Bug 1323955 (CVE-2016-3960, xsa173)

ISSUE DESCRIPTION
=================

In the x86 shadow pagetable code, the guest frame number of a
superpage mapping is stored in a 32-bit field.  If a shadowed guest
can cause a superpage mapping of a guest-physical address at or above
2^44 to be shadowed, the top bits of the address will be lost, causing
an assertion failure or NULL dereference later on, in code that
removes the shadow.


IMPACT
======

A HVM guest using shadow pagetables can cause the host to crash.

A PV guest using shadow pagetables (i.e. being migrated) with PV
superpages enabled (which is not the default) can crash the host, or
corrupt hypervisor memory, and so a privilege escalation cannot be
ruled out.


VULNERABLE SYSTEMS
==================

Xen versions from 3.4 onwards are affected.

Only x86 variants of Xen are susceptible.  ARM variants are not
affected.

HVM guests using shadow mode paging can expose this vulnerability.  HVM
guests using Hardware Assisted Paging (HAP) are unaffected.

Systems running PV guests with PV superpages enabled are vulnerable if
those guests undergo live migration.  PV superpages are disabled by
default, so systems are not vulnerable in this way unless
"allowsuperpage" is on the Xen command line.

To discover whether your HVM guests are using HAP, or shadow page
tables: request debug key `q' (from the Xen console, or with
`xl debug-keys q').  This will print (to the console, and visible in
`xl dmesg'), debug information for every domain, containing something
like this:

(XEN) General information for domain 2:
(XEN)     refcnt=1 dying=2 pause_count=2
(XEN)     nr_pages=2 xenheap_pages=0 shared_pages=0 paged_pages=0 dirty_cpus={} max_pages=262400
(XEN)     handle=ef58ef1a-784d-4e59-8079-42bdee87f219 vm_assist=00000000
(XEN)     paging assistance: hap refcounts translate external
^^^
The presence of `hap' here indicates that the host is not
vulnerable to this domain.  For an HVM domain the presence of `shadow'
indicates that the domain can exploit the vulnerability.


MITIGATION
==========

Running only PV guests will avoid this vulnerability, unless PV
superpage support is enabled (see above).

Running HVM guests with Hardware Assisted Paging (HAP) enabled will
also avoid this vulnerability.  This is the default mode on hardware
supporting HAP, but can be overridden by hypervisor command line
option and guest configuration setting.  Such overrides ("hap=0" in
either case, with variants like "no-hap" being possible in the
hypervisor command line case) would need to be removed to avoid this
vulnerability.


External References:

http://xenbits.xen.org/xsa/advisory-173.html

Acknowledgements:

Name: the Xen project