CVE-2019-15792: Access of Resource Using Incompatible Type

In shiftfs, a non-upstream patch to the Linux kernel included in the Ubuntu 5.0 and 5.3 kernel series, shiftfs_btrfs_ioctl_fd_replace() calls fdget(oldfd), then without further checks passes the resulting file* into shiftfs_real_fdget(), which casts file->private_data, a void* that points to a filesystem-dependent type, to a "struct shiftfs_file_info *". As the private_data is not required to be a pointer, an attacker can use this to cause a denial of service or possibly execute arbitrary code.