Re: [PATCH v4] perf: Fix null pointer access in is_include_guest_event()

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

On 9/1/2026 7:50 AM, Vinay Belgaumkar wrote:
> A kernel panic is observed when an application has a perf connection open
> and the driver managing the pmu event unbinds. The issue occurs when
> perf_pmu_unregister() is called while there is an active perf session. The
> null pointer error is seen when the event is freed through __free_event()
> and mediated_pmu_unaccount_event is called. There is a missing null check
> inside is_include_guest_event(). Also call mediated_pmu_unaccount_event()
> inside __pmu_detach_event() similar to __free_event().
>
> Signature of the failure:
>
>  Oops: general protection fault, kernel NULL pointer dereference
>  ...
>  RIP: 0010:_free_event+0x3e/0x370
>  ...
>  Call Trace:
>  ...
>  perf_event_release_kernel+0x260/0x2d0
>  perf_release+0x12/0x20
>
> While here, also add a null check inside perf_event_read_local() which can
> cause a similar crash. Sashiko pointed out a possible race between when we
> check for revoked state here and then actually access event-pmu inside
> __perf_event_read_cpu(). So, add another null check there.
>
> v2: Add another check to prevent null access (sashiko)
> v3: Use READ_ONCE for checking pmu state (sashiko)
> v4: READ_ONCE for the pmu check as well (sashiko)

Thanks for finding and reporting this issue.

Please rewrite the change log and explicitly show the call-chain and tell
why NULL pointer is accessed.

e.g, pmu_detach_event() clears the event->pmu to null and then put_event()
calls mediated_pmu_unaccount_event() and then trigger the NULL pointer access.


>
> Fixes: eff95e170275 ("perf: Add APIs to create/release mediated guest vPMUs")
> Cc: Alexander Kanevskiy <alexander.kanevskiy@intel.com>
> Cc: Dapeng Mi <dapeng1.mi@linux.intel.com>
> Assisted-by: Claude:Claude-Sonnet-5
> Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
> ---
>  kernel/events/core.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 94ca277e0819..4f8c5191ef2c 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4794,6 +4794,9 @@ static int __perf_event_read_cpu(struct perf_event *event, int event_cpu)
>  	if ((unsigned)event_cpu >= nr_cpu_ids)
>  		return event_cpu;
>  
> +	if (!READ_ONCE(event->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);
>  
> @@ -4917,6 +4920,11 @@ 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;
> +	}
> +

The above two changes has nothing to do with the fix of null pointer
access, better move them into a separate patch.
Thanks.


>  	/*
>  	 * Get the event CPU numbers, and adjust them to local if the event is
>  	 * a per-package event that can be read locally
> @@ -6350,6 +6358,9 @@ static DEFINE_MUTEX(perf_mediated_pmu_mutex);
>  /* !exclude_guest event of PMU with PERF_PMU_CAP_MEDIATED_VPMU */
>  static inline bool is_include_guest_event(struct perf_event *event)
>  {
> +	if (!event->pmu)
> +		return false;
> +
>  	if ((event->pmu->capabilities & PERF_PMU_CAP_MEDIATED_VPMU) &&
>  	    !event->attr.exclude_guest)
>  		return true;
> @@ -12977,6 +12988,7 @@ static void __pmu_detach_event(struct pmu *pmu, struct perf_event *event,
>  	exclusive_event_destroy(event);
>  	module_put(pmu->module);
>  
> +	mediated_pmu_unaccount_event(event);
>  	event->pmu = NULL; /* force fault instead of UAF */
>  }
>