A domain's primary array of vcpu pointers can be allocated by a toolstack exactly once in the lifetime of a domain via the XEN_DOMCTL_max_vcpus hypercall. This array is leaked on domain teardown. This memory leak could -- over time -- exhaust the host's memory.
A domain given partial management control via XEN_DOMCTL_max_vcpus can mount a denial of service attack affecting the whole system. The ability to also restart or create suitable domains is also required to fully exploit the issue. Without this the leak is limited to a small multiple of the maximum number of vcpus for the domain. The maximum leak is 64kbytes per domain (re)boot (less on ARM).
This issue is only relevant to systems which intend to increase security through the use of advanced disaggregated management techniques. This does not include systems using libxl, libvirt, or OpenStack (unless substantially modified or supplemented, as compared to versions supplied by the respective upstreams). Versions of Xen from 4.0 onwards are vulnerable. All architectures are affected.