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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Lee Jones <lee.jones@linaro.org>
Subject: [PATCH 4.4 14/28] futex: Sanitize exit state handling
Date:   Tue,  2 Feb 2021 14:38:34 +0100
Message-Id: <20210202132941.759808490@linuxfoundation.org>
In-Reply-To: <20210202132941.180062901@linuxfoundation.org>
References: <20210202132941.180062901@linuxfoundation.org>
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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>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
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
@@ -3276,16 +3276,19 @@ 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);