Affected: Linux

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()…