iio: fix potential out-of-bound write

The buffer is set to 20 characters. If a caller write more characters,
count is truncated to the max available space in "simple_write_to_buffer".
To protect from OoB access, check that the input size fit into buffer and
add a zero terminator after copy to the end of the copied data.

Fixes: 6d5dd48 iio: core: make use of simple_write_to_buffer()
Signed-off-by: Markus Burri <markus.burri@mt.com>
Link: https://patch.msgid.link/20250508130612.82270-4-markus.burri@mt.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: Markus Burri

drivers/iio/industrialio-core.c

@@ -411,12 +411,15 @@ static ssize_t iio_debugfs_write_reg(struct file *file,
 	char buf[80];
 	int ret;
 
+	if (count >= sizeof(buf))
+		return -EINVAL;
+
 	ret = simple_write_to_buffer(buf, sizeof(buf) - 1, ppos, userbuf,
 				     count);
 	if (ret < 0)
 		return ret;
 
-	buf[count] = '\0';
+	buf[ret] = '\0';
 
 	ret = sscanf(buf, "%i %i", &reg, &val);