thu, 22 Oct 2020 19:53:32 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        linux-arm-msm@...r.kernel.org, coresight@...ts.linaro.org,
        linux-kernel@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Ingo Molnar <mingo@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        linux-arm-kernel@...ts.infradead.org,
        Mike Leach <mike.leach@...aro.org>
Subject: Re: [PATCHv2 2/4] coresight: tmc-etf: Fix NULL ptr dereference in
 tmc_enable_etf_sink_perf()

On 2020-10-22 19:04, Peter Zijlstra wrote:
> On Thu, Oct 22, 2020 at 06:19:37PM +0530, Sai Prakash Ranjan wrote:
>> On 2020-10-22 17:02, Peter Zijlstra wrote:
>> > On Thu, Oct 22, 2020 at 04:27:52PM +0530, Sai Prakash Ranjan wrote:
>> >
>> > > Looking at the ETR and other places in the kernel, ETF and the
>> > > ETB are the only places trying to dereference the task(owner)
>> > > in tmc_enable_etf_sink_perf() which is also called from the
>> > > sched_in path as in the call trace.
>> >
>> > > @@ -391,6 +392,10 @@ static void *tmc_alloc_etf_buffer(struct
>> > > coresight_device *csdev,
>> > >  {
>> > >  	int node;
>> > >  	struct cs_buffers *buf;
>> > > +	struct task_struct *task = READ_ONCE(event->owner);
>> > > +
>> > > +	if (!task || is_kernel_event(event))
>> > > +		return NULL;
>> >
>> >
>> > This is *wrong*... why do you care about who owns the events?
>> >
>> 
>> The original issue was the owner being NULL and causing
>> a NULL pointer dereference. I did ask some time back
>> if it is valid for the owner to be NULL [1] and should
>> probably be handled in events core?
> 
> No, what I asked is why do you care about ->owner to begin with? That
> seems wrong. A driver should not touch ->owner _at_all_.
> 

Ah ok, so Suzuki explained that in other reply and if there is
some other better way?

Thanks,
Sai

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