The CWE-787 vulnerability, also known as "Out-of-bounds Write," occurs when the software writes data past the end, or before the beginning, of the intended buffer. This can lead to buffer overflow, memory corruption, or other unintended behavior which might compromise the security or stability of the system.

In the provided code snippet from the function `skb_copy_expand`, the vulnerability specifically arises at line 8:
```c
struct sk_buff *n = __alloc_skb(newheadroom + skb->len + newtailroom,
```

Here's the breakdown of the root cause:

1. **Buffer Size Calculation**: The function `__alloc_skb` is called with the argument `newheadroom + skb->len + newtailroom`. This expression is intended to calculate the total size of a new buffer based on the existing `skb` buffer's length (`skb->len`) and additional space specified by `newheadroom` and `newtailroom`.

2. **Assumption of Valid `skb->len`**: The code assumes that the `skb->len` (length of the original `sk_buff` structure's buffer) is a valid, non-negative number. If `skb->len` is unexpectedly large (either due to a logical error earlier in the code or due to malicious manipulation), the calculation for the size of the new buffer might result in an integer overflow.

3. **Integer Overflow**: If `newheadroom`, `skb->len`, and `newtailroom` together exceed the maximum value that can be held by the data type (typically an integer), an integer overflow can occur. This overflow would result in a smaller buffer being allocated than intended.

4. **Insufficient Buffer Allocation**: Due to the potential integer overflow, `__alloc_skb` might allocate a buffer smaller than required. Subsequent operations that assume the buffer is large enough to hold `newheadroom + skb->len + newtailroom` bytes of data could then write beyond the bounds of the allocated buffer, leading to memory corruption.

5. **Lack of Validation**: There is no explicit validation or checking of the values of `newheadroom`, `skb->len`, or `newtailroom` before their use in the buffer size calculation. This lack of validation increases the risk of overflow and the associated vulnerabilities.

To mitigate this vulnerability, the code should include:
- Checks to ensure that `newheadroom`, `skb->len`, and `newtailroom` are within reasonable limits and that their sum does not cause an integer overflow.
- Use of safe arithmetic operations that can detect and handle overflows.
- Proper error handling if the size calculation exceeds expected limits or if memory allocation fails.

By addressing these issues, the code can be made more robust against buffer overflow attacks and other related security threats.