Messages in this thread | | | Date | Wed, 14 Oct 2020 21:29:13 +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 2020-10-14 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
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