x86 pv: Race condition in typeref acquisition Xen maintains a type reference count for pages, in addition to a regular reference count. This scheme is used to maintain invariants required for Xens safety, e.g. PV guests may not have direct writeable access to pagetables; updates need auditing by Xen. Unfortunately, the logic for acquiring a type reference has a race condition, whereby a safely TLB flush is issued too early and creates a window where the guest can re-establish the read/write mapping before writeability is prohibited.
The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Xen | Xen | * | * |
| Xen | Ubuntu | bionic | * |
| Xen | Ubuntu | focal | * |
| Xen | Ubuntu | impish | * |
| Xen | Ubuntu | kinetic | * |
| Xen | Ubuntu | lunar | * |
| Xen | Ubuntu | mantic | * |
| Xen | Ubuntu | oracular | * |
| Xen | Ubuntu | trusty | * |
| Xen | Ubuntu | xenial | * |
A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:
A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.