Xen Security Advisory 69 (CVE-2013-4370) - misplaced free in ocaml xc_vcpu_getaffinity stub

[Xen.org security team <security@xen.org>]

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             Xen Security Advisory CVE-2013-4370 / XSA-69
                               version 2

           misplaced free in ocaml xc_vcpu_getaffinity stub

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

Public release.

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

The ocaml binding for the xc_vcpu_getaffinity function incorrectly
frees a pointer before using it and subsequently freeing it again
afterwards. The code therefore contains a use-after-free and
double-free flaws.

IMPACT
======

An attacker may be able to cause a multithreaded toolstack written in
ocaml and using this function to race against itself leading to heap
corruption and a potential DoS.

Depending on the malloc implementation code execution cannot be ruled
out.

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

The flaw is present in Xen 4.2 onwards.

Systems using an ocaml based toolstack (e.g. xapi) are vulnerable.

MITIGATION
==========

Not calling the vcpu_getaffinity function will avoid this issue.

Not allowing untrusted users access to toolstack functionality will
avoid this issue.

CREDITS
=======

This issue was discovered by Coverity Scan and Matthew Daley.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa69.patch             Xen 4.3.x, Xen 4.2.x, xen-unstable


$ sha256sum xsa69*.patch
d3beb662aacf628b6a25ff6cfcd9526ab689aa43a56cf25e792a001f89b4edbc  xsa69.patch
$
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From 067c122873c67bd1d9620f8340f9c9c209135388 Mon Sep 17 00:00:00 2001
From: Matthew Daley <mattjd@gmail.com>
Date: Tue, 10 Sep 2013 23:12:45 +1200
Subject: [PATCH] tools/ocaml: fix erroneous free of cpumap in
 stub_xc_vcpu_getaffinity

Not sure how it got there...

Coverity-ID: 1056196

This is CVE-2013-4370 / XSA-69

Signed-off-by: Matthew Daley <mattjd@gmail.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
---
 tools/ocaml/libs/xc/xenctrl_stubs.c |    2 --
 1 file changed, 2 deletions(-)

diff --git a/tools/ocaml/libs/xc/xenctrl_stubs.c b/tools/ocaml/libs/xc/xenctrl_stubs.c
index df756ad..f5cf0ed 100644
--- a/tools/ocaml/libs/xc/xenctrl_stubs.c
+++ b/tools/ocaml/libs/xc/xenctrl_stubs.c
@@ -461,8 +461,6 @@ CAMLprim value stub_xc_vcpu_getaffinity(value xch, value domid,
 
 	retval = xc_vcpu_getaffinity(_H(xch), _D(domid),
 	                             Int_val(vcpu), c_cpumap);
-	free(c_cpumap);
-
 	if (retval < 0) {
 		free(c_cpumap);
 		failwith_xc(_H(xch));
-- 
1.7.10.4


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