[PATCH 1/7] perf/core: Add pmu get/put

[Ian Rogers]

On Mon, Jul 22, 2024 at 2:07 PM Lucas De Marchi
<lucas.demarchi at intel.com> wrote:
>
> If a pmu is unregistered while there's an active event, perf will still
> access the pmu via event->pmu, even after the event is destroyed. This
> makes it difficult for drivers like i915 that take a reference on the
> device when the event is created and put it when it's destroyed.
> Currently the following use-after-free happens just after destroying the
> event:
>
>         BUG: KASAN: use-after-free in exclusive_event_destroy+0xd8/0xf0
>         Read of size 4 at addr ffff88816e2bb63c by task perf/7748
>
> Whenever and event is created, get a pmu reference to use in event->pmu
> and just before calling module_put(), drop the reference..
>
> Signed-off-by: Lucas De Marchi <lucas.demarchi at intel.com>
> ---
>  include/linux/perf_event.h |  3 +++
>  kernel/events/core.c       | 32 ++++++++++++++++++++++++++++----
>  2 files changed, 31 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index a5304ae8c654..7048a505e93c 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -540,6 +540,9 @@ struct pmu {
>          * Check period value for PERF_EVENT_IOC_PERIOD ioctl.
>          */
>         int (*check_period)             (struct perf_event *event, u64 value); /* optional */
> +
> +       struct pmu *(*get)              (struct pmu *pmu); /* optional: get a reference */
> +       void (*put)                     (struct pmu *pmu); /* optional: put a reference */
>  };
>
>  enum perf_addr_filter_action_t {
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 1b6f5dc7ed32..cc7541b644b0 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -5208,6 +5208,8 @@ static void perf_addr_filters_splice(struct perf_event *event,
>
>  static void _free_event(struct perf_event *event)
>  {
> +       struct module *module;
> +
>         irq_work_sync(&event->pending_irq);
>
>         unaccount_event(event);
> @@ -5259,7 +5261,13 @@ static void _free_event(struct perf_event *event)
>                 put_ctx(event->ctx);
>
>         exclusive_event_destroy(event);
> -       module_put(event->pmu->module);
> +
> +       module = event->pmu->module;
> +       event->pmu->put(event->pmu);
> +       /* can't touch pmu anymore */
> +       event->pmu = NULL;
> +
> +       module_put(module);
>
>         call_rcu(&event->rcu_head, free_event_rcu);
>  }
> @@ -11331,6 +11339,11 @@ static int perf_pmu_nop_int(struct pmu *pmu)
>         return 0;
>  }
>
> +static struct pmu *perf_pmu_nop_pmu(struct pmu *pmu)
> +{
> +       return pmu;
> +}
> +
>  static int perf_event_nop_int(struct perf_event *event, u64 value)
>  {
>         return 0;
> @@ -11617,6 +11630,12 @@ int perf_pmu_register(struct pmu *pmu, const char *name, int type)
>         if (!pmu->event_idx)
>                 pmu->event_idx = perf_event_idx_default;
>
> +       if (!pmu->get)
> +               pmu->get = perf_pmu_nop_pmu;
> +
> +       if (!pmu->put)
> +               pmu->put = perf_pmu_nop_void;
> +
>         list_add_rcu(&pmu->entry, &pmus);
>         atomic_set(&pmu->exclusive_cnt, 0);
>         ret = 0;
> @@ -11695,7 +11714,8 @@ static int perf_try_init_event(struct pmu *pmu, struct perf_event *event)
>                 BUG_ON(!ctx);
>         }
>
> -       event->pmu = pmu;
> +       event->pmu = pmu->get(pmu);
> +
>         ret = pmu->event_init(event);
>
>         if (ctx)
> @@ -11714,8 +11734,12 @@ static int perf_try_init_event(struct pmu *pmu, struct perf_event *event)
>                         event->destroy(event);
>         }
>
> -       if (ret)
> -               module_put(pmu->module);
> +       if (ret) {
> +               struct module *module = pmu->module;
> +
> +               pmu->put(pmu);

I think this is a great fix, a nit here, wouldn't it be good to do:

event->pmu = NULL;

Thanks,
Ian

> +               module_put(module);
> +       }
>
>         return ret;
>  }
> --
> 2.43.0
>
>