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      Xen Security Advisory CVE-2015-5307,CVE-2015-8104 / XSA-156
                              version 2

              x86: CPU lockup during exception delivery

UPDATES IN VERSION 2
====================

Minor title and text adjustment.

CVE-2015-8104 has been assigned for the problem with #DB.
(The #AC issue remains CVE-2015-5307.)

Public release.

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

When a benign exception occurs while delivering another benign
exception, it is architecturally specified that these would be
delivered sequentially. There are, however, cases where this results in
an infinite loop inside the CPU, which (in the virtualized case) can be
broken only by intercepting delivery of the respective exception.

Architecturally, at least some of these cases should also be
resolvable by an arriving NMI or external interrupt, but empirically
this has been determined to not be the case.

The cases affecting Xen are:

#AC (Alignment Check Exception, CVE-2015-5307): When a 32-bit guest
sets up the IDT entry corresponding to this exception to reference a
ring-3 handler, and when ring 3 code triggers the exception while
running with an unaligned stack pointer, delivering the exception will
re-encounter #AC, ending in an infinite loop.

#DB (Debug Exception, CVE-2015-8104): When a guest sets up a hardware
breakpoint covering a data structure involved in delivering #DB, upon
completion of the delivery of the first exception another #DB will
need to be delivered. The effects slightly differ depending on further
guest characteristics:

- - Guests running in 32-bit mode would be expected to sooner or later
  encounter another fault due to the stack pointer decreasing during
  each iteration of the loop. The most likely case would be #PF (Page
  Fault) due to running into unmapped virtual space. However, an
  infinite loop cannot be excluded (e.g. when the guest is running with
  paging disabled).

- - Guests running in long mode, but not using the IST (Interrupt Stack
  Table) feature for the IDT entry corresponding to #DB would behave
  similarly to guests running in 32-bit mode, just that the larger
  virtual address space allows for a much longer loop. The loop can't,
  however, be infinite, as eventually the stack pointer would move into
  non-canonical address space, causing #SS (Stack Fault) instead.

- - Guests running in long mode and using IST for the IDT entry
  corresponding to #DB would enter an infinite loop, as the stack
  pointer wouldn't change between #DB instances.

IMPACT
======

A malicious HVM guest administrator can cause a denial of service.
Specifically, prevent use of a physical CPU for a significant, perhaps
indefinite period.

If a host watchdog (Xen or dom0) is in use, this can lead to a
watchdog timeout and consequently a reboot of the host.  If another,
innocent, guest, is configured with a watchdog, this issue can lead to
a reboot of such a guest.

It is possible that a guest kernel might expose the #AC vulnerability
to malicious unprivileged guest users (by permitting #AC to be handled
in guest user mode).  However, we believe that almost all ordinary
operating system kernels do not permit this; we are not aware of any
exceptions.  (A guest kernel which exposed the #AC vulnerability to
guest userspace would be vulnerable when running on baremetal, without
Xen involved.)


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

The vulnerability is exposed to any x86 HVM guest.

ARM is not vulnerable.  x86 PV VMs are not vulnerable.

All versions of Xen are affected.

x86 CPUs from all manufacturers are affected.

MITIGATION
==========

Running only PV guests will avoid this issue.

Running only kernels which avoid exposing the #AC problem to userspace
(as discussed in Impact) will prevent untrusted guest users from
exploiting this issue.

With such good kernels, the vulnerability can be avoided altogether if
the guest kernel is controlled by the host rather than guest
administrator, provided that further steps are taken to prevent the
guest administrator 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.  In Xen HVM, controlling
the guest's kernel would involve locking down the bootloader.


CREDITS
=======

These issues were discovered by Ben Serebrin from Google and
Jan Beulich from SUSE.

RESOLUTION
==========

To correctly support the intended uses of the relevant CPU features
would require architectural changes to the CPU specification, design
and implementation.  This is not practical as a security response.

Applying the appropriate attached patch works around the issue in
software.

xsa156.patch        xen-unstable, Xen 4.6.x
xsa156-4.5.patch    Xen 4.5.x
xsa156-4.4.patch    Xen 4.4.x
xsa156-4.3.patch    Xen 4.3.x

$ sha256sum xsa156*.patch
ffc8153cdf4e69ff2feced6ea4988b594b5cb724e9909300209f9ae35fe0e618  xsa156-4.3.patch
c2001aed46840b044a066b9ca79a8c53aca26fc637125016ccfebafa5ace5475  xsa156-4.4.patch
af8edc5cfb2fe54d8c195b8748e80ffad0f32c37c50a16fa5005fec461cdb6ff  xsa156-4.5.patch
d92729ca9174f7d1d8c6fd31321d1a58696c0630e87420539c32f7718b9e8ee8  xsa156.patch
$


NOTE REGARDING EMBARGO DURATION
===============================

We have released this advisory as soon as possible after we obtained
firm confirmation of the embargo end date from the discoverer.


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
administrators.

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
Team.


(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:
  http://www.xenproject.org/security-policy.html
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