kvmtool through 39181fc allows an out-of-bounds write, related to virtio/balloon.c and virtio/pci.c. This allows a guest OS user to execute arbitrary code on the host machine.
https://www.kalmarunionen.dk/writeups/2021/hxp-2021/lkvm/
https://git.kernel.org/pub/scm/linux/kernel/git/will/kvmtool.git/log/
https://cdn.discordapp.com/attachments/921419715170164776/921882173517230100/exploit.c