net: dsa: realtek: fix out-of-bounds access

The probe function sets priv->chip_data to (void *)priv + sizeof(*priv)
with the expectation that priv has enough trailing space.

However, only realtek-smi actually allocated this chip_data space.
Do likewise in realtek-mdio to fix out-of-bounds accesses.

These accesses likely went unnoticed so far, because of an (unused)
buf[4096] member in struct realtek_priv, which caused kmalloc to
round up the allocated buffer to a big enough size, so nothing of
value was overwritten. With a different allocator (like in the barebox
bootloader port of the driver) or with KASAN, the memory corruption
becomes quickly apparent.

Fixes: aac94001 ("net: dsa: realtek: add new mdio interface for drivers")
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.kernel.org/r/20230323103735.2331786-1-a.fatoum@pengutronix.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/realtek/realtek-mdio.c

@@ -21,6 +21,7 @@
 
 #include <linux/module.h>
 #include <linux/of_device.h>
+#include <linux/overflow.h>
 #include <linux/regmap.h>
 
 #include "realtek.h"
@@ -152,7 +153,9 @@ static int realtek_mdio_probe(struct mdio_device *mdiodev)
 	if (!var)
 		return -EINVAL;
 
-	priv = devm_kzalloc(&mdiodev->dev, sizeof(*priv), GFP_KERNEL);
+	priv = devm_kzalloc(&mdiodev->dev,
+			    size_add(sizeof(*priv), var->chip_data_sz),
+			    GFP_KERNEL);
 	if (!priv)
 		return -ENOMEM;