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            Xen Security Advisory CVE-2020-27671 / XSA-346
                              version 3

                  undue deferral of IOMMU TLB flushes

UPDATES IN VERSION 3
====================

CVE assigned.

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

To efficiently change the physical to machine address mappings of a
larger range of addresses for fully virtualized guests, Xen contains
an optimization to coalesce per-page IOMMU TLB flushes into a single,
wider flush after all adjustments have been made.  While this is fine
to do for newly introduced page mappings, the possible removal of
pages from such guests during this operation should not be "optimized"
in the same way.  This is because the (typically) final reference of
such pages is dropped before the coalesced flush, and hence the pages
may have been put to a different use even though DMA initiated by
their original owner mightstill be in progress.

IMPACT
======

A malicious guest might be able to cause data corruption and data
leaks.  Host or guest Denial of Service (DoS), and privilege
escalation, cannot be ruled out.

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

All Xen versions from 4.2 onwards are vulnerable.  Xen versions 4.1 and
earlier are not vulnerable.

Only x86 HVM and PVH guests can leverage the vulnerability.  Arm guests
as well as x86 PV ones cannot leverage the vulnerability.

Only x86 HVM and PVH guests which have physical devices passed through
to them can leverage the vulnerability.

Only x86 HVM and PVH guests configured to not share IOMMU and CPU
page tables can leverage the vulnerability.  Sharing these page tables
is the default on capable Intel (VT-d) hardware.  On AMD hardware
sharing is not possible.  On Intel (VT-d) hardware sharing may also not
be possible, depending on hardware properties.  Whether it is possible
can be seen from the presence (or absence) of "iommu_hap_pt_share" on
the "virt_caps" line of "xl info" output.  Guests run in shadow mode
can leverage the vulnerability.

MITIGATION
==========

Not passing through physical devices to untrusted guests will avoid
the vulnerability.

On systems permitting page table sharing, not suppressing use of the
functionality will allow to avoid the vulnerability. This means guests
should not be run in
* shadow mode, i.e. hardware needs to be HAP (Hardware Assisted Paging)
  capable, there should not be "hap=0" in the guest's xl configuration
  file, and there should not be "hap=0" or equivalent on Xen's command
  line,
* non-shared page table mode, i.e. hardware needs to be capable of
  sharing, there should not be "passthrough=sync_pt" in the guest's xl
  configuration file, and there should not be "iommu=no-sharept" or
  equivalent on Xen's command line.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

RESOLUTION
==========

Applying the appropriate pair of attached patches resolves this issue.

Note that patches for released versions are generally prepared to
apply to the stable branches, and may not apply cleanly to the most
recent release tarball.  Downstreams are encouraged to update to the
tip of the stable branch before applying these patches.

xsa346/xsa346-?.patch           Xen 4.14 - xen-unstable
xsa346/xsa346-4.13-?.patch      Xen 4.13
xsa346/xsa346-4.12-?.patch      Xen 4.12
xsa346/xsa346-4.11-?.patch      Xen 4.11
xsa346/xsa346-4.10-?.patch      Xen 4.10

$ sha256sum xsa346* xsa346*/*
ba560d34cb46f45d6da0ba5d672cb896c173e90de5c022d22415ace20c5e47b8  xsa346.meta
5f8b3e5565bc7d87283af173f5f2b35975e4ab6bff502780799d14fb263f730d  xsa346/xsa346-1.patch
9de89ca360f303e7aa3b42529cdf4191b0700ee7cb6928a22068195e047a4db7  xsa346/xsa346-2.patch
f3612bfad219160917a3bc46ea5b31673137593d62ae4f819a8e80ade0339c5b  xsa346/xsa346-4.10-1.patch
734ed82d583bbce342ffabeb9dd84e300f2717ec71e3de866670b0ddf18d57aa  xsa346/xsa346-4.10-2.patch
7a41bf06e19590cfc69d4f2ac132a23843dcec2ea5f98d86c4be971f9eec86af  xsa346/xsa346-4.11-1.patch
1359801b8f64ac62dc8de4e3acc15ec42c040f692f3a1ee9986acb478ee330cd  xsa346/xsa346-4.11-2.patch
190f594bb77dd044af8f0a051ab1d4143c348da192206da9b390af91c0a2cdec  xsa346/xsa346-4.12-1.patch
5bcb65dc45f6d74c644ee6b6add518044c9875e6759254773d3816e718c2be28  xsa346/xsa346-4.12-2.patch
69e0158276a922829eb60dc5bb13e60a71a232ace808843f45dac407716b107b  xsa346/xsa346-4.13-1.patch
eb8132a02c252dc65be1f334939f252db0c30ae2db8aa23f0d9e67f8148e2d2d  xsa346/xsa346-4.13-2.patch
$

DEPLOYMENT DURING EMBARGO
=========================

Deployment of the patches described above (or others which are
substantially similar) is permitted during the embargo, even on
public-facing systems with untrusted guest users and administrators.

HOWEVER, deployment of the mitigations is NOT permitted (except where
all the affected systems and VMs are administered and used only by
organisations which are members of the Xen Project Security Issues
Predisclosure List).  Specifically, deployment on public cloud systems
is NOT permitted.

This is because removal of pass-through devices or their replacement by
emulated devices is a guest visible configuration change, which may lead
to re-discovery of the issue.  Similarly the possible guest
configuration changes can't be excluded to be noticeable to guests.

Deployment of this mitigation is permitted only AFTER the embargo ends.

AND: 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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