With HVM domains, libxl's setup of PCI passthrough devices does the IOMMU setup after giving (via the device model) the guest access to the hardware and advertising it to the guest.
If the IOMMU is disabled the overall setup fails, but after the device has been made available to the guest; subsequent DMA instructions from the guest to the device will cause wild DMA.
A HVM domain, given access to a device which bus mastering capable in the absence of a functioning IOMMU, can mount a privilege escalation or denial of service attack affecting the whole system.
References:
http://lists.xen.org/archives/html/xen-announce/2013-09/msg00001.html
Acknowledgements:
Red Hat would like to thank the Xen project for reporting this issue.
Statement:
Not vulnerable.
This issue does not affect the versions of the xen package as shipped with Red Hat Enterprise Linux 5 as it does not provide support for the libxl toolstack.
This issue does not affect Red Hat Enterprise Linux 6 and Red Hat Enterprise MRG 2.