This issue was addressed with improved checks. This issue is fixed in tvOS 14.6, Security Update 2021-004 Mojave, iOS 14.6 and iPadOS 14.6, Security Update 2021-003 Catalina, macOS Big Sur 11.4, watchOS 7.5. A local attacker may be able to elevate their privileges.
Another race in XENMAPSPACE_grant_table handling Guests are permitted access to certain Xen-owned pages of memory. The majority of such pages remain allocated / associated with a guest for its entire lifetime. Grant table v2 status pages, however, are de-allocated when a guest switches (back) from v2 to v1. Freeing such pages requires that the hypervisor enforce that no parallel request can result in the addition of a mapping of such a page to a guest. That enforcement was missing, allowing guests to retain access to pages that were freed and perhaps re-used for other purposes. Unfortunately, when XSA-379 was being prepared, this similar issue was not noticed.
The issue was addressed with improved permissions logic. This issue is fixed in macOS Big Sur 11.3, Security Update 2021-002 Catalina, Security Update 2021-003 Mojave. A local attacker may be able to elevate their privileges.