CVE-2019-15792: Type confusion in shiftfs

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