CVE-2020-11739

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
don’t 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.

Notes