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

[Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>]

On 2020-10-22 14:57, Suzuki Poulose wrote:
> On 10/22/20 9:02 AM, Sai Prakash Ranjan wrote:
>> On 2020-10-21 15:38, Suzuki Poulose wrote:
>>> On 10/21/20 8:29 AM, Sai Prakash Ranjan wrote:
>>>> On 2020-10-20 21:40, Sai Prakash Ranjan wrote:
>>>>> On 2020-10-14 21:29, Sai Prakash Ranjan wrote:
>>>>>> 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.
>>>>>> 
>>>>> 
>>>>> I have stress tested with the original patch many times
>>>>> now, i.e., without a check for event->owner and is_kernel_event()
>>>>> and didn't observe any crash. Plus on ETR where this was already
>>>>> done, no crashes were reported till date and with ETF, the issue
>>>>> was quickly reproducible, so I am fairly confident that this
>>>>> doesn't just delay the original issue but actually fixes
>>>>> it. I will run an overnight test again to confirm this.
>>>>> 
>>>> 
>>>> I ran the overnight test which collected aroung 4G data(see below),
>>>> with the following small change to see if the two cases
>>>> (event->owner=NULL and is_kernel_event()) are triggered
>>>> with suggested changes and it didn't trigger at all.
>>>> Do we still need those additional checks?
>>>> 
>>> 
>>> Yes. Please see perf_event_create_kernel_event(), which is
>>> an exported function allowing any kernel code (including modules)
>>> to use the PMU (just like the userspace perf tool would do).
>>> Just because your use case doesn't trigger this (because
>>> you don't run something that can trigger this) doesn't mean
>>> this can't be triggered.
>>> 
>> 
>> Thanks for that pointer, I will add them in the next version.
>> 
> 
> And instead of redefining TASK_TOMBSTONE in the driver, you
> may simply use IS_ERR_OR_NULL(tsk) to cover both NULL case
> and kernel event.
> 

Ugh sorry, sent out v2 exporting is_kernel_event() before seeing
this comment, I will resend.

Thanks,
Sai

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