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

[Suzuki Poulose]

On 10/22/20 12:07 PM, Sai Prakash Ranjan wrote:
> On 2020-10-22 14:57, Suzuki Poulose wrote:
> > 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.
>
> Ugh sorry, sent out v2 exporting is_kernel_event() before seeing
> this comment, I will resend.

Saw that. I would say, wait until someone complains about that. If
people are Ok with exporting it, it is fine. I guess it will be useful.
You could fall back to this approach if there is resistance.

Cheers
Suzuki