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2fd5ad3f31
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:
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.