Xen Security Advisory 275 v2 - insufficient TLB flushing / improper large page mappings with AMD IOMMUs

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Xen Security Advisory 275 v2 - insufficient TLB flushing / improper large page mappings with AMD IOMMUs

Xen.org security team
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                    Xen Security Advisory XSA-275
                              version 2

  insufficient TLB flushing / improper large page mappings with AMD IOMMUs

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

Public release.

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

In order to be certain that no undue access to memory is possible
anymore after IOMMU mappings of this memory have been removed,
Translation Lookaside Buffers (TLBs) need to be flushed after most
changes to such mappings.  Xen bypassed certain IOMMU flushes on AMD
x86 hardware.

Furthermore logic exists Xen to re-combine small page mappings
into larger ones.  Such re-combination could have occured in cases
when it was not really safe/correct to do so.

IMPACT
======

A malicious or buggy guest may be able to escalate its privileges, may
cause a Denial of Service (DoS) affecting the entire host, or may be
able to access data it is not supposed to access (information leak).

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

Xen versions from at least 3.2 onwards are affected.  Note that the
situation is worse in 4.1 and earlier, in that there's no flushing of
the TLB at all.

Only systems with AMD x86 hardware with enabled IOMMU are affected.

ARM and Intel x86 systems, and AMD x86 systems without enabled IOMMU,
are not affected.

Only systems where physical PCI devices are assigned to untrusted guests
are vulnerable.

MITIGATION
==========

There is no known mitigation for affected system/guest combinations.

CREDITS
=======

This issue was discovered by Paul Durrant of Citrix.

RESOLUTION
==========

Applying the appropriate set of attached patches resolves this issue.

xsa275-?.patch           xen-unstable
xsa275-4.11-?.patch      Xen 4.11.x ... Xen 4.8.x
xsa275-4.7-?.patch       Xen 4.7.x

$ sha256sum xsa275*
b5a02598cd2cffcc2cb59c724eeabb50220fa55f2cbe571726a5228909bf7bfe  xsa275.meta
7a3360e61fbb088f7d9f2b92921c9dceb08a1e01563c42ba4cf4a9999fe42fc4  xsa275-1.patch
4783a3abd2d87386ce9a7b790666ad398c5e027a6a146fce6424f0bcbfd8a7c6  xsa275-2.patch
49844d06f24ea129f1a501b4b0d5cb6ec3b288f3a2b41377ce793cc6fc81a788  xsa275-4.7-1.patch
7ea8bf2ff2c8c92cb064a70959a1148229c4577109015bd5aab72603ccb8f7e3  xsa275-4.7-2.patch
15d1aa7528368ed92caf8ea9baf77a406e1de26d0697dafd8a85da0d66eb95dc  xsa275-4.11-1.patch
0806e8c904ac9e8eb89404dffd227fcd56da84b7eb0150ee1e9b4bee54a05b4e  xsa275-4.11-2.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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xsa275.meta (2K) Download Attachment
xsa275-1.patch (6K) Download Attachment
xsa275-2.patch (3K) Download Attachment
xsa275-4.7-1.patch (5K) Download Attachment
xsa275-4.7-2.patch (4K) Download Attachment
xsa275-4.11-1.patch (5K) Download Attachment
xsa275-4.11-2.patch (3K) Download Attachment