net/mlx5e: Fix nullptr in mlx5e_tc_add_fdb_flow()

The result of __dev_get_by_index() is not checked for NULL, which then
passed to mlx5e_attach_encap() and gets dereferenced.

Also, in case of a successful lookup, the net_device reference count is
not incremented, which may result in net_device pointer becoming invalid
at any time during mlx5e_attach_encap() execution.

Fix by using dev_get_by_index(), which does proper reference counting on
the net_device pointer. Also, handle nullptr return value when mirred
device is not found.

It's safe to call dev_put() on the mirred net_device pointer, right
after mlx5e_attach_encap() call, because it's not being saved/copied
down the call chain.

Fixes: 3c37745e ("net/mlx5e: Properly deal with encap flows add/del under neigh update")
Addresses-Coverity: ("Dereference null return value")
Signed-off-by: Dima Chumak <dchumak@nvidia.com>
Reviewed-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>

drivers/net/ethernet/mellanox/mlx5/core/en_tc.c

@@ -1322,10 +1322,10 @@ mlx5e_tc_add_fdb_flow(struct mlx5e_priv *priv,
 		      struct netlink_ext_ack *extack)
 {
 	struct mlx5_eswitch *esw = priv->mdev->priv.eswitch;
-	struct net_device *out_dev, *encap_dev = NULL;
 	struct mlx5e_tc_flow_parse_attr *parse_attr;
 	struct mlx5_flow_attr *attr = flow->attr;
 	bool vf_tun = false, encap_valid = true;
+	struct net_device *encap_dev = NULL;
 	struct mlx5_esw_flow_attr *esw_attr;
 	struct mlx5_fc *counter = NULL;
 	struct mlx5e_rep_priv *rpriv;
@@ -1371,16 +1371,22 @@ mlx5e_tc_add_fdb_flow(struct mlx5e_priv *priv,
 	esw_attr = attr->esw_attr;
 
 	for (out_index = 0; out_index < MLX5_MAX_FLOW_FWD_VPORTS; out_index++) {
+		struct net_device *out_dev;
 		int mirred_ifindex;
 
 		if (!(esw_attr->dests[out_index].flags & MLX5_ESW_DEST_ENCAP))
 			continue;
 
 		mirred_ifindex = parse_attr->mirred_ifindex[out_index];
-		out_dev = __dev_get_by_index(dev_net(priv->netdev),
-					     mirred_ifindex);
+		out_dev = dev_get_by_index(dev_net(priv->netdev), mirred_ifindex);
+		if (!out_dev) {
+			NL_SET_ERR_MSG_MOD(extack, "Requested mirred device not found");
+			err = -ENODEV;
+			goto err_out;
+		}
 		err = mlx5e_attach_encap(priv, flow, out_dev, out_index,
 					 extack, &encap_dev, &encap_valid);
+		dev_put(out_dev);
 		if (err)
 			goto err_out;