The root cause of the CWE-125 (Out-of-bounds Read) vulnerability in the given code slice is:

The use of `memdup_user` function with an incorrect buffer size calculation. The vulnerability occurs because the function attempts to allocate and copy `count + 1` bytes from the user-provided buffer, which may lead to reading beyond the bounds of the input buffer.

Explanation:

1. The `memdup_user` function is used to copy data from user space to kernel space.

2. The size argument passed to `memdup_user` is `count + 1`, where `count` is the size of the user-provided buffer.

3. By adding 1 to `count`, the code assumes there's an extra byte available in the source buffer, which may not be the case.

4. If the user-provided buffer is exactly `count` bytes long, attempting to read `count + 1` bytes will result in an out-of-bounds read, potentially accessing memory beyond the allocated buffer.

This vulnerability can lead to information disclosure or system crashes if exploited. To fix this issue, the correct approach would be to use `count` instead of `count + 1` as the size argument for `memdup_user`, ensuring that only the actual available data is copied.