The blk_rq_map_user_iov function in block/blk-map.c in the Linux kernel before 4.8.14 does not properly restrict the type of iterator, which allows local users to read or write to arbitrary kernel memory locations or cause a denial of service (use-after-free) by leveraging access to a /dev/sg device.
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.