An issue was discovered in Xen through 4.12.x allowing x86 PV guest OS users to gain host OS privileges by leveraging race conditions in pagetable promotion and demotion operations. There are issues with restartable PV type change operations. 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. A malicious PV guest administrator may be able to escalate their privilege to that of the host. All x86 systems with untrusted PV guests are vulnerable. HVM and PVH guests cannot exercise this vulnerability.
The product contains a code sequence that can run concurrently with other code, and the code sequence 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 that is operating concurrently.
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Xen | Xen | * | 4.12.1 (including) |
| Xen | Ubuntu | bionic | * |
| Xen | Ubuntu | disco | * |
| Xen | Ubuntu | eoan | * |
| Xen | Ubuntu | esm-infra/bionic | * |
| Xen | Ubuntu | esm-infra/xenial | * |
| Xen | Ubuntu | trusty | * |
| Xen | Ubuntu | xenial | * |
This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider. A race condition occurs within concurrent environments, and 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. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single “x++” statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x). 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.