Re: [PATCH 3/4] perf: Fix event leak upon exit

[Peter Zijlstra <peterz@infradead.org>]

On Wed, May 15, 2024 at 04:43:10PM +0200, Frederic Weisbecker wrote:
> When a task is scheduled out, pending sigtrap deliveries are deferred
> to the target task upon resume to userspace via task_work.
> 
> However failures while adding en event's callback to the task_work
> engine are ignored. And since the last call for events exit happen
> after task work is eventually closed, there is a small window during
> which pending sigtrap can be queued though ignored, leaking the event
> refcount addition such as in the following scenario:
> 
>     TASK A
>     -----
> 
>     do_exit()
>        exit_task_work(tsk);
> 
>        <IRQ>
>        perf_event_overflow()
>           event->pending_sigtrap = pending_id;
>           irq_work_queue(&event->pending_irq);
>        </IRQ>
>     =========> PREEMPTION: TASK A -> TASK B
>        event_sched_out()
>           event->pending_sigtrap = 0;
>           atomic_long_inc_not_zero(&event->refcount)
>           // FAILS: task work has exited
>           task_work_add(&event->pending_task)
>        [...]
>        <IRQ WORK>
>        perf_pending_irq()
>           // early return: event->oncpu = -1
>        </IRQ WORK>
>        [...]
>     =========> TASK B -> TASK A
>        perf_event_exit_task(tsk)
>           perf_event_exit_event()
>              free_event()
>                 WARN(atomic_long_cmpxchg(&event->refcount, 1, 0) != 1)
>                 // leak event due to unexpected refcount == 2
> 
> As a result the event is never released while the task exits.

Urgh...

> 
> Fix this with appropriate task_work_add()'s error handling.
> 
> Fixes: 517e6a301f34 ("perf: Fix perf_pending_task() UaF")
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
>  kernel/events/core.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 724e6d7e128f..c1632e69c69d 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -2289,10 +2289,11 @@ event_sched_out(struct perf_event *event, struct perf_event_context *ctx)
>  		event->pending_sigtrap = 0;
>  		if (state != PERF_EVENT_STATE_OFF &&
>  		    !event->pending_work) {
> -			event->pending_work = 1;
> -			dec = false;
> -			WARN_ON_ONCE(!atomic_long_inc_not_zero(&event->refcount));
> -			task_work_add(current, &event->pending_task, TWA_RESUME);
> +			if (task_work_add(current, &event->pending_task, TWA_RESUME) >= 0) {

AFAICT the thing is a return 0 on success -Efoo on fail, no? That is,
should this not simply be '== 0' ?

> +				WARN_ON_ONCE(!atomic_long_inc_not_zero(&event->refcount));
> +				dec = false;
> +				event->pending_work = 1;
> +			}

Also, do we want to write it like so and save an indent?

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2288,11 +2288,11 @@ event_sched_out(struct perf_event *event
 
 		event->pending_sigtrap = 0;
 		if (state != PERF_EVENT_STATE_OFF &&
-		    !event->pending_work) {
+		    !event->pending_work &&
+		    !task_work_add(current, &event->pending_task, TWA_RESUME)) {
 			event->pending_work = 1;
 			dec = false;
 			WARN_ON_ONCE(!atomic_long_inc_not_zero(&event->refcount));
-			task_work_add(current, &event->pending_task, TWA_RESUME);
 		}
 		if (dec)
 			local_dec(&event->ctx->nr_pending);