Xen Security Advisory 264 (CVE-2018-12891) - preemption checks bypassed in x86 PV MM handling
-----BEGIN PGP SIGNED MESSAGE-----
Xen Security Advisory CVE-2018-12891 / XSA-264
preemption checks bypassed in x86 PV MM handling
UPDATES IN VERSION 3
Certain PV MMU operations may take a long time to process. For that
reason Xen explicitly checks for the need to preempt the current vCPU at
certain points. A few rarely taken code paths did bypass such checks.
By suitably enforcing the conditions through its own page table
contents, a malicious guest may cause such bypasses to be used for an
unbounded number of iterations.
A malicious or buggy PV guest may cause a Denial of Service (DoS)
affecting the entire host. Specifically, it may prevent use of a
physical CPU for an indeterminate period of time.
All Xen versions from 3.4 onwards are vulnerable. Xen versions 3.3 and
earlier are vulnerable to an even wider class of attacks, due to them
lacking preemption checks altogether in the affected code paths.
Only x86 systems are affected. ARM systems are not affected.
Only multi-vCPU x86 PV guests can leverage the vulnerability. x86 HVM
or PVH guests as well as x86 single-vCPU PV ones cannot leverage the
Running only HVM, PVH, or single-vCPU PV guests will avoid this
For PV guests, the vulnerability can be avoided if the guest kernel is
controlled by the host rather than guest administrator, provided that
further steps are taken to prevent the guest administrator from loading
code into the kernel (e.g. by disabling loadable modules etc) or from
using other mechanisms which allow them to run code at kernel privilege.
This issue was discovered by Jan Beulich of SUSE.
Applying the appropriate attached patch resolves this issue.
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is then no longer applicable.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.)