CVE-2025-37747

In the Linux kernel, the following vulnerability has been resolved:

perf: Fix hang while freeing sigtrap event

Perf can hang while freeing a sigtrap event if a related deferred

signal hadn't managed to be sent before the file got closed:

perf_event_overflow()

task_work_add(perf_pending_task)

fput()

task_work_add(____fput())

task_work_run()

____fput()

perf_release()

perf_event_release_kernel()

_free_event()

perf_pending_task_sync()

task_work_cancel() -> FAILED

rcuwait_wait_event()

Once task_work_run() is running, the list of pending callbacks is

removed from the task_struct and from this point on task_work_cancel()

can't remove any pending and not yet started work items, hence the

task_work_cancel() failure and the hang on rcuwait_wait_event().

Task work could be changed to remove one work at a time, so a work

running on the current task can always cancel a pending one, however

the wait / wake design is still subject to inverted dependencies when

remote targets are involved, as pictured by Oleg:

T1 T2

fd = perf_event_open(pid => T2->pid); fd = perf_event_open(pid => T1->pid);

close(fd) close(fd)

<IRQ> <IRQ>

perf_event_overflow() perf_event_overflow()

task_work_add(perf_pending_task) task_work_add(perf_pending_task)

</IRQ> </IRQ>

fput() fput()

task_work_add(____fput()) task_work_add(____fput())

task_work_run() task_work_run()

____fput() ____fput()

perf_release() perf_release()

perf_event_release_kernel() perf_event_release_kernel()

_free_event() _free_event()

perf_pending_task_sync() perf_pending_task_sync()

rcuwait_wait_event() rcuwait_wait_event()

Therefore the only option left is to acquire the event reference count

upon queueing the perf task work and release it from the task work, just

like it was done before 3a5465418f5f ("perf: Fix event leak upon exec and file release")

but without the leaks it fixed.

Some adjustments are necessary to make it work:

* A child event might dereference its parent upon freeing. Care must be

taken to release the parent last.

* Some places assuming the event doesn't have any reference held and

therefore can be freed right away must instead put the reference and

let the reference counting to its job.