net/mlx4_core: Use-after-free causes a resource leak in flow-steering detach

commit 3b01fe7f upstream.

mlx4_QP_FLOW_STEERING_DETACH_wrapper first removes the steering
rule (which results in freeing the rule structure), and then
references a field in this struct (the qp number) when releasing the
busy-status on the rule's qp.

Since this memory was freed, it could reallocated and changed.
Therefore, the qp number in the struct may be incorrect,
so that we are releasing the incorrect qp. This leaves the rule's qp
in the busy state (and could possibly release an incorrect qp as well).

Fix this by saving the qp number in a local variable, for use after
removing the steering rule.

Fixes: 2c473ae7 ("net/mlx4_core: Disallow releasing VF QPs which have steering rules")
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/net/ethernet/mellanox/mlx4/resource_tracker.c

@@ -4091,6 +4091,7 @@ int mlx4_QP_FLOW_STEERING_DETACH_wrapper(struct mlx4_dev *dev, int slave,
 	int err;
 	struct res_qp *rqp;
 	struct res_fs_rule *rrule;
+	int qpn;
 
 	if (dev->caps.steering_mode !=
 	    MLX4_STEERING_MODE_DEVICE_MANAGED)
@@ -4099,9 +4100,10 @@ int mlx4_QP_FLOW_STEERING_DETACH_wrapper(struct mlx4_dev *dev, int slave,
 	err = get_res(dev, slave, vhcr->in_param, RES_FS_RULE, &rrule);
 	if (err)
 		return err;
+	qpn = rrule->qpn;
 	/* Release the rule form busy state before removal */
 	put_res(dev, slave, vhcr->in_param, RES_FS_RULE);
-	err = get_res(dev, slave, rrule->qpn, RES_QP, &rqp);
+	err = get_res(dev, slave, qpn, RES_QP, &rqp);
 	if (err)
 		return err;
 
@@ -4117,7 +4119,7 @@ int mlx4_QP_FLOW_STEERING_DETACH_wrapper(struct mlx4_dev *dev, int slave,
 	if (!err)
 		atomic_dec(&rqp->ref_count);
 out:
-	put_res(dev, slave, rrule->qpn, RES_QP);
+	put_res(dev, slave, qpn, RES_QP);
 	return err;
 }