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

[Frederic Weisbecker <frederic@kernel.org>]

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.

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) {
+				WARN_ON_ONCE(!atomic_long_inc_not_zero(&event->refcount));
+				dec = false;
+				event->pending_work = 1;
+			}
 		}
 		if (dec)
 			local_dec(&event->ctx->nr_pending);
-- 
2.44.0