Xen Security Advisory CVE-2020-11739 / XSA-314
Missing memory barriers in read-write unlock paths
UPDATES IN VERSION 3
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
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
A malicous 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.
Systems running all versions of Xen are affected.
Whether an individual Arm-based CPU is vulnerable depends on its memory
re-ordering properties. Consult your CPU vendor.
x86 systems are not vulnerable.
There is no known mitigation.
This issue was discovered by Julien Grall of Amazon.
Applying the appropriate attached patch resolves this issue.
Note that patches for released versions are generally prepared to
apply to the stable branches, and may not apply cleanly to the most
recent release tarball. Downstreams are encouraged to update to the
tip of the stable branch before applying these patches.
DEPLOYMENT DURING EMBARGO
Deployment of the patches and/or mitigations described above (or
others which are substantially similar) is permitted during the
embargo, even on public-facing systems with untrusted guest users and
But: Distribution of updated software is prohibited (except to other
members of the predisclosure list).
Predisclosure list members who wish to deploy significantly different
patches and/or mitigations, please contact the Xen Project Security
(Note: this during-embargo deployment notice is retained in
post-embargo publicly released Xen Project advisories, even though it
is then no longer applicable. This is to enable the community to have
oversight of the Xen Project Security Team's decisionmaking.)