The root cause of the CWE-787 (Out-of-bounds Write) vulnerability in the given code is that the `kmalloc` function is used to allocate memory for the `event` structure without checking the size of the `acpi_bus_event` structure.

If the size of the `acpi_bus_event` structure is larger than the maximum size that `kmalloc` can allocate, it can lead to a buffer overflow vulnerability. This is because the subsequent string copy operations (`strcpy`) on lines 14 and 15 can write beyond the allocated memory boundaries, resulting in an out-of-bounds write.

To mitigate this vulnerability, the code should ensure that the size of the `acpi_bus_event` structure is within the limits of the memory allocation function (`kmalloc` in this case) before proceeding with the string copy operations.