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Date: Thu, 24 Nov 2016 16:19:41 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
cc: Ingo Molnar <mingo@kernel.org>, dvhart@infradead.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] futex: Fix potential use-after-free in
 FUTEX_REQUEUE_PI
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On Thu, 24 Nov 2016, Peter Zijlstra wrote:

> 
> While working on the futex code, I stumbled over this potential
> use-after-free scenario.
> 
> pi_mutex is a pointer into pi_state, which we drop the reference on in
> unqueue_me_pi(). So any access to that pointer after that is bad.
> 
> Since other sites already do rt_mutex_unlock() with hb->lock held, see
> for example futex_lock_pi(), simply move the unlock before
> unqueue_me_pi().
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/futex.c | 22 +++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 2c4be467fecd..d5a81339209f 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -2813,7 +2813,6 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
>  {
>  	struct hrtimer_sleeper timeout, *to = NULL;
>  	struct rt_mutex_waiter rt_waiter;
> -	struct rt_mutex *pi_mutex = NULL;
>  	struct futex_hash_bucket *hb;
>  	union futex_key key2 = FUTEX_KEY_INIT;
>  	struct futex_q q = futex_q_init;
> @@ -2905,6 +2904,8 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
>  			spin_unlock(q.lock_ptr);

In this path the fixup can return -EFAIL as well, so it should drop rtmutex
too if it owns it. We should move the rtmutex drop into the fixup functions...