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AUSCERT External Security Bulletin Redistribution
Xen Security Advisory CVE-2015-7835 / XSA-148
30 October 2015
AusCERT Security Bulletin Summary
Operating System: Xen
UNIX variants (UNIX, Linux, OSX)
Impact/Access: Root Compromise -- Existing Account
CVE Names: CVE-2015-7835
Comment: Internet hosting service companies such as Amazon EC2, IBM
SoftLayer, Liquid Web, Fujitsu Global Cloud Platform, Linode,
OrionVM and Rackspace Cloud use Xen as the primary VM hypervisor for
their product offerings, and are likely to be impacted by this
Users of these product offerings are advised to ensure hosting
companies have updated their systems appropriately.
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Xen Security Advisory CVE-2015-7835 / XSA-148
x86: Uncontrolled creation of large page mappings by PV guests
UPDATES IN VERSION 4
The code to validate level 2 page table entries is bypassed when
certain conditions are satisfied. This means that a PV guest can
create writeable mappings using super page mappings.
Such writeable mappings can violate Xen intended invariants for pages
which Xen is supposed to keep read-only.
This is possible even if the "allowsuperpage" command line option is
Malicious PV guest administrators can escalate privilege so as to
control the whole system.
Xen 3.4 and onward are vulnerable.
Only x86 systems are vulnerable. ARM systems are not vulnerable.
Only PV guests can exploit the vulnerability. Both 32-bit and 64-bit
PV guests can do so.
Running only HVM guests will avoid this vulnerability.
On systems where the guest kernel is controlled by the host rather
than guest administrator, running only kernels which do not call these
hypercalls will also prevent untrusted guest users from exploiting
this issue. However untrusted guest administrators can still trigger
it unless further steps are taken to prevent them 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 ??? (??) of Alibaba.
Applying the appropriate attached patch resolves this issue.
xsa148.patch xen-unstable, Xen 4.6.x
xsa148-4.5.patch Xen 4.5.x
xsa148-4.4.patch Xen 4.4.x, Xen 4.3.x
$ sha256sum xsa148*.patch
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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