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

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Message-ID: <707b7860-0daa-d3e3-1f0f-17e1b05feae2@arm.com>
Date:   Wed, 14 Oct 2020 14:16:36 +0100
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     saiprakash.ranjan@...eaurora.org
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 10/14/2020 10:36 AM, Sai Prakash Ranjan wrote:
> 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?

Because the event->owner could be :

  = NULL
  = -1UL  // kernel event
  = valid.


Kind regards
Suzuki