Before correction, the copy_file_range system call checked only for the CAP_READ and CAP_WRITE capabilities on the input and output file descriptors, respectively. Using an offset is logically equivalent to seeking, and the system call must additionally require the CAP_SEEK capability.
This incorrect privilege check enabled sandboxed processes with only read or write but no seek capability on a file descriptor to read data from or write data to an arbitrary location within the file corresponding to that file descriptor.