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Advisory

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            Xen Security Advisory CVE-2017-12135 / XSA-226
                               version 7

               multiple problems with transitive grants

UPDATES IN VERSION 7
====================

First patch provided in version 6 regressed 32-bit Dom0 or backend
domains. The updated patch includes a fix for this.

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

1) Code to handle copy operations on transitive grants has built in
   retry logic, involving a function reinvoking itself with unchanged
   parameters.  Such use assumes that the compiler would also translate
   this to a so called "tail call" when generating machine code.
   Empirically, this is not commonly the case, allowing for
   theoretically unbounded nesting of such function calls.

2) The reference counting and locking discipline for transitive grants
   is broken.  Concurrent use of the transitive grant can leak
   references on the transitively-referenced grant.

IMPACT
======

A malicious or buggy guest may be able to crash Xen.  Privilege
escalation and information leaks cannot be ruled out.  A malicious or
buggy guest can leak references on grants it has been given, amounting
to a DoS against the grantee.

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

All versions of Xen are vulnerable.

MITIGATION
==========

There is no known mitigation.

CREDITS
=======

This issue was discovered by Jan Beulich of SUSE.

The security team would also like to thank Amazon for helping to identify that
the problems with transitive grants were deeper than originally believed.

RESOLUTION
==========

Applying the appropriate attached pair of patches from the list below
addresses this issue:

xsa226-unstable/*.patch     xen-unstable
xsa226-4.9/*.patch          Xen 4.9.x, Xen 4.8.x, Xen 4.7.x
xsa226-4.6/*.patch          Xen 4.6.x
xsa226-4.5/*.patch          Xen 4.5.x

Note that these patches have already been applied to the respective staging
trees.

Alternatively, applying the appropriate attached patch from the list
below works around this issue by disabling transitive grants by default:

xsa226.patch           xen-unstable, Xen 4.9.x, Xen 4.8.x
xsa226-4.7.patch       Xen 4.7.x
xsa226-4.6.patch       Xen 4.6.x
xsa226-4.5.patch       Xen 4.5.x

$ sha256sum xsa226* xsa226*/*
b09e07aaf422ae04a4ece5e2c5b5e54036cfae5b5c632bfc6953a0cacd6f60ff  xsa226.patch
d999767014501d3ac62def06ccd43b97bbbf0ef7d402d3bd70ca96ac9997a14d  xsa226-unstable/0001-gnttab-dont-use-possibly-unbounded-tail-calls.patch
4473fd96ce4fdea5e19e0b502d65f20bd279d82473ac34ff404ce2b2cbc10be1  xsa226-unstable/0002-gnttab-fix-transitive-grant-handling.patch
ca8b92b2ff58b87e8bec137a34784cbf11e2820659046df6e1d71e23bf7e7dee  xsa226-4.5.patch
ca77d01172abf263b5b731f26f5e3f74b0b8c75b3e29bee3f65a9318236daba7  xsa226-4.5/0001-gnttab-dont-use-possibly-unbounded-tail-calls.patch
de6359e50fd2bb710469da74a596013ce275edb43d3d1c36d41452f88eee9b7d  xsa226-4.5/0002-gnttab-fix-transitive-grant-handling.patch
28c7df7edabb91fb2f1fa3fc7d6906bfae75a6e701f1cd335baafaae3e087696  xsa226-4.6.patch
0186f78e99f5f6eec913da8355e0c28946a14a6099a7219bd4e0d385fdf8c306  xsa226-4.6/0001-gnttab-dont-use-possibly-unbounded-tail-calls.patch
e34dbba7b94942faeb3e6b7630ba06f01998e2b56be1035d76e67aa47e77457d  xsa226-4.6/0002-gnttab-fix-transitive-grant-handling.patch
fffcc0a4428723e6aea391ff4f1d27326b5a3763d2308cbde64e6a786502c702  xsa226-4.7.patch
3878c27b77ba24012599289e0e0fb1e5198b1e4efe2f87f7c46def5f335f2fd5  xsa226-4.9/0001-gnttab-dont-use-possibly-unbounded-tail-calls.patch
01d773c5bb4cafe54daf0d14e8a3af899a7c5863513d18927c4a570a74afdb15  xsa226-4.9/0002-gnttab-fix-transitive-grant-handling.patch
$

(The .meta file is a prototype machine-readable file for describing
which patches are to be applied how.)

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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