Commit f90fff1e authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
Browse files

posix-cpu-timers: fix race between handle_posix_cpu_timers() and posix_cpu_timer_del() 



If an exiting non-autoreaping task has already passed exit_notify() and
calls handle_posix_cpu_timers() from IRQ, it can be reaped by its parent
or debugger right after unlock_task_sighand().

If a concurrent posix_cpu_timer_del() runs at that moment, it won't be
able to detect timer->it.cpu.firing != 0: cpu_timer_task_rcu() and/or
lock_task_sighand() will fail.

Add the tsk->exit_state check into run_posix_cpu_timers() to fix this.

This fix is not needed if CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y, because
exit_task_work() is called before exit_notify(). But the check still
makes sense, task_work_add(&tsk->posix_cputimers_work.work) will fail
anyway in this case.

Cc: stable@vger.kernel.org
Reported-by: default avatarBenoît Sevens <bsevens@google.com>
Fixes: 0bdd2ed4 ("sched: run_posix_cpu_timers: Don't check ->exit_state, use lock_task_sighand()")
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 3ca933aa

kernel/time/posix-cpu-timers.c

+9 −0
Original line number Diff line number Diff line
@@ -1405,6 +1405,15 @@ void run_posix_cpu_timers(void) 


	lockdep_assert_irqs_disabled();



	/*

	 * Ensure that release_task(tsk) can't happen while

	 * handle_posix_cpu_timers() is running. Otherwise, a concurrent

	 * posix_cpu_timer_del() may fail to lock_task_sighand(tsk) and

	 * miss timer->it.cpu.firing != 0.

	 */

	if (tsk->exit_state)

		return;



	/*

	 * If the actual expiry is deferred to task work context and the

	 * work is already scheduled there is no point to do anything here.