With its original introduction, GNTTAB_cache_flush was made available
uniformly. XSA-232 (CVE-2017-14318) then covered an omitted NULL
pointer check, yet the addition of that check failed to adjust the
subsequent error path. That error path fails to drop a page reference in
case the specified page is owned by a special system domain (DOM_IO and
DOM_COW are system domains which can have memory pages assigned to them.)
Credits
This issue was discovered independently by Andrew Cooper of Citrix and
Yehyeong Lee.
