Xen Security Advisory 253 - x86: memory leak with MSR emulation

Xen Security Advisory 253 - x86: memory leak with MSR emulation

[Xen.org security team]

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                    Xen Security Advisory XSA-253
                              version 2

                  x86: memory leak with MSR emulation

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

Public release.

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

In Xen 4.10, new infrastructure was introduced as part of an overhaul to
how MSR emulation happens for guests.  Unfortunately, one tracking
structure isn't freed when a vcpu is destroyed.

IMPACT
======

A memory allocation of 8 bytes is leaked each time a vcpu is destroyed.

A malicious guest may, by frequently rebooting over extended periods of
time, run the system out of memory, resulting in a Denial of Service
(DoS).

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

Xen versions 4.10 and later are affected.  Xen 4.9 and earlier are not
affected.

Only x86 systems are affected.  ARM systems are not.

All guest kinds can exploit this vulnerability.

MITIGATION
==========

Limiting the frequency with which a guest is able to reboot, will
limit the memory leak.

Rebooting each host (after migrating its guests) periodically will
reclaim the leaked space.

CREDITS
=======

This issue was discovered by Andrew Cooper of Citrix.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa253.patch           Xen 4.10, xen-unstable

$ sha256sum xsa253*
bba1abb5e4368421de29385e37f8477bf3534d3ba3ff7e2aae9c9d3da53f1393  xsa253.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is then no longer applicable.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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From: Andrew Cooper <andrew.cooper3@citrix.com>
Subject: x86/msr: Free msr_vcpu_policy during vcpu destruction

c/s 4187f79dc7 "x86/msr: introduce struct msr_vcpu_policy" introduced a
per-vcpu memory allocation, but failed to free it in the clean vcpu
destruction case.

This is XSA-253

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index b17468c..0ae715d 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -382,6 +382,9 @@ void vcpu_destroy(struct vcpu *v)
 
     vcpu_destroy_fpu(v);
 
+    xfree(v->arch.msr);
+    v->arch.msr = NULL;
+
     if ( !is_idle_domain(v->domain) )
         vpmu_destroy(v);
 

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