From 54ad1e0ea42c3dc25de39b09220af408e4913820 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Fri, 11 Sep 2026 11:09:40 +0200 Subject: [PATCH] posix-cpu-timers: Prevent enqueueing when PF_EXITING is set To prepare for cleaning up POSIX CPU timers in do_exit(), prevent enqueueing POSIX CPU timers on a task which has PF_EXITING set. Queueing a timer on such a task is pointless because the task won't expire the timer anymore. The same applies to process wide timers when tsk::signal::flags has SIGNAL_GROUP_EXIT set. Pretending that the timer is armed allows to keep the POSIX timer mechanism "working" so that the timer stays accessible up to the point where a task is unhashed. Signed-off-by: Thomas Gleixner Tested-by: Kijo Park Reviewed-by: Oleg Nesterov Reviewed-by: Frederic Weisbecker Link: https://patch.msgid.link/20260911090541.892445443@kernel.org --- kernel/time/posix-cpu-timers.c | 28 ++++++++++++++++++++++++++++ 1 file changed, 28 insertions(+) diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c index cf4bd1f9d74c..ddaf81bb23fa 100644 --- a/kernel/time/posix-cpu-timers.c +++ b/kernel/time/posix-cpu-timers.c @@ -628,6 +628,7 @@ static int posix_cpu_timer_del(struct k_itimer *timer) } if (!ret) { + WARN_ON_ONCE(cpu_timer_queued(&timer->it.cpu)); put_pid(timer->it.cpu.pid); timer->it_status = POSIX_TIMER_DISARMED; } @@ -674,6 +675,27 @@ void posix_cpu_timers_exit_group(struct task_struct *tsk) cleanup_timers(&tsk->signal->posix_cputimers); } +/* + * This function validates that POSIX CPU timers can be safely enqueued on the + * target task. + * + * Enqueue is allowed when PF_EXITING is not set. If set then it is only allowed + * for process shared timers (type = PIDTYPE_TGID) as long as tsk::signal::flags + * does not have SIGNAL_GROUP_EXIT set. PIDTYPE_PID targets are not allowed at + * all when the task has PF_EXITING set. + * + * This guarantees that after the POSIX timer cleanup in posixtimer_exit() no + * POSIX CPU timers are queued on the task or in case of a group exit on the + * process. + */ +static inline bool task_can_enqueue_timer(struct task_struct *tsk, enum pid_type type) +{ + if (likely(!(tsk->flags & PF_EXITING))) + return true; + + return type == PIDTYPE_TGID && !(tsk->signal->flags & SIGNAL_GROUP_EXIT); +} + /* * Insert the timer on the appropriate list before any timers that * expire later. This must be called with the sighand lock held. @@ -684,7 +706,13 @@ static void arm_timer(struct k_itimer *timer, struct task_struct *p) struct cpu_timer *ctmr = &timer->it.cpu; u64 newexp = cpu_timer_getexpires(ctmr); + lockdep_assert_held(&p->sighand->siglock); + timer->it_status = POSIX_TIMER_ARMED; + + if (unlikely(!task_can_enqueue_timer(p, clock_pid_type(timer->it_clock)))) + return; + if (!cpu_timer_enqueue(&base->tqhead, ctmr)) return;