The CWE-787 vulnerability is a "Out-of-bounds Write" vulnerability. 

In this specific case, the vulnerability is likely due to the fact that `nlmsg_new` allocates a buffer of a certain size, but the size calculation does not take into account the actual size of the data that will be written to the buffer.

The `NLMSG_ALIGN` macro is used to align the size of the `struct ifaddrmsg` to a certain boundary, but it does not guarantee that the allocated buffer will be large enough to hold all the data that will be written to it.

The root cause of the vulnerability is likely that the code does not properly validate the size of the data that will be written to the buffer, and does not check for buffer overflows. This can lead to a situation where more data is written to the buffer than it can hold, resulting in a buffer overflow and potentially allowing an attacker to execute arbitrary code.

To fix this vulnerability, the code should be modified to properly validate the size of the data that will be written to the buffer, and to check for buffer overflows. This can be done by using functions such as `skb_put` and `skb_tailroom` to ensure that the buffer is large enough to hold the data, and by checking the return value of `nlmsg_new` to ensure that the allocation was successful.

Here is an example of how the code could be modified to fix the vulnerability:
```c
skb = nlmsg_new(NLMSG_ALIGN(sizeof(struct ifaddrmsg)) + data_len, GFP_KERNEL);
if (!skb) {
    // handle allocation failure
}

// Check if the buffer is large enough to hold the data
if (skb_tailroom(skb) < data_len) {
    // handle buffer overflow
}

// Write data to the buffer using skb_put
skb_put(skb, data_len);
```
Note that this is just an example, and the actual fix will depend on the specific requirements of the code.