net: dsa: mxl862xx: don't read out-of-bounds

The write loop in mxl862xx_api_wrap() computes the word count as
(size + 1) / 2, rounding up for odd-sized structs.

On the last iteration of an odd-sized buffer it reads a full __le16
from data[i], accessing one byte past the end of the caller's struct.
KASAN catches this as a stack-out-of-bounds read during probe (e.g.
from mxl862xx_bridge_config_fwd() because of the odd length of
sizeof(struct mxl862xx_bridge_config) == 49).

The read-back loop already handles this case, it writes only a single
byte when (i * 2 + 1) == size. The write loop lacked the same guard.

In practice the over-read is harmless: the extra stack byte is sent to
the firmware which ignores trailing data beyond the command's declared
payload size.

Apply the same odd-size last-byte handling to the write path: when the
final word contains only one valid byte, send *(u8 *)&data[i] instead
of le16_to_cpu(data[i]). This is endian-safe because data is
__le16-encoded and the low byte is always at the lowest address
regardless of host byte order.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/83356ad9c9a4470dd49b6b3d661c2a8dd85cc6a1.1773803190.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/mxl862xx/mxl862xx-host.c

@@ -175,8 +175,14 @@ int mxl862xx_api_wrap(struct mxl862xx_priv *priv, u16 cmd, void *_data,
 				goto out;
 		}
 
-		ret = mxl862xx_reg_write(priv, MXL862XX_MMD_REG_DATA_FIRST + off,
-					 le16_to_cpu(data[i]));
+		if ((i * 2 + 1) == size)
+			ret = mxl862xx_reg_write(priv,
+						 MXL862XX_MMD_REG_DATA_FIRST + off,
+						 *(u8 *)&data[i]);
+		else
+			ret = mxl862xx_reg_write(priv,
+						 MXL862XX_MMD_REG_DATA_FIRST + off,
+						 le16_to_cpu(data[i]));
 		if (ret < 0)
 			goto out;
 	}