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            Xen Security Advisory CVE-2019-17346 / XSA-292
                              version 3

            x86: insufficient TLB flushing when using PCID

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

CVE assigned.

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

Use of Process Context Identifiers (PCID) was introduced into Xen in
order to improve performance after XSA-254 (and in particular its
Meltdown sub-issue).  This enablement implied changes to the TLB
flushing logic.  The particular case of context switch to a vCPU of a
PCID-enabled guest left open a time window between the full TLB flush,
and the actual address space switch, during which additional TLB
entries (from the address space about to be switched away from) can be
accumulated, which will not subsequently be purged.

IMPACT
======

Malicious PV guests may be able to cause a host crash (Denial of
Service) or to gain access to data pertaining to other guests.
Privilege escalation opportunities cannot be ruled out.

Additionally, vulnerable configurations are likely to be unstable even
in the absence of an attack.

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

Only x86 systems are vulnerable.  ARM systems are not vulnerable.

Only systems running x86 PV guests are vulnerable.  Systems running
only x86 HVM or PVH guests are not vulnerable.

Only systems with at least one PCID-enabled PV guest are vulnerable.

Systems where PCID or INVPCID are unavailable or entirely disabled are
not vulnerable.

Note that PCID is enabled by default for both 64-bit dom0 and 64-bit
domU when hardware supports it.  PCID acceleration has been backported
to the following versions:
 - Xen 4.11.x,
 - Xen 4.10.2 and onwards,
 - Xen 4.9.3 and onwards,
 - Xen 4.8.4 and onwards,
 - Xen 4.7.6.

To exploit this vulnerability, problematic TLB entries must be created
between the full TLB flush and the address space switch.  The NMI
watchdog handler (enabled via the "watchdog" command line option) is
known to create such entries; other vectors cannot be ruled out.

MITIGATION
==========

Running only HVM or PVH guests will avoid this vulnerability.

Running only 32-bit PV guests alongside the other two types mentioned
above will also avoid this vulnerability, provided Dom0 is also 32-bit
or is not using PCID.  Making a 64-bit Dom0 not use PCID can be achieved
by e.g. "xpti=no-dom0 pcid=xpti".

Disabling use of PCID entirely, by passing "pcid=0" or "invpcid=0" as a
command line option to the hypervisor, will also avoid this
vulnerability (albeit re-introducing the XPTI performance regression
use of PCID was intended to reduce).

Disabling the watchdog timer will remove the only known way of reliably
creating problematic TLB entries, potentially reducing the risk of a
successful attack.

CREDITS
=======

This issue was discovered by Sergey Dyasli and Andrew Cooper of Citrix.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa292.patch           xen-unstable, Xen 4.11.x ... Xen 4.7.6

$ sha256sum xsa292*
c515e98e5ae8a16bc5c894741eea5523a7e568f81ee8a570626dcc0f58f40b40  xsa292.meta
f42cb5e1eae5a5c6f0fd84e38df4db9f09a4e1176905c37f292fef9855c82fea  xsa292.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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