[PATCH 1/2] coresight: tmc-etf: Fix NULL ptr dereference in tmc_enable_etf_sink_perf()

[Sai Prakash Ranjan]

On 2020-10-14 18:46, Suzuki K Poulose wrote:
> 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.
> 

Yes I understood that part, but here we were trying to
fix the NULL pointer dereference right and hence the
question as to why we need to check for kernel events?
I am no expert in perf but I don't see anywhere in the
kernel checking for is_kernel_event(), so I am a bit
skeptical if exporting that is actually right or not.

Thanks,
Sai

-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a 
member
of Code Aurora Forum, hosted by The Linux Foundation