Description of the problem: It is possible that subpage mmio is registered over existing memory page. When this happens "memory" will have real memory address and not index into io_mem array so next access to the page will generate segfault. It is uncommon to have some part of a page to be accessed as memory and some as mmio, but qemu shouldn't crash even when guest does stupid things. So lets just pretend that the rest of the page is unassigned if guest configure part of the memory page as mmio. Public post: http://www.spinics.net/lists/kvm/msg39173.html
This issue has been addressed in following products: Red Hat Enterprise Linux 5 Via RHSA-2010:0627 https://rhn.redhat.com/errata/RHSA-2010-0627.html
This issue has been addressed in following products: Red Hat Enterprise Virtualization for RHEL-5 Via RHSA-2010:0622 https://rhn.redhat.com/errata/RHSA-2010-0622.html