[kernel-xen] Xen Security Advisory 84 - integer overflow in several XSM/Flask hypercalls

[Steven Haigh]

                     Xen Security Advisory XSA-84
                              version 2

           integer overflow in several XSM/Flask hypercalls

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

Public release.

The patch for 4.1 was extended to cover a few further similar issues.

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

The FLASK_{GET,SET}BOOL, FLASK_USER and FLASK_CONTEXT_TO_SID
suboperations of the flask hypercall are vulnerable to an integer
overflow on the input size. The hypercalls attempt to allocate a
buffer which is 1 larger than this size and is therefore vulnerable to
integer overflow and an attempt to allocate then access a zero byte
buffer.

Xen 3.3 through 4.1, while not affected by the above overflow, have a
different overflow issue on FLASK_{GET,SET}BOOL and expose unreasonably
large memory allocation to aribitrary guests.

Xen 3.2 (and presumably earlier) exhibit both problems, with the
overflow issue being present for more than just the suboperations
listed above.

The FLASK_GETBOOL op is available to all domains.

The FLASK_SETBOOL op is only available to domains which are granted
access via the Flask policy.  However the permissions check is
performed only after running the vulnerable code and the vulnerability
via this subop is exposed to all domains.

The FLASK_USER and FLASK_CONTEXT_TO_SID ops are only available to
domains which are granted access via the Flask policy.

IMPACT
======

Attempting to access the result of a zero byte allocation results in
a processor fault leading to a denial of service.

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

All Xen versions back to at least 3.2 are vulnerable to this issue when
built with XSM/Flask support. XSM support is disabled by default and is
enabled by building with XSM_ENABLE=y.

We have not checked earlier versions of Xen, but it is likely that
they are vulnerable to this or related vulnerabilities.

All Xen versions built with XSM_ENABLE=y are vulnerable.

MITIGATION
==========

There is no useful mitigation available in installations where XSM
support is actually in use.

In other systems, compiling it out (with XSM_ENABLE=n) will avoid the
vulnerability.

CREDITS
=======

This issue was discovered by Matthew Daley.

RESOLUTION
==========

Fixed in xen-4.2.3-13