Re: [PATCH] perf: Fix dangling cgroup pointer in cpuctx

[Chris Arges <carges@cloudflare.com>]

On 2025-10-07 11:45:52, Chris J Arges wrote:
> From: Yeoreum Yun <yeoreum.yun@arm.com>
> 
> [ Upstream commit 3b7a34aebbdf2a4b7295205bf0c654294283ec82 ]
> 
> Commit a3c3c66670ce ("perf/core: Fix child_total_time_enabled accounting
> bug at task exit") moves the event->state update to before
> list_del_event(). This makes the event->state test in list_del_event()
> always false; never calling perf_cgroup_event_disable().
> 
> As a result, cpuctx->cgrp won't be cleared properly; causing havoc.
> 
> Cc: stable@vger.kernel.org # 6.6.x, 6.12.x
> Fixes: a3c3c66670ce ("perf/core: Fix child_total_time_enabled accounting bug at task exit")
> Signed-off-by: Chris J Arges <carges@cloudflare.com>
> ---
>  kernel/events/core.c | 21 ++++++---------------
>  1 file changed, 6 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 3cc06ffb60c1..6688660845d2 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2100,18 +2100,6 @@ list_del_event(struct perf_event *event, struct perf_event_context *ctx)
>  	if (event->group_leader == event)
>  		del_event_from_groups(event, ctx);
>  
> -	/*
> -	 * If event was in error state, then keep it
> -	 * that way, otherwise bogus counts will be
> -	 * returned on read(). The only way to get out
> -	 * of error state is by explicit re-enabling
> -	 * of the event
> -	 */
> -	if (event->state > PERF_EVENT_STATE_OFF) {
> -		perf_cgroup_event_disable(event, ctx);
> -		perf_event_set_state(event, PERF_EVENT_STATE_OFF);
> -	}
> -
>  	ctx->generation++;
>  	event->pmu_ctx->nr_events--;
>  }
> @@ -2456,11 +2444,14 @@ __perf_remove_from_context(struct perf_event *event,
>  	 */
>  	if (flags & DETACH_EXIT)
>  		state = PERF_EVENT_STATE_EXIT;
> -	if (flags & DETACH_DEAD) {
> -		event->pending_disable = 1;
> +	if (flags & DETACH_DEAD)
>  		state = PERF_EVENT_STATE_DEAD;
> -	}
> +
>  	event_sched_out(event, ctx);
> +
> +	if (event->state > PERF_EVENT_STATE_OFF)
> +		perf_cgroup_event_disable(event, ctx);
> +
>  	perf_event_set_state(event, min(event->state, state));
>  	if (flags & DETACH_GROUP)
>  		perf_group_detach(event);
> -- 
> 2.43.0
>

Hi, can this patch be applied to stable 6.6.x and 6.12.x?

Thanks,
--chris