iommu/iommufd: Fix NULL pointer deref in iommufd_ioas_change_process when racing with iopt_map_file_pages
In the Linux kernel, the following vulnerability has been resolved: iommu/iommufd: Fix NULL pointer deref in iommufd_ioas_change_process when racing with iopt_map_file_pages iommufd_ioas_change_process() iterates every IOAS area while only holding every IOAS iova_rwsem, so it assumes every area has a non-NULL pages pointer. That assumption can be false when it runs concurrently with iopt_map_file_pages(). iopt_map_pages() executes in two phases. It first creates the area and inserts it into the interval tree under iova_rwsem, with area->pages still NULL. It then drops iova_rwsem and later fills area->pages under domains_rwsem. This leaves