|Public release ||2016-11-22 12:00|
|Updated ||2016-11-22 12:00|
|Title ||x86 task switch to VM86 mode mis-handled|
Filesadvisory-192.txt (signed advisory file)
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Xen Security Advisory CVE-2016-9382 / XSA-192
x86 task switch to VM86 mode mis-handled
UPDATES IN VERSION 3
LDTR, just like TR, is purely a protected mode facility. Hence even
when switching to a VM86 mode task, LDTR loading needs to follow
protected mode semantics. This was violated by the code.
On SVM (AMD hardware): a malicious unprivileged guest process can
escalate its privilege to that of the guest operating system.
On both SVM and VMX (Intel hardware): a malicious unprivileged guest
process can crash the guest.
Only 32-bit x86 HVM guests are vulnerable. Furthermore, only guest
operating systems which actually make use of hardware task switching,
and allow a new task to start in VM86 mode, are vulnerable. We are
not aware of any such operating systems.
The vulnerability is NOT exposed on any PV guests.
The vulnerability is NOT exposed on any 64-bit guests,
ARM systems are NOT vulnerable.
Xen versions from 4.0 onwards are affected. Xen versions 3.4 and
earlier are not affected.
For guests which are affected, the vulnerability could possibly be
mitigated by disabling access to VM86 mode by unprivileged guest
programs. Details would depend on the (so far hypothetical)
vulnerable guest kernel.
This issue was discovered by Jan Beulich of SUSE.
Applying the appropriate attached patch resolves this issue.
xsa192.patch xen-unstable, Xen 4.7.x, Xen 4.6.x
xsa192-4.5.patch Xen 4.5.x, Xen 4.4.x
$ sha256sum xsa192*
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is then no longer applicable.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.)
For more information about permissible uses of embargoed information,
consult the Xen Project community's agreed Security Policy:
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Xenproject.org Security Team