The root cause of the CWE-787 (Out-of-bounds Write) vulnerability in this code is a buffer overflow issue stemming from an incorrect buffer size check in the while loop condition on line 20.

Here's a detailed analysis of the root cause:

1. Buffer allocation:
   - On line 6, `bufsize` is calculated as `1 + size * 8 + 1 + 1`.
   - On line 14, `buf` is allocated with this `bufsize` using `kzalloc()`.

2. Incorrect size check:
   - The while loop on line 20 uses `sizeof(buf)` as a condition to check if there's space left in the buffer.
   - However, `sizeof(buf)` returns the size of the pointer `buf` (typically 4 or 8 bytes depending on the architecture), not the actual allocated size of the buffer.

3. Buffer overflow:
   - Due to this incorrect size check, the loop can continue writing beyond the allocated buffer size.
   - The loop will keep writing until it reaches `sizeof(buf)` bytes (e.g., 8 bytes), which is likely much smaller than the actual allocated size.
   - This allows writing beyond the end of the allocated buffer, causing an out-of-bounds write.

4. Consequences:
   - Writing beyond the allocated buffer can corrupt adjacent memory.
   - This can lead to unpredictable behavior, crashes, or potential security vulnerabilities if an attacker can control the input.

To fix this vulnerability, the correct allocated size (`bufsize`) should be used in the loop condition instead of `sizeof(buf)`. The correct check should be:

```c
while (i < bufsize - 1 && (pg - page < len) && ...)
```

This ensures that the writing stops when the actual buffer limit is reached, preventing the out-of-bounds write.