Messages in this thread | | | Subject | Re: [PATCH 1/2] coresight: tmc-etf: Fix NULL ptr dereference in tmc_enable_etf_sink_perf() | From | Suzuki K Poulose <> | Date | Wed, 14 Oct 2020 14:16:36 +0100 |
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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.
Kind regards Suzuki
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