x86 PV guests can free memory pages while still keeping a stale TLB entry
pointing to them. A TLB flush is only issued by Xen (if needed) when the
page is re-used. Since it's possible for the page to be scrubbed ahead of
the TLB flush, there's a window where a PV guest can modify an already
scrubbed page.
All Xen versions from 4.13 onwards are vulnerable. Xen versions 4.12 and
earlier are not vulnerable as they lack the scrub-domheap command line
option.
Only x86 PV guests can exploit the vulnerability.
There is no known mitigation.