Use of Process Context Identifiers (PCID) was introduced into Xen in order to improve performance after XSA-254 (and in particular its Meltdown sub-issue). This enablement implied changes to the TLB flushing logic. One aspect which was overlooked is the safety of switching between shadow pagetables, which previously relied on the unconditional flushing of a write to CR3. With PCID enabled, a switch of shadow pagetable for a 64bit PV guest fails to invalidate the linear mappings of the previous shadow pagetable. As a result, subsequent accesses to the shadow pagetables may be deemed to be safe by the shadow logic (based on the old shadow pagetable) but fault when made in practice.
References: https://seclists.org/oss-sec/2019/q1/165
Created xen tracking bugs for this issue: Affects: fedora-all [bug 1685577]