The Floppy Disk Controller (FDC) in QEMU, as used in Xen 4.5.x and earlier and KVM, allows local guest users to cause a denial of service (out-of-bounds write and guest crash) or possibly execute arbitrary code via the (1) FDCMDREADID, (2) FDCMDDRIVESPECIFICATION_COMMAND, or other unspecified commands, aka VENOM.