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SubjectRe: [PATCH] futex: Fix potential use-after-free in FUTEX_REQUEUE_PI
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...

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