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

The function nlmsg_new() is called with an incomplete size calculation. The size argument is only partially computed, ending with a '+' operator, which means the actual allocated size for the sk_buff structure is undefined or potentially smaller than intended. This can lead to buffer overflow situations when writing data to the allocated buffer, as the size might not be sufficient to hold all the required information.

To fix this issue, the size calculation should be completed properly, ensuring that enough memory is allocated for the entire message, including any additional data that needs to be stored. The correct size calculation should account for all components of the message structure and any payload data.