Xen Security Advisory XSA-116

xen crash due to use after free on hvm guest teardown

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

Certain data accessible (via hypercalls) by the domain controlling the
execution of a HVM domain is being freed prematurely, leading to the
respective memory regions to possibly be read from and written to in
ways unexpected by their new owner(s).

IMPACT
======

Malicious or buggy stub domain kernels or tool stacks otherwise living
outside of Domain0 can mount a denial of service attack which, if
successful, can affect the whole system.

Only domains controlling HVM guests can exploit this vulnerability.
(This includes domains providing hardware emulation services to HVM
guests.)

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

Xen versions from 4.2 onwards are vulnerable on x86 systems.
ARM systems are not vulnerable.

This vulnerability is only applicable to Xen systems using stub domains
or other forms of disaggregation of control domains for HVM guests.

MITIGATION
==========

Running only PV guests will avoid this issue.

(The security of a Xen system using stub domains 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.)

CREDITS
=======

The issue was discovered by Mihai Donțu from Bitdefender who also
supplied the fix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa116.patch xen-unstable, Xen 4.4.x
xsa116-4.3-4.2.patch Xen 4.3.x, Xen 4.2.x

$ sha256sum xsa116*.patch
84b5a7bb2386e3d95d9d836a4a2504870723694ddaf537f1b59db75b7c63e9bd xsa116.patch
3aed6d157f62343a806347ea7c37bb8cdf50ee68002449bded9c7c1712810201 xsa116-4.3-4.2.patch
$