iio: core: fix uninitialized data in debugfs

commit ab92ed2 upstream.

If *ppos is non-zero then simple_write_to_buffer() will not initialize
the start of buf[].  Non zero values for *ppos aren't going to work
anyway.  Test for them at the start of the function and return -EINVAL.

Fixes: 6d5dd48 ("iio: core: make use of simple_write_to_buffer()")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Maxwell Doose <m32285159@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: error27

drivers/iio/industrialio-core.c

@@ -418,7 +418,7 @@ static ssize_t iio_debugfs_write_reg(struct file *file,
 	char buf[80];
 	int ret;
 
-	if (count >= sizeof(buf))
+	if (*ppos != 0 || count >= sizeof(buf))
 		return -EINVAL;
 
 	ret = simple_write_to_buffer(buf, sizeof(buf) - 1, ppos, userbuf,