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Advisory

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            Xen Security Advisory CVE-2018-12892 / XSA-266
                               version 3

      libxl fails to honour readonly flag on HVM emulated SCSI disks

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

Public release.

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

libxl fails to pass the readonly flag to qemu when setting up a SCSI
disk, due to what was probably an erroneous merge conflict resolution.

IMPACT
======

Malicious guest administrators or (in some situations) users may be
able to write to supposedly read-only disk images.

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

Only emulated SCSI disks (specified as "sd" in the libxl disk
configuration, or an equivalent) are affected.  IDE disks ("hd") are
not affected (because attempts to make them readonly are rejected).

Additionally, CDROM devices (that is, devices specified to be
presented to the guest as CDROMs, regardless of the nature of the
backing storage on the host) are not affected; they are always
readonly.

Only systems using qemu-xen (rather than qemu-xen-traditional) as the
device model version are vulnerable.

Only systems using libxl or libxl-based toolstacks are vulnerable.
(This includes xl, and libvirt with the libxl driver.)

The vulnerability is present in Xen versions 4.7 and later.
(In earlier versions, provided that the patch for XSA-142 has been
applied, attempts to create readonly disks are rejected.)

If the host and guest together usually support PVHVM, the issue is
exploitable only if the malicious guest administrator has control of
the guest kernel or guest kernel command line.

MITIGATION
==========

Switching to qemu-xen-traditional will avoid this vulnerability.
This can be done with
   device_model_version="qemu-xen-traditional"
in the xl configuration file.

Using stub domain device models (which necessarily involves switching
to qemu-xen-traditional) will also avoid this vulnerability.
This can be done with
   device_model_stubdomain_override=true
in the xl configuration file.

All of these mitigations are liable to have other guest-visible
effects or even regressions.

It may be possible, depending on the configuration, to make the
underlying storage object readonly, or to make it reject writes.

CREDITS
=======

This issue was discovered by Andrew Reimers of OrionVM.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa266/*.patch           xen-unstable
xsa266-4.10/*.patch      Xen 4.10.x
xsa266-4.9/*.patch       Xen 4.9.x
xsa266-4.8/*.patch       Xen 4.8.x
xsa266-4.7/*.patch       Xen 4.7.x
xsa266-4.6/*.patch       Xen 4.6.x

$ sha256sum xsa266* xsa266*/*
d0d998bb3c2f36b0795cdf86d52aa2da3eee72218f9073f398fc6fd2cf5719cd  xsa266.meta
0e5634c9b730e2e022bfef9ded2bb81b7740d05911dae6499671db5cb90663c0  xsa266-4.7/0001-libxl-qemu_disk_scsi_drive_string-Break-out-common-p.patch
e6dcef1bdd890a245cb9181266fc1378d77b08cf06c063f35a0835ab3b99cf91  xsa266-4.7/0002-libxl-restore-passing-readonly-to-qemu-for-SCSI-disk.patch
19ce6f236702219eb4831ed597f82dc81122fd517131e826643cee95b53d9f1c  xsa266-4.8/0001-libxl-qemu_disk_scsi_drive_string-Break-out-common-p.patch
e0a4c616218bc42abada75aa5fa0c3e35da6b6334fe50d6104a5892ffebcdb04  xsa266-4.8/0002-libxl-restore-passing-readonly-to-qemu-for-SCSI-disk.patch
9fd48f20da140731bb71dde07035b938cf0966339449a0b6833787767c588c0a  xsa266-4.9/0001-libxl-qemu_disk_scsi_drive_string-Break-out-common-p.patch
f23d0e76f15b1f6af487adc36a84cf2591197548ca7cab8ee84be72a87424cf7  xsa266-4.9/0002-libxl-restore-passing-readonly-to-qemu-for-SCSI-disk.patch
3d857f38d11b5531a651a45c2f151ac1493260524d4f49ead6833b5f1d599e64  xsa266-4.10/0001-libxl-qemu_disk_scsi_drive_string-Break-out-common-p.patch
e380976abd77b5b46d69c9564aca3acf9bf467b36645ac34e035aba89d081591  xsa266-4.10/0002-libxl-restore-passing-readonly-to-qemu-for-SCSI-disk.patch
160dc8c8a918cae7259c252af098206f9eff357e52bdfc0b15553e9c31c587e6  xsa266/0001-libxl-qemu_disk_scsi_drive_string-Break-out-common-p.patch
2b44fd6baac094c82145667a16d9b1530b97fa342d0e635c831425b53a336266  xsa266/0002-libxl-restore-passing-readonly-to-qemu-for-SCSI-disk.patch
$

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

Deployment of patches or 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 all of the patches and mitigations make significant
guest-visible changes.  In particular, applying the patch will cause
the emulated SCSI disk object to be reported to the guest as readonly,
when previously it was reported as writeable.

Deployment is permitted only AFTER the embargo ends.

(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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