Re: [PATCH 2/2] perf: Add checks to prevent null ptr access

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Sep 17, 2026 at 01:10:15PM -0700, Belgaumkar, Vinay wrote:
> 
> On 9/17/2026 1:39 AM, Peter Zijlstra wrote:
> > On Fri, Sep 04, 2026 at 11:16:25AM -0700, Vinay Belgaumkar wrote:
> > > Sashiko recommended some additional checks to prevent null pointer
> > > access. Check for revoked states inside perf_event_read_local(), as
> > > the pmu event may have already been freed at this point. Add a null
> > > check inside __perf_event_read_cpu() as well before accessing the pmu
> > > ptr.
> > > 
> > > Cc: Dapeng Mi <dapeng1.mi@linux.intel.com>
> > > Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
> > > ---
> > >   kernel/events/core.c | 12 +++++++++++-
> > >   1 file changed, 11 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > > index 7777e82aad5e..059f82f0cadd 100644
> > > --- a/kernel/events/core.c
> > > +++ b/kernel/events/core.c
> > > @@ -4788,14 +4788,19 @@ static inline const struct cpumask *perf_scope_cpu_topology_cpumask(unsigned int
> > >   static int __perf_event_read_cpu(struct perf_event *event, int event_cpu)
> > >   {
> > > +	struct pmu *pmu = READ_ONCE(event->pmu);
> > >   	int local_cpu = smp_processor_id();
> > >   	u16 local_pkg, event_pkg;
> > >   	if ((unsigned)event_cpu >= nr_cpu_ids)
> > >   		return event_cpu;
> > > +	if (!pmu)
> > > +		return -ENODEV;
> > > +
> > >   	if (event->group_caps & PERF_EV_CAP_READ_SCOPE) {
> > > -		const struct cpumask *cpumask = perf_scope_cpu_topology_cpumask(event->pmu->scope, event_cpu);
> > > +		const struct cpumask *cpumask = perf_scope_cpu_topology_cpumask(pmu->scope,
> > > +										event_cpu);
> > >   		if (cpumask && cpumask_test_cpu(local_cpu, cpumask))
> > >   			return local_cpu;
> > > @@ -4917,6 +4922,11 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
> > >   		goto out;
> > >   	}
> > > +	if (READ_ONCE(event->state) <= PERF_EVENT_STATE_REVOKED) {
> > > +		ret = -ENODEV;
> > > +		goto out;
> > > +	}
> > > +
> > >   	/*
> > >   	 * Get the event CPU numbers, and adjust them to local if the event is
> > >   	 * a per-package event that can be read locally
> > I don't think any of this is right.
> > 
> > When unregistered, the event is de-scheduled, this means event->oncpu
> > will be -1, therefore __perf_event_read_cpu() will already exit early.
> > 
> > And perf_event_read_local() will then already do the right thing, by
> > returning the old value.
> > 
> > So AFAICT, there is nothing to fix here.
> 
> yeah, I think this was more of a defensive fix which Sashiko suggested-
> 
> CPU A                                                  CPU B
> 
> perf_event_read_local                     ...
> 
> __perf_event_read_cpu                  perf_pmu_unregister
> 
> I don't think it is easy to repro this situation, but there is a theoretical
> possibility of a race between these two functions. I don't think even the
> changes above can guarantee to work in any case. We can drop this second
> patch if that is the case.

perf_event_read_local() has IRQs disabled, perf_pmu_unregister() will
eventually have to de-schedule the event, which involves IPIs.

If you have IRQs disabled, those IPIs will wait.

IOW, as long as you have IRQs disabled, your event->oncpu is stable,
provided of course that event->cpu is the local CPU, otherwise having
called perf_event_read_local() was a bug in the first place.

Hmm... I think I see a problem though.

The verification of that last condition, it being a local event. That
uses event->cpu as argument to __perf_event_read_cpu(), and that *can*
indeed hit the pmu.

I'm thinking __pmu_detach_event() should probably clear
PERF_EV_CAP_READ_SCOPE or something from all the
event->{event,group}_caps fields.