net/mlx4_core: Fix access to uninitialized index

commit 2bb07e15 upstream.

Prevent using uninitialized or negative index when handling
steering entries.

Fixes: b12d93d6 ('mlx4: Add support for promiscuous mode in the new steering model.')
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

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

@@ -615,7 +615,7 @@ int mlx4_qp_attach_common(struct mlx4_dev *dev, struct mlx4_qp *qp, u8 gid[16],
 	struct mlx4_mgm *mgm;
 	u32 members_count;
 	u16 hash;
-	int index, prev;
+	int index = -1, prev;
 	int link = 0;
 	int i;
 	int err;
@@ -693,7 +693,7 @@ int mlx4_qp_attach_common(struct mlx4_dev *dev, struct mlx4_qp *qp, u8 gid[16],
 		goto out;
 
 out:
-	if (prot == MLX4_PROT_ETH) {
+	if (prot == MLX4_PROT_ETH && index != -1) {
 		/* manage the steering entry for promisc mode */
 		if (new_entry)
 			new_steering_entry(dev, 0, port, steer, index, qp->qpn);