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

[Suzuki Poulose <suzuki.poulose@arm.com>]

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.

Cheers
Suzuki