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: Benoît Sevens <bsevens@google.com>
Fixes: 0bdd2ed4138e ("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>

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 50e8d04ab661f42283a820910fa6fd83e9d96e2a..2e5b89d7d8660585460490557021dfbf7799740d 100644 (file)
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -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.