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AUSCERT External Security Bulletin Redistribution
Xen Security Advisory CVE-2014-3124 / XSA-92 version 3
7 May 2014
AusCERT Security Bulletin Summary
Operating System: Xen
UNIX variants (UNIX, Linux, OSX)
Impact/Access: Execute Arbitrary Code/Commands -- Existing Account
Denial of Service -- Existing Account
CVE Names: CVE-2014-3124
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Xen Security Advisory CVE-2014-3124 / XSA-92
HVMOP_set_mem_type allows invalid P2M entries to be created
UPDATES IN VERSION 3
This issue has been assigned CVE-2014-3124.
The implementation in Xen of the HVMOP_set_mem_type HVM control
operations attempts to exclude transitioning a page from an
inappropriate memory type. However, only an inadequate subset of
memory types is excluded.
There are certain other types that don't correspond to a particular
valid page, whose page table translation can be inappropriately
changed (by HVMOP_set_mem_type) from not-present (due to the lack of
valid memory page) to present. If this occurs, an invalid translation
will be established.
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 crash leading to a Denial of Service.
Arbitrary code execution, and therefore privilege escalation, cannot
be entirely excluded: On a system with a RAM page present immediately
below the 52-bit address boundary, this would be possible. However,
we are not aware of any systems with such a memory layout.
All Xen versions from 4.1 onwards are vulnerable.
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
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 exercise 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.
xsa92.patch xen-unstable, Xen 4.4.x, Xen 4.3.x
xsa92-4.2.patch Xen 4.2.x
xsa92-4.1.patch Xen 4.1.x
$ sha256sum xsa92*.patch
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