Commit message: futex: Fix errors in nested key ref-counting

futex_wait() is leaking key references due to futex_wait_setup()
acquiring an additional reference via the queue_lock() routine. The
nested key ref-counting has been masking bugs and complicating code
analysis. queue_lock() is only called with a previously ref-counted
key, so remove the additional ref-counting from the queue_(un)lock()
functions.

Also futex_wait_requeue_pi() drops one key reference too many in
unqueue_me_pi(). Remove the key reference handling from
unqueue_me_pi(). This was paired with a queue_lock() in
futex_lock_pi(), so the count remains unchanged.

Document remaining nested key ref-counting sites.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Reported-and-tested-by: Matthieu Fertré<matthieu.fertre@kerlabs.com>
Reported-by: Louis Rilling<louis.rilling@kerlabs.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
LKML-Reference: <4CBB17A8.70401@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@kernel.org
CVE: CVE-2014-0205
CWE: CWE-119
Vulnerability description: The futex_wait function in kernel/futex.c in the Linux kernel before 2.6.37 does not properly maintain a certain reference count during requeue operations, which allows local users to cause a denial of service (use-after-free and system crash) or possibly gain privileges via a crafted application that triggers a zero count.

Project: linux
File: futex.c

static int futex_wait(u32 __user *uaddr, int fshared,
		      u32 val, ktime_t *abs_time, u32 bitset, int clockrt)
{
	struct hrtimer_sleeper timeout, *to = NULL;
	struct restart_block *restart;
	struct futex_hash_bucket *hb;
	struct futex_q q;
	int ret;

	if (!bitset)
		return -EINVAL;

	q.pi_state = NULL;
	q.bitset = bitset;
	q.rt_waiter = NULL;
	q.requeue_pi_key = NULL;

	if (abs_time) {
		to = &timeout;

		hrtimer_init_on_stack(&to->timer, clockrt ? CLOCK_REALTIME :
				      CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
		hrtimer_init_sleeper(to, current);
		hrtimer_set_expires_range_ns(&to->timer, *abs_time,
					     current->timer_slack_ns);
 	}
 
 retry:
	/* Prepare to wait on uaddr. */
 	ret = futex_wait_setup(uaddr, val, fshared, &q, &hb);
 	if (ret)
 		goto out;

	/* queue_me and wait for wakeup, timeout, or a signal. */
	futex_wait_queue_me(hb, &q, to);
 
 	/* If we were woken (and unqueued), we succeeded, whatever. */
 	ret = 0;
 	if (!unqueue_me(&q))
		goto out_put_key;
 	ret = -ETIMEDOUT;
 	if (to && !to->task)
		goto out_put_key;
 
 	/*
 	 * We expect signal_pending(current), but we might be the
 	 * victim of a spurious wakeup as well.
 	 */
	if (!signal_pending(current)) {
		put_futex_key(fshared, &q.key);
 		goto retry;
	}
 
 	ret = -ERESTARTSYS;
 	if (!abs_time)
		goto out_put_key;
 
 	restart = &current_thread_info()->restart_block;
 	restart->fn = futex_wait_restart;
	restart->futex.uaddr = (u32 *)uaddr;
	restart->futex.val = val;
	restart->futex.time = abs_time->tv64;
	restart->futex.bitset = bitset;
	restart->futex.flags = FLAGS_HAS_TIMEOUT;

	if (fshared)
		restart->futex.flags |= FLAGS_SHARED;
	if (clockrt)
		restart->futex.flags |= FLAGS_CLOCKRT;
 
 	ret = -ERESTART_RESTARTBLOCK;
 
out_put_key:
	put_futex_key(fshared, &q.key);
 out:
 	if (to) {
 		hrtimer_cancel(&to->timer);
		destroy_hrtimer_on_stack(&to->timer);
	}
	return ret;
}