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

jira VULN-136687
cve CVE-2025-38352
commit-author Oleg Nesterov <[email protected]>
commit f90fff1
upstream-diff Applying to kernel/posix-cpu-timers.c instead of
              kernel/time/posix-cpu-timers.c in upstream.  There were
              minor merge conflicts due to differences in surrounding
              code but the code added is the same.

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: [email protected]
	Reported-by: Benoît Sevens <[email protected]>
Fixes: 0bdd2ed ("sched: run_posix_cpu_timers: Don't check ->exit_state, use lock_task_sighand()")
	Signed-off-by: Oleg Nesterov <[email protected]>
	Signed-off-by: Linus Torvalds <[email protected]>
(cherry picked from commit f90fff1)
	Signed-off-by: Brett Mastbergen <[email protected]>

fixup

Author: bmastbergen

kernel/posix-cpu-timers.c

@@ -1325,6 +1325,15 @@ void run_posix_cpu_timers(struct task_struct *tsk)
 
 	BUG_ON(!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.