[PATCH 4.14 193/209] futex: Sanitize exit state handling

Greg Kroah-Hartman
2019-12-04T18:09:50Z

From: Thomas Gleixner <tglx@linutronix.de>

commit 4a8e991b91aca9e20705d434677ac013974e0e30 upstream.

Instead of having a smp_mb() and an empty lock/unlock of task::pi_lock move
the state setting into to the lock section.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191106224556.645603214@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/futex.c | 17 ++++++++++-------
1 file changed, 10 insertions(+), 7 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3726,16 +3726,19 @@ void futex_exit_recursive(struct task_st

void futex_exit_release(struct task_struct *tsk)
{
- tsk->futex_state = FUTEX_STATE_EXITING;
- /*
- * Ensure that all new tsk->pi_lock acquisitions must observe
- * FUTEX_STATE_EXITING. Serializes against attach_to_pi_owner().
- */
- smp_mb();
/*
- * Ensure that we must observe the pi_state in exit_pi_state_list().
+ * Switch the state to FUTEX_STATE_EXITING under tsk->pi_lock.
+ *
+ * This ensures that all subsequent checks of tsk->futex_state in
+ * attach_to_pi_owner() must observe FUTEX_STATE_EXITING with
+ * tsk->pi_lock held.
+ *
+ * It guarantees also that a pi_state which was queued right before
+ * the state change under tsk->pi_lock by a concurrent waiter must
+ * be observed in exit_pi_state_list().
*/
raw_spin_lock_irq(&tsk->pi_lock);
+ tsk->futex_state = FUTEX_STATE_EXITING;
raw_spin_unlock_irq(&tsk->pi_lock);

futex_exec_release(tsk);