nfp: validate rx offset from the BAR and size down it's field

NFP_NET_CFG_RX_OFFSET is 32bit wide, make sure what we read from
there is reasonable for packet headroom.  This allows us to store
the rx_offset in a 8bit variable.
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/netronome/nfp/nfp_net.h

@@ -439,9 +439,9 @@ struct nfp_stat_pair {
  * @bpf_offload_xdp:	Offloaded BPF program is XDP
  * @chained_metadata_format:  Firemware will use new metadata format
  * @rx_dma_dir:		Mapping direction for RX buffers
+ * @rx_offset:		Offset in the RX buffers where packet data starts
  * @ctrl:		Local copy of the control register/word.
  * @fl_bufsz:		Currently configured size of the freelist buffers
- * @rx_offset:		Offset in the RX buffers where packet data starts
  * @xdp_prog:		Installed XDP program
  * @tx_rings:		Array of pre-allocated TX ring structures
  * @rx_rings:		Array of pre-allocated RX ring structures
@@ -466,11 +466,11 @@ struct nfp_net_dp {
 
 	u8 rx_dma_dir;
 
+	u8 rx_offset;
+
 	u32 ctrl;
 	u32 fl_bufsz;
 
-	u32 rx_offset;
-
 	struct bpf_prog *xdp_prog;
 
 	struct nfp_net_tx_ring *tx_rings;

drivers/net/ethernet/netronome/nfp/nfp_net_common.c

@@ -3124,10 +3124,18 @@ int nfp_net_netdev_init(struct net_device *netdev)
 	nfp_net_write_mac_addr(nn);
 
 	/* Determine RX packet/metadata boundary offset */
-	if (nn->fw_ver.major >= 2)
-		nn->dp.rx_offset = nn_readl(nn, NFP_NET_CFG_RX_OFFSET);
-	else
+	if (nn->fw_ver.major >= 2) {
+		u32 reg;
+
+		reg = nn_readl(nn, NFP_NET_CFG_RX_OFFSET);
+		if (reg > NFP_NET_MAX_PREPEND) {
+			nn_err(nn, "Invalid rx offset: %d\n", reg);
+			return -EINVAL;
+		}
+		nn->dp.rx_offset = reg;
+	} else {
 		nn->dp.rx_offset = NFP_NET_RX_OFFSET;
+	}
 
 	/* Set default MTU and Freelist buffer size */
 	if (nn->max_mtu < NFP_NET_DEFAULT_MTU)