i40e: i40e_main: fix a missing check on list iterator

The bug is here:
	ret = i40e_add_macvlan_filter(hw, ch->seid, vdev->dev_addr, &aq_err);

The list iterator 'ch' will point to a bogus position containing
HEAD if the list is empty or no element is found. This case must
be checked before any use of the iterator, otherwise it will
lead to a invalid memory access.

To fix this bug, use a new variable 'iter' as the list iterator,
while use the origin variable 'ch' as a dedicated pointer to
point to the found element.

Cc: stable@vger.kernel.org
Fixes: 1d8d80b ("i40e: Add macvlan support on i40e")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Tested-by: Gurucharan <gurucharanx.g@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Link: https://lore.kernel.org/r/20220510204846.2166999-1-anthony.l.nguyen@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Xiaomeng Tong

drivers/net/ethernet/intel/i40e/i40e_main.c

@@ -7549,42 +7549,43 @@ static void i40e_free_macvlan_channels(struct i40e_vsi *vsi)
 static int i40e_fwd_ring_up(struct i40e_vsi *vsi, struct net_device *vdev,
 			    struct i40e_fwd_adapter *fwd)
 {
+	struct i40e_channel *ch = NULL, *ch_tmp, *iter;
 	int ret = 0, num_tc = 1,  i, aq_err;
-	struct i40e_channel *ch, *ch_tmp;
 	struct i40e_pf *pf = vsi->back;
 	struct i40e_hw *hw = &pf->hw;
 
-	if (list_empty(&vsi->macvlan_list))
-		return -EINVAL;
-
 	/* Go through the list and find an available channel */
-	list_for_each_entry_safe(ch, ch_tmp, &vsi->macvlan_list, list) {
-		if (!i40e_is_channel_macvlan(ch)) {
-			ch->fwd = fwd;
+	list_for_each_entry_safe(iter, ch_tmp, &vsi->macvlan_list, list) {
+		if (!i40e_is_channel_macvlan(iter)) {
+			iter->fwd = fwd;
 			/* record configuration for macvlan interface in vdev */
 			for (i = 0; i < num_tc; i++)
 				netdev_bind_sb_channel_queue(vsi->netdev, vdev,
 							     i,
-							     ch->num_queue_pairs,
-							     ch->base_queue);
-			for (i = 0; i < ch->num_queue_pairs; i++) {
+							     iter->num_queue_pairs,
+							     iter->base_queue);
+			for (i = 0; i < iter->num_queue_pairs; i++) {
 				struct i40e_ring *tx_ring, *rx_ring;
 				u16 pf_q;
 
-				pf_q = ch->base_queue + i;
+				pf_q = iter->base_queue + i;
 
 				/* Get to TX ring ptr */
 				tx_ring = vsi->tx_rings[pf_q];
-				tx_ring->ch = ch;
+				tx_ring->ch = iter;
 
 				/* Get the RX ring ptr */
 				rx_ring = vsi->rx_rings[pf_q];
-				rx_ring->ch = ch;
+				rx_ring->ch = iter;
 			}
+			ch = iter;
 			break;
 		}
 	}
 
+	if (!ch)
+		return -EINVAL;
+
 	/* Guarantee all rings are updated before we update the
 	 * MAC address filter.
 	 */