Issues with restartable PV type change operations

ISSUE DESCRIPTION

To avoid using shadow pagetables for PV guests, Xen exposes the actual hardware pagetables to the guest. In order to prevent the guest from modifying these page tables directly, Xen keeps track of how pages are used using a type system; pages must be “promoted” before being used as a pagetable, and “demoted” before being used for any other type. Xen also allows for “recursive” promotions: i.e., an operating system promoting a page to an L4 pagetable may end up causing pages to be promoted to L3s, which may in turn cause pages to be promoted to L2s, and so on. These operations may take an arbitrarily large amount of time, and so must be re-startable.
Unfortunately, making recursive pagetable promotion and demotion operations restartable is incredibly complicated, and the code contains several races which, if triggered, can cause Xen to drop or retain extra type counts, potentially allowing guests to get write access to in-use pagetables.

IMPACT

A malicious PV guest administrator may be able to escalate their privilege to that of the host.

VULNERABLE SYSTEMS

All x86 systems with untrusted PV guests are vulnerable.
HVM and PVH guests cannot exercise this vulnerability. ARM systems are not vulnerable because ARM guests are all PVH.
All security-supported Xen versions are vulnerable.
Note that these attacks require very precise timing, which may be difficult to exploit in practice.