bus: moxtet: fix potential stack buffer overflow

The input_read function declares the size of the hex array relative to
sizeof(buf), but buf is a pointer argument of the function. The hex
array is meant to contain hexadecimal representation of the bin array.

Link: https://lore.kernel.org/r/20200215142130.22743-1-marek.behun@nic.cz
Fixes: 5bc7f990 ("bus: Add support for Moxtet bus")
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reported-by: sohu0106 <sohu0106@126.com>
Signed-off-by: Olof Johansson <olof@lixom.net>

drivers/bus/moxtet.c

@@ -465,7 +465,7 @@ static ssize_t input_read(struct file *file, char __user *buf, size_t len,
 {
 	struct moxtet *moxtet = file->private_data;
 	u8 bin[TURRIS_MOX_MAX_MODULES];
-	u8 hex[sizeof(buf) * 2 + 1];
+	u8 hex[sizeof(bin) * 2 + 1];
 	int ret, n;
 
 	ret = moxtet_spi_read(moxtet, bin);