xen-tools -- QEMU: Banked access to VGA memory (VBE) uses inconsistent bounds checks

The Xen Project reports:

Qemu VGA module allows banked access to video memory using the
window at 0xa00000 and it supports different access modes with
different address calculations.
Qemu VGA module allows guest to edit certain registers in ‘vbe’
and ‘vga’ modes.
A privileged guest user could use CVE-2016-3710 to exceed the bank
address window and write beyond the said memory area, potentially
leading to arbitrary code execution with privileges of the Qemu
process. If the system is not using stubdomains, this will be in
domain 0.
A privileged guest user could use CVE-2016-3712 to cause potential
integer overflow or OOB read access issues in Qemu, resulting in a DoS
of the guest itself. More dangerous effect, such as data leakage or
code execution, are not known but cannot be ruled out.