The root cause of the CWE-787 (Out-of-bounds Write) vulnerability in the given code is that the `sizeof(fsnotify_group)` is used to allocate memory for the `group` pointer without checking the actual size of the `fsnotify_group` structure.

If the size of the `fsnotify_group` structure is larger than expected, the memory allocated by `kmalloc` may not be sufficient, leading to a potential out-of-bounds write when initializing or using the `group` structure.

Without knowing the exact definition of the `fsnotify_group` structure, it's difficult to determine if the size calculation is correct or not. However, it's generally a good practice to use a safer method for calculating the required memory size, such as using `sizeof(struct fsnotify_group)` or defining a constant for the size of the structure.