Re: [PATCH v2] perf/core: Fix cgroup event list management

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Dec 12, 2021 at 10:59:36PM -0800, Namhyung Kim wrote:
> The active cgroup events are managed in the per-cpu cgrp_cpuctx_list.
> This list is accessed from current cpu and not protected by any locks.
> But from the commit ef54c1a476ae ("perf: Rework
> perf_event_exit_event()"), this assumption does not hold true anymore.
> 
> In the perf_remove_from_context(), it can remove an event from the
> context without an IPI when the context is not active.  I think it

"I tihnk" just doesn't cut it. That means I have to completely reverse
engineer your patch and it's assumptions. Which is more work for me :-(

> assumes task event context, but it's possible for cpu event context
> only with cgroup events can be inactive at the moment - and it might
> become active soon.
> 
> If the event is enabled when it's about to be closed, it might call
> perf_cgroup_event_disable() and list_del() with the cgrp_cpuctx_list
> on a different cpu.
> 
> This resulted in a crash due to an invalid list pointer access during
> the cgroup list traversal on the cpu which the event belongs to.
> 
> The following program can crash my box easily..

Unless that's already public, you've just given the script kiddos ammo,
surely we don't need that.

> Let's use IPI to prevent such crashes.

Let's just not do random things and hope stuff 'works'. Either it is
correct or it is not.

> Similarly, I think perf_install_in_context() should use IPI for the
> cgroup events too.

Let's be sure, ok?

> Reported-by: kernel test robot <lkp@intel.com>  # for build error

That's complete garbage, please don't do that.

> Cc: Marco Elver <elver@google.com>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
> v2) simply use IPI for cgroup events
> 
>  kernel/events/core.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 30d94f68c5bd..9460c083acd9 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2388,7 +2388,7 @@ static void perf_remove_from_context(struct perf_event *event, unsigned long fla
>  	 * event_function_call() user.
>  	 */
>  	raw_spin_lock_irq(&ctx->lock);
> -	if (!ctx->is_active) {
> +	if (!ctx->is_active && !is_cgroup_event(event)) {
>  		__perf_remove_from_context(event, __get_cpu_context(ctx),
>  					   ctx, (void *)flags);
>  		raw_spin_unlock_irq(&ctx->lock);
> @@ -2857,11 +2857,14 @@ perf_install_in_context(struct perf_event_context *ctx,
>  	 * perf_event_attr::disabled events will not run and can be initialized
>  	 * without IPI. Except when this is the first event for the context, in
>  	 * that case we need the magic of the IPI to set ctx->is_active.
> +	 * Similarly, cgroup events for the context also needs the IPI to
> +	 * manipulate the cgrp_cpuctx_list.
>  	 *
>  	 * The IOC_ENABLE that is sure to follow the creation of a disabled
>  	 * event will issue the IPI and reprogram the hardware.
>  	 */
> -	if (__perf_effective_state(event) == PERF_EVENT_STATE_OFF && ctx->nr_events) {
> +	if (__perf_effective_state(event) == PERF_EVENT_STATE_OFF &&
> +	    ctx->nr_events && !is_cgroup_event(event)) {
>  		raw_spin_lock_irq(&ctx->lock);
>  		if (ctx->task == TASK_TOMBSTONE) {
>  			raw_spin_unlock_irq(&ctx->lock);
> 
> base-commit: 73743c3b092277febbf69b250ce8ebbca0525aa2

What's junk like that doing ?