The root cause of the CWE-787 (Out-of-bounds Write) vulnerability in this code slice is:

Integer overflow in buffer size calculation

Explanation:
1. The vulnerability occurs in the calculation of the buffer size for the new sk_buff structure.

2. The size is computed by adding three values: newheadroom + skb->len + newtailroom

3. If these values are large enough, their sum could overflow the integer type used to store the result, leading to a smaller-than-intended buffer allocation.

4. This smaller allocation could then result in out-of-bounds writes when the buffer is used, as the actual data might exceed the allocated space.

5. The __alloc_skb function likely uses the calculated size to allocate memory, but due to the potential overflow, it may allocate less memory than required.

To mitigate this vulnerability, the code should include checks to prevent integer overflow before performing the allocation, such as:

1. Validating input parameters (newheadroom, newtailroom) to ensure they are within reasonable bounds.
2. Using safe arithmetic operations that detect overflow.
3. Implementing a maximum size limit for the total buffer allocation.

By addressing these issues, the code can prevent potential out-of-bounds writes caused by insufficient memory allocation due to integer overflow.