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

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commit f90fff1e152dedf52b932240ebbd670d83330eca upstream.

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: Benoît Sevens <bsevens@google.com>
Fixes: 0bdd2ed413 ("sched: run_posix_cpu_timers: Don't check ->exit_state, use lock_task_sighand()")
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Oleg Nesterov 2025-06-17 19:15:50 +02:00 • committed by Greg Kroah-Hartman
commit 78a4b8e379
1 changed files with 9 additions and 0 deletions
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9
kernel/time/posix-cpu-timers.c
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@ -1119,6 +1119,15 @@ 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;
/*
* The fast path checks that there are no expired thread or thread
* group timers. If that's so, just return.