net: vertexcom: mse102x: Add range check for CMD_RTS

Since there is no protection in the SPI protocol against electrical
interferences, the driver shouldn't blindly trust the length payload
of CMD_RTS. So introduce a bounds check for incoming frames.

Fixes: 2f207cb ("net: vertexcom: Add MSE102x SPI support")
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250430133043.7722-4-wahrenst@gmx.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: lategoodbye

drivers/net/ethernet/vertexcom/mse102x.c

@@ -6,6 +6,7 @@
 
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
+#include <linux/if_vlan.h>
 #include <linux/interrupt.h>
 #include <linux/module.h>
 #include <linux/kernel.h>
@@ -337,8 +338,9 @@ static void mse102x_rx_pkt_spi(struct mse102x_net *mse)
 	}
 
 	rxlen = cmd_resp & LEN_MASK;
-	if (!rxlen) {
-		net_dbg_ratelimited("%s: No frame length defined\n", __func__);
+	if (rxlen < ETH_ZLEN || rxlen > VLAN_ETH_FRAME_LEN) {
+		net_dbg_ratelimited("%s: Invalid frame length: %d\n", __func__,
+				    rxlen);
 		mse->stats.invalid_len++;
 		return;
 	}