An issue was discovered in Xen through 4.13.x, allowing guest OS users to cause a denial of service or possibly gain privileges because of missing memory barriers in read-write unlock paths. The read-write unlock paths dont contain a memory barrier. On Arm, this means a processor is allowed to re-order the memory access with the preceding ones. In other words, the unlock may be seen by another processor before all the memory accesses within the critical section. As a consequence, it may be possible to have a writer executing a critical section at the same time as readers or another writer. In other words, many of the assumptions (e.g., a variable cannot be modified after a check) in the critical sections are not safe anymore. The read-write locks are used in hypercalls (such as grant-table ones), so a malicious guest could exploit the race. For instance, there is a small window where Xen can leak memory if XENMAPSPACE_grant_table is used concurrently. A malicious guest may be able to leak memory, or cause a hypervisor crash resulting in a Denial of Service (DoS). Information leak and privilege escalation cannot be excluded.
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 | * | 4.13.0 (including) |
| Xen | Xen | 4.13.0-rc1 (including) | 4.13.0-rc1 (including) |
| Xen | Xen | 4.13.0-rc2 (including) | 4.13.0-rc2 (including) |
| Xen | Ubuntu | bionic | * |
| Xen | Ubuntu | eoan | * |
| Xen | Ubuntu | esm-apps/focal | * |
| Xen | Ubuntu | esm-infra/bionic | * |
| Xen | Ubuntu | esm-infra/xenial | * |
| Xen | Ubuntu | focal | * |
| Xen | Ubuntu | groovy | * |
| Xen | Ubuntu | hirsute | * |
| Xen | Ubuntu | impish | * |
| 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.