Q: Given the following code slice:
```
1 static ssize_t
2 bnad_debugfs_write_regrd(struct file *file, const char __user *buf,
3 		size_t nbytes, loff_t *ppos)
13 	void *kern_buf;
16 	kern_buf = memdup_user(buf, nbytes);```
which has a CWE-125 vulnerability at line:
```
16 	kern_buf = memdup_user(buf, nbytes);
```
Please analyze the root cause of the vulnerability.