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Date: Wed, 02 Apr 2014 11:48:31 +0000
From: security team <>
CC: security team <>
Subject: Xen Security Advisory 89 (CVE-2014-2599) - HVMOP_set_mem_access
 is not preemptible

Hash: SHA1

             Xen Security Advisory CVE-2014-2599 / XSA-89
                              version 3

              HVMOP_set_mem_access is not preemptible


This issue has been assigned CVE-2014-2599.


Processing of the HVMOP_set_mem_access HVM control operations does not
check the size of its input and can tie up a physical CPU for extended
periods of time.


In a configuration where device models run with limited privilege (for
example, stubdom device models), a guest attacker who successfully
finds and exploits an unfixed security flaw in qemu-dm could leverage
the other flaw into a Denial of Service affecting the whole host.

In the more general case, in more abstract terms: a malicious
administrator of a domain privileged with regard to an HVM guest can
cause Xen to become unresponsive leading to a Denial of Service.


All Xen versions from 4.1 onwards are vulnerable. In 4.2 only 64-bit
versions of the hypervisor are vulnerable (HVMOP_set_mem_access is not
available in 32-bit hypervisors).

The vulnerability is only exposed to service domains for HVM guests
which have privilege over the guest.  In a usual configuration that
means only device model emulators (qemu-dm).

In the case of HVM guests whose device model is running in an
unrestricted dom0 process, qemu-dm already has the ability to cause
problems for the whole system.  So in that case the vulnerability is
not applicable.

The situation is more subtle for an HVM guest with a stub qemu-dm.
That is, where the device model runs in a separate domain (in the case
of xl, as requested by "device_model_stubdomain_override=1" in the xl
domain configuration file).  The same applies with a qemu-dm in a dom0
process subjected to some kind kernel-based process privilege
limitation (eg the chroot technique as found in some versions of

In those latter situations this issue means that the extra isolation
does not provide as good a defence (against denial of service) as
intended.  That is the essence of this vulnerability.

However, the security is still better than with a qemu-dm running as
an unrestricted dom0 process.  Therefore users with these
configurations should not switch to an unrestricted dom0 qemu-dm.

Finally, in a radically disaggregated system: where the HVM service
domain software (probably, the device model domain image) is not
always supplied by the host administrator, a malicious service domain
administrator can excercise this vulnerability.


Running only PV guests will avoid this vulnerability.

In a radically disaggregated system, restricting HVM service domains
to software images approved by the host administrator will avoid the


This issue was discovered by Jan Beulich.


Applying the appropriate attached patch resolves this issue.

xsa89.patch        xen-unstable, Xen 4.4.x, Xen 4.3.x, Xen 4.2.x
xsa89-4.1.patch    Xen 4.1.x

$ sha256sum xsa89*.patch
741c8fbbfa8e425d8debba17135d4c2e1e962d15717769bc93d68a65b5dc5ea6  xsa89.patch
7d965e9bf1894b7d909bfaddbc6b7bdcee0ba91b86942ce85e0ae80464f2463e  xsa89-4.1.patch
Version: GnuPG v1.4.12 (GNU/Linux)


Download attachment "xsa89.patch" of type "application/octet-stream" (3011 bytes)

Download attachment "xsa89-4.1.patch" of type "application/octet-stream" (1247 bytes)

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