The root cause of the CWE-125 vulnerability in the given code slice is a potential out-of-bounds read from the user space.

Here's a breakdown of the issue:

- The `memdup_user` function is used to duplicate a user-space buffer into kernel space.
- The `count` variable represents the size of the user-space buffer.
- The `+ 1` in `count + 1` is likely intended to account for the null-terminator character at the end of the string.
- However, if the user-space buffer is not null-terminated or is larger than the specified `count`, this can lead to an out-of-bounds read.

The CWE-125 vulnerability occurs because the `memdup_user` function does not perform any bounds checking on the user-space buffer. If the buffer is not properly null-terminated or is larger than the specified `count`, the `memdup_user` function will read beyond the bounds of the buffer, potentially accessing sensitive data or causing a crash.

To fix this vulnerability, you should ensure that the user-space buffer is properly null-terminated and that the `count` variable accurately represents the size of the buffer. Additionally, you should consider using a safer function, such as `strncpy_from_user`, which performs bounds checking and can help prevent out-of-bounds reads.

Here's an example of how you could modify the code to fix the vulnerability:
```c
size_t bytes_copied = strncpy_from_user(cmd_buf, buffer, count);
if (bytes_copied != count) {
    // Handle error: buffer not null-terminated or too large
}
```
In this example, the `strncpy_from_user` function is used to copy the user-space buffer into kernel space, and the `bytes_copied` variable is used to check if the buffer was properly null-terminated. If the buffer was not null-terminated or was too large, an error is handled accordingly.