Messages in this thread | | | Date | Thu, 22 Oct 2020 13:32:29 +0530 | From | Sai Prakash Ranjan <> | Subject | Re: [PATCH 1/2] coresight: tmc-etf: Fix NULL ptr dereference in tmc_enable_etf_sink_perf() |
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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.
Thanks, Sai
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