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

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Message-ID: <20201022133414.GH2611@hirez.programming.kicks-ass.net>
Date:   Thu, 22 Oct 2020 15:34:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.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 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_.