Xen Security Advisory 298 v3 (CVE-2019-18425) - missing descriptor table limit checking in x86 PV emulation
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Xen Security Advisory CVE-2019-18425 / XSA-298
missing descriptor table limit checking in x86 PV emulation
UPDATES IN VERSION 3
When emulating certain PV guest operations, descriptor table accesses
are performed by the emulating code. Such accesses should respect the
guest specified limits, unless otherwise guaranteed to fail in such a
case. Without this, emulation of 32-bit guest user mode calls through
call gates would allow guest user mode to install and then use
descriptors of their choice, as long as the guest kernel did not
itself install an LDT. (Most OSes don't install any LDT by default).
32-bit PV guest user mode can elevate its privileges to that of the
Xen versions from at least 3.2 onwards are affected.
Only 32-bit PV guest user mode can leverage this vulnerability.
HVM, PVH, as well as 64-bit PV guests cannot leverage this
Arm systems are unaffected.
Running only HVM, PVH, or 64-bit PV guests will avoid this
This issue was discovered by Andrew Cooper of Citrix.
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is then no longer applicable.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.)