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perf: Fix event leak upon exit

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commit 2fd5ad3f31 upstream.

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 an 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: 517e6a301f ("perf: Fix perf_pending_task() UaF")
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240621091601.18227-4-frederic@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Frederic Weisbecker 2024-06-21 11:16:00 +02:00 committed by Greg Kroah-Hartman
parent cde43031df
commit 67fad724f1
1 changed files with 5 additions and 8 deletions
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13
kernel/events/core.c
View File

@ -2395,18 +2395,15 @@ event_sched_out(struct perf_event *event,
}
if (event->pending_sigtrap) {
bool dec = true;
event->pending_sigtrap = 0;
if (state != PERF_EVENT_STATE_OFF &&
!event->pending_work) {
event->pending_work = 1;
dec = false;
!event->pending_work &&
!task_work_add(current, &event->pending_task, TWA_RESUME)) {
WARN_ON_ONCE(!atomic_long_inc_not_zero(&event->refcount));
task_work_add(current, &event->pending_task, TWA_RESUME);
}
if (dec)
event->pending_work = 1;
} else {
local_dec(&event->ctx->nr_pending);
}
}
perf_event_set_state(event, state);