x86 pv: Insufficient care with non-coherent mappings

ISSUE DESCRIPTION

Xen maintains a type reference count for pages, in addition to a regular reference count. This scheme is used to maintain invariants required for Xen’s safety, e.g. PV guests may not have direct writeable access to pagetables; updates need auditing by Xen.
Unfortunately, Xen’s safety logic doesn’t account for CPU-induced cache non-coherency; cases where the CPU can cause the content of the cache to be different to the content in main memory. In such cases, Xen’s safety logic can incorrectly conclude that the contents of a page is safe.

IMPACT

Malicious x86 PV guest administrators can escalate privilege so as to control the whole system.

VULNERABLE SYSTEMS

All versions of Xen are vulnerable.
Only x86 PV guests can trigger this vulnerability.
Only x86 PV guests configured with access to devices (e.g. PCI Passthrough) can trigger the vulnerability.
Only CPUs which can issue non-coherent memory accesses are impacted. CPUs which enumerate the SelfSnoop feature are not impacted, except as noted in errata. Therefore, we believe that Xen running on Intel IvyBridge or later CPUs is not impacted by the vulnerability.