Re: [PATCH] perf/core: Do not resurrect cpuctx->cgrp after last cgroup event exit

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Nov 06, 2026 at 05:16:42PM +0700, Phat Dinh wrote:
> perf_cgroup_switch() relies on:
> 
>     /*
>      * cpuctx->cgrp is set when the first cgroup event enabled,
>      * and is cleared when the last cgroup event disabled.
>      */
> 
> It computes the next task's cgroup, schedules out the CPU context and
> then assigns cpuctx->cgrp unconditionally:
> 
>     ctx_sched_out(&cpuctx->ctx, NULL, EVENT_ALL|EVENT_CGROUP);
>     /*
>      * must not be done before ctxswout due
>      * to update_cgrp_time_from_cpuctx() in
>      * ctx_sched_out()
>      */
>     cpuctx->cgrp = cgrp;
> 
> However, ctx_sched_out() can disable events itself. If a cgroup event
> has a pending disable from perf_event_disable_inatomic() (e.g. its
> PERF_EVENT_IOC_REFRESH limit ran out) that the irq_work has not
> handled yet, event_sched_out() consumes it inline:
> 
>     if (event->pending_disable) {
>         event->pending_disable = 0;
>         perf_cgroup_event_disable(event, ctx);
>         state = PERF_EVENT_STATE_OFF;
>     }
> 
> If that was the last cgroup event, perf_cgroup_event_disable() clears
> cpuctx->cgrp:
> 
>     if (--ctx->nr_cgroups)
>         return;
> 
>     cpuctx->cgrp = NULL;
> 
> and perf_cgroup_switch() then sets it again. cpuctx->cgrp is left
> non-NULL with ctx.nr_cgroups == 0, so the NULL check in
> perf_cgroup_switch() no longer catches it and the next switch trips:
> 
>     WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0);
> 
> cpuctx->cgrp does not hold a reference on the cgroup. Once that cgroup
> is removed, the next ctx_sched_in() dereferences the stale pointer:
> 
>   BUG: KASAN: slab-use-after-free in ctx_sched_in+0xb1f/0xfa0
>   Read of size 8 at addr ffff8881008972d0 by task swapper/3/0
> 
>   Tainted: [W]=WARN
>   Call Trace:
>    <IRQ>
>    ctx_sched_in+0xb1f/0xfa0
>    perf_event_sched_in+0x1a/0x70
>    ctx_resched+0x32b/0xac0
>    __perf_install_in_context+0x475/0xab0
>    remote_function+0x105/0x200
>    __flush_smp_call_function_queue+0x179/0x8d0
>    __sysvec_call_function_single+0x1b/0x1c0
>    sysvec_call_function_single+0x66/0x80
>    </IRQ>
>    <TASK>
>    asm_sysvec_call_function_single+0x1a/0x20
> 
> Only assign cpuctx->cgrp when cgroup events remain after the context
> has been scheduled out, so that it stays NULL as the invariant
> requires.
> 
> Fixes: e19cd0b6fa59 ("perf/core: Always set cpuctx cgrp when enable cgroup event")
> Cc: stable@vger.kernel.org
> Signed-off-by: Phat Dinh <phatdt1116@gmail.com>

Indeed, thanks!

> ---
>  kernel/events/core.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index f36a3bbaa33c..54b87584c752 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -1072,8 +1072,13 @@ static void perf_cgroup_switch(struct task_struct *task)
>  	 * must not be done before ctxswout due
>  	 * to update_cgrp_time_from_cpuctx() in
>  	 * ctx_sched_out()
> +	 *
> +	 * ctx_sched_out() can consume a pending disable and drop the last cgroup
> +	 * event, which clear the cpuctx->cgrp. Cgroup tracking follow active cgroup
> +	 * event, so do not resurrect it
>  	 */
> -	cpuctx->cgrp = cgrp;
> +	if (cpuctx->ctx.nr_cgroups)
> +		cpuctx->cgrp = cgrp;
>  	/*
>  	 * set cgrp before ctxsw in to allow
>  	 * perf_cgroup_set_timestamp() in ctx_sched_in()
> 
> base-commit: d095321349222961813333277881b3c74f10b17b
> -- 
> 2.56.0
>