linux-kernel - Re: [PATCH 1/2] coresight: tmc-etf: Fix NULL ptr dereference in tmc_enable_etf_sink

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Date:   Wed, 14 Oct 2020 15:06:28 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     mathieu.poirier@...aro.org, mike.leach@...aro.org,
        coresight@...ts.linaro.org, swboyd@...omium.org,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, denik@...gle.com,
        leo.yan@...aro.org, peterz@...radead.org
Subject: Re: [PATCH 1/2] coresight: tmc-etf: Fix NULL ptr dereference in
 tmc_enable_etf_sink_perf()

On 2020-10-13 22:05, Suzuki K Poulose wrote:
> On 10/07/2020 02:00 PM, Sai Prakash Ranjan wrote:
>> There was a report of NULL pointer dereference in ETF enable
>> path for perf CS mode with PID monitoring. It is almost 100%
>> reproducible when the process to monitor is something very
>> active such as chrome and with ETF as the sink and not ETR.
>> Currently in a bid to find the pid, the owner is dereferenced
>> via task_pid_nr() call in tmc_enable_etf_sink_perf() and with
>> owner being NULL, we get a NULL pointer dereference.
>> 
>> 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. Owner(task) is NULL even
>> in the case of ETR in tmc_enable_etr_sink_perf(), but since we
>> cache the PID in alloc_buffer() callback and it is done as part
>> of etm_setup_aux() when allocating buffer for ETR sink, we never
>> dereference this NULL pointer and we are safe. So lets do the
> 
> The patch is necessary to fix some of the issues. But I feel it is
> not complete. Why is it safe earlier and not later ? I believe we are
> simply reducing the chances of hitting the issue, by doing this earlier 
> than
> later. I would say we better fix all instances to make sure that the
> event->owner is valid. (e.g, I can see that the for kernel events
> event->owner == -1 ?)
> 
> struct task_struct *tsk = READ_ONCE(event->owner);
> 
> if (!tsk || is_kernel_event(event))
>    /* skip ? */
> 

Looking at it some more, is_kernel_event() is not exposed
outside events core and probably for good reason. Why do
we need to check for this and not just tsk?

Thanks,
Sai

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