[Xen-devel] Xen Security Advisory 91 - Hardware timer context is not properly context switched on ARM

[Xen.org security team <security@xxxxxxx>]

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                     Xen Security Advisory XSA-91
                               version 2

    Hardware timer context is not properly context switched on ARM

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

Public release.

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

When running on an ARM platform Xen was not context switching the
CNTKCTL_EL1 register, which is used by the guest kernel to control
access by userspace processes to the hardware timers. This meant that
any guest can reconfigure these settings for the entire system.

IMPACT
======

A malicious guest kernel can reconfigure CNTKCTL_EL1 to block
userspace access to the timer hardware for all domains, including
control domains. Depending on the other guest kernels in use this may
cause an unexpected exception in those guests which may lead to a
kernel crash and therefore a denial of service.

64-bit ARM Linux is known to be susceptible to crashing in this way.

A malicious guest kernel can also enable userspace access to the timer
control registers, which may not be expected by kernels running in
other domains. This can allow user processes to reprogram timer
interrupts and therefore lead to unexpected behaviour, potentially up
to and including crashing the guest. Userspace processes will also be
able to read the current timestamp value for the domain perhaps
leaking information to those processes.

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

Both 32- and 64-bit ARM systems are vulnerable from Xen 4.4 onwards.

x86 systems are not vulnerable.

MITIGATION
==========

None.

CREDITS
=======

Chen Baozi discovered this issue as a bug which was then diagnosed by
Julien Grall.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa91-unstable.patch                  xen-unstable
xsa91-4.4.patch                       Xen 4.4.x

$ sha256sum xsa91*.patch
8a3dc1f001274550acfe929a0a443b09f8164001f6eea76821bd87292b8732e0  
xsa91-4.4.patch
327ccd88f2d9bc21daf51f3e5c81cbae2e779a6f997715d9d0d95285c509ecbd  
xsa91-unstable.patch
$
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Attachment: xsa91-4.4.patch
Description: Binary data

Attachment: xsa91-unstable.patch
Description: Binary data

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