[Xen-devel] Xen Security Advisory 60 (CVE-2013-2212) - Excessive time to disable caching with HVM guests with PCI passthrough

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

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             Xen Security Advisory CVE-2013-2212 / XSA-60
                             version 6

   Excessive time to disable caching with HVM guests with PCI passthrough

UPDATES IN VERSION 6
====================

Since the issue of this advisory, various fixes have been applied to
the public Xen trees.

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

HVM guests are able to manipulate their physical address space such that
processing a subsequent request by that guest to disable caches takes an
extended amount of time changing the cachability of the memory pages assigned
to this guest. This applies only when the guest has been granted access to
some memory mapped I/O region (typically by way of assigning a passthrough
PCI device).

This can cause the CPU which processes the request to become unavailable,
possibly causing the hypervisor or a guest kernel (including the domain 0 one)
to halt itself ("panic").

IMPACT
======

A malicious domain, given access to a device with memory mapped I/O
regions, can cause the host to become unresponsive for a period of
time, potentially leading to a DoS affecting the whole system.

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

Xen version 3.3 onwards is vulnerable.

Only systems using the Intel variant of Hardware Assisted Paging (aka EPT) are
vulnerable.

MITIGATION
==========

This issue can be avoided by not assigning PCI devices to untrusted guests, or
by running HVM guests with shadow mode paging (through adding "hap=0" to the
domain configuration file).

CREDITS
=======

Zhenzhong Duan found the issue as a bug, which on examination by the
Xenproject.org Security Team turned out to be a security problem.

RESOLUTION
==========

This issue has been fixed in the public xen.git trees.

For xen-unstable (#staging, #master), in these git commits:
  c13b0d65ddedd745 VMX: disable EPT when !cpu_has_vmx_pat
  1c84d046735102e0 VMX: remove the problematic set_uc_mode logic
  62652c00efa55fb4 VMX: fix cr0.cd handling
  86d60e855fe118df VMX: flush cache when vmentry back to UC guest
  f1c9658d6802c433 Revert "VMX: flush cache when vmentry back to UC guest"
(Earliest commit is listed first.  Note that f1c9658d reverts
not only 86d60e85 but also part of 62652c00.)

For Xen 4.2 (#staging-4.2, #stable-4.2):
  f1e0df14412c VMX: disable EPT when !cpu_has_vmx_pat
  644e6c5c7106 VMX: remove the problematic set_uc_mode logic
  0fffcffeb594 VMX: fix cr0.cd handling
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