EGI CSIRT:Alerts/kernel-2013-05-14

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EGI CSIRT ADVISORY [EGI-ADV-20130514]

Title:       Linux kernel perf_event vulnerability (CVE-2013-2094) [EGI-ADV-20130514]
Date:        2013-05-14
Updated:     2013-05-15

URL:         https://wiki.egi.eu/wiki/EGI_CSIRT:Alerts/kernel-2013-05-14


Update Summary
==============

 + 2013-05-14: Initial revision.
 + 2013-05-15: Made mitigation drawbacks more explicit.


Introduction
============

A recently-discovered vulnerability in the Linux kernel allows a local user
to escalate their privilege level and gain root access.  Working exploit code
is publicly available.


Details
=======

The performance measurement subsystem in the Linux kernel incorrectly casts a
64-bit integer into a 32-bit integer which is subsequently used for array
dereferencing.  Providing carefully chosen integers as input allows arbitrary
code to be executed.

The erroneous code has been introduced in kernel version 2.6.37 (commit
b0a873ebbf87bf38bf70b5e39a7cadc96099fa13 on 2010-09-09) and is fixed in kernel
version 3.8.9 (commit 8176cced706b5e5d15887584150764894e94e02f on 2013-04-15).
Additionally, the vulnerability was backported to 2.6.32 kernels by Red Hat.

Working exploit code is publicly available.  This code will not work on all
vulnerable distributions; however, it appears to work on RHEL 6 and derived
systems.


Risk Category
=============

This issue has been assessed as CRITICAL risk by the EGI CSIRT as a working
exploit is publicly available.


Affected Software
=================

 + Linux kernels 2.6.36-3.8.8 through 3.8.9.
 + Linux kernels 2.6.32 with Red Hat backports.


Mitigation
==========

To mitigate the issue, install the systemtap package and create a
file /root/mitigation.stp containing the following (without the
BEGIN/END marker lines):
---BEGIN FILE---
%{
#include <linux/perf_event.h>
%}

function sanitize_config:long (event:long) %{
        struct perf_event *event;
        event = (struct perf_event *) (long) STAP_ARG_event;
        event->attr.config &= INT_MAX;
%}

probe kernel.function("perf_swevent_init@kernel/events/core.c").call {
        sanitize_config($event);
}
---END FILE---

Then, run the command
  stap -g /root/mitigation.stp

Note that this needs to be re-run after every reboot.

A much easier mitigation that will only(!) prevent the published exploit
code from working correctly can be performed by disabling user-level
kernel profiling:
  sysctl kernel.perf_event_paranoid=2

This is also not persistent across reboots, so it is necessary either to
re-run the command after each boot or to add the line
  kernel.perf_event_paranoid=2
to /etc/sysctl.conf.

Both mitigations are discussed in more detail at
  https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2013-2094

Component Installation information
==================================

For many distributions, patched kernel packages are available.  Refer to your
distro's information channels.


Recommendations
===============

It is recommended that sites implement the mitigation described above unless
kernel profiling is essential and upgrade their kernels as soon as possible
as they become available for their respective distributions.


References
==========

 + Mitre: http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2013-2094
 + NIST NVD: http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2013-2094
 + OSS-Sec: http://marc.info/?s=CVE-2013-2094&l=oss-security
 + Debian: https://security-tracker.debian.org/tracker/CVE-2013-2094
 + Red Hat: https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2013-2094
 + Ubuntu: http://people.canonical.com/~ubuntu-security/cve/CVE-2013-2094