An internal flag is used to temporarily suppress IOMMU TLB flushes, in
order to consolidate multiple single page flushes into one wider
flush.  This flag is not cleared again, on certain error paths.  This
can result in TLB flushes not happening when they are needed.
Retaining stale TLB entries could allow guests access to memory that
ought to have been revoked, or grant greater access than intended.

Malicious guest administrators might be able to cause host-wide denial of
service, or escalate their privilege to that of the host.

Acknowledgements:

Red Hat would like to thank the Xen project for reporting this issue.