qemu -- code execution on host machine

Petr Matousek of Red Hat Inc. reports:

Due converting PIO to the new memory read/write api we no longer
provide separate I/O region lenghts for read and write operations.
As a result, reading from PIT Mode/Command register will end with
accessing pit->channels with invalid index and potentially cause
memory corruption and/or minor information leak.
A privileged guest user in a guest with QEMU PIT emulation enabled
could potentially (tough unlikely) use this flaw to execute
arbitrary code on the host with the privileges of the hosting QEMU
process.
Please note that by default QEMU/KVM guests use in-kernel (KVM) PIT
emulation and are thus not vulnerable to this issue.