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Advisory

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    Xen Security Advisory CVE-2019-19581,CVE-2019-19582 / XSA-307
                              version 4

                        find_next_bit() issues

UPDATES IN VERSION 4
====================

Canonicalize version ranges for easier parsing.

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

In a number of places bitmaps are being used by the hypervisor to track
certain state.  Iteration over all bits involves functions which may
misbehave in certain corner cases:
- - On 32-bit Arm accesses to bitmaps with bit a count which is a multiple
  of 32, an out of bounds access may occur.  (CVE-2019-19581)
- - On x86 accesses to bitmaps with a compile time known size of 64 may
  incur undefined behavior, which may in particular result in infinite
  loops. (CVE-2019-19582)

IMPACT
======

A malicious guest may cause a hypervisor crash or hang, resulting in a
Denial of Service (DoS).

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

All versions of Xen are vulnerable.

32-bit Arm systems are vulnerable.

x86 systems with 64 or more nodes are vulnerable.  We are unaware of any
such systems that Xen would run on.

64-bit Arm systems as well as x86 systems with less than 64 nodes are
not vulnerable.

MITIGATION
==========

There is no known mitigation for 32-bit Arm systems.

For x86 systems the issue can be avoided by suppressing the use of NUMA
information provided by firmware, via the "numa=off" command line
option.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa307.patch           xen-unstable, Xen 4.13.x - 4.8.x

$ sha256sum xsa307*
e589e96a0b3ec66f1d2d6393b82fab13ed18fd9fb112044a12263336b8499c68  xsa307.meta
7df052768cc05329bc44bf724897227885da8bb2cde9ff01d0ba2a34611bde97  xsa307.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
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is then no longer applicable.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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