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

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

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 ? */

Suzuki