Date: Fri, 17 Jul 2009 12:06:56 +0200
From: Akita Software Security <lists@...tasecurity.nl>
To: full-disclosure@...ts.grok.org.uk
Subject: PulseAudio local race condition privilege
	escalation vulnerability

------------------------------------------------------------------------
PulseAudio local race condition privilege escalation vulnerability
------------------------------------------------------------------------
Yorick Koster, June 2009

------------------------------------------------------------------------
Abstract
------------------------------------------------------------------------

The PulseAudio binary is affected by a local race condition. If the 
binary is installed as SUID root, it is possible to exploit this 
vulnerability to gain root privileges. This attack requires that a local
attacker can create hard links on the same hard disk partition on which
PulseAudio is installed (i.e. /usr/bin and /tmp reside on the same 
partition).

------------------------------------------------------------------------
See also
------------------------------------------------------------------------

- CVE-2009-1894 [2]
- GLSA 200907-13 [3] PulseAudio: Local privilege escalation
- USN-804-1 [4] PulseAudio vulnerability

------------------------------------------------------------------------
Tested version
------------------------------------------------------------------------

This issue was successfully verified on the following Linux 
distributions:

- Ubuntu 9.04 running PulseAudio version 0.9.14
- Debian 5.0 running PulseAudio version 0.9.10
- Mandriva Linux 2009 Spring running PulseAudio version 0.9.15

------------------------------------------------------------------------
Fix
------------------------------------------------------------------------

A patch for PulseAudio was released that addresses this issue. This 
patch can be obtained from the following location:

http://git.0pointer.de/?p=pulseaudio.git;a=commit;h=84200b423ebfa7e2dad9b1b65f64eac7bf3d2114

As a temporary workaround, remove the SUID bit from the PulseAudio 
binary.

$ chmod u-s `which pulseaudio`

------------------------------------------------------------------------
Introduction
------------------------------------------------------------------------

PulseAudio [5] is a sound server for POSIX and Win32 systems. A sound 
server is basically a proxy for your sound applications. It allows you 
to do advanced operations on your sound data as it passes between your 
application and your hardware.

On some systems, the PulseAudio binary is installed SUID root to enable 
real-time scheduling. If set, the daemon will drop root privileges 
immediately on startup, however it will retain the CAP_NICE capability 
(on systems that support it), but only if the calling user is a member 
of the pulse-rt group. For all other users all capabilities are dropped 
immediately.

------------------------------------------------------------------------
Race condition
------------------------------------------------------------------------

If the PulseAudio binary is started on Linux systems, it checks if the 
LD_BIND_NOW environment variable is set. If this is not the case, 
PulseAudio will set the variable and it will reload itself. It tries to 
determine its path name by looking at the /proc/self/exe symbolic link. 
This symbolic link will point to the full path name of the current 
process.

int main(int argc, char *argv[]) {
[...]
#if defined(__linux__) && defined(__OPTIMIZE__)
	/*
		Disable lazy relocations to make usage of external libraries
		more deterministic for our RT threads. We abuse __OPTIMIZE__ as
		a check whether we are a debug build or not.
	*/
	
	if (!getenv("LD_BIND_NOW")) {
		char *rp;
	
		/* We have to execute ourselves, because the libc caches the
		* value of $LD_BIND_NOW on initialization. */
	
		pa_set_env("LD_BIND_NOW", "1");
		pa_assert_se(rp = pa_readlink("/proc/self/exe"));
		pa_assert_se(execv(rp, argv) == 0);
	}
#endif

Normally, /proc/self/exe will point to something like 
/usr/bin/pulseaudio. However by using hard links, it is possible to 
cause /proc/self/exe to point to a different location.

$ cd /tmp
$ ls -la /proc/self/exe
lrwxrwxrwx 1 yorick yorick 0 2009-06-09 16:31 /proc/self/exe -> 
/bin/ls
$ ln `which ls` ls
$ ./ls -la /proc/self/exe
lrwxrwxrwx 1 yorick yorick 0 2009-06-09 16:31 /proc/self/exe -> 
/tmp/ls

In addition, if a hard link is created, the SUID bit is preserved.

$ ln `which pulseaudio` pulseaudio
$ ls -la pulseaudio 
-rwsr-xr-x 2 root root 71616 2009-04-09 02:12 pulseaudio

A race condition exists in the reload mechanism of PulseAudio. An 
attacker can exploit this issue by creating a hard link pointing to the 
PulseAudio binary. After this it can execute this binary through the 
hard link. At this moment /proc/sef/exe will point to the hard link. 
Before PulseAudio is restarted, the attacker can replace the hard link 
with a different (executable) file or (symbolic) link. If PulseAudio is 
restarted, it will use a path name that at this moment points to a 
different file, for example a command shell. Root privileges are not 
dropped when PulseAudio is reloading, thus allowing a local attacker to 
gain root privileges.

Please note, this attack is only possible if the attacker can create 
hard links on the same hard disk partition on which PulseAudio is 
installed (i.e. /usr/bin and /tmp reside on the same partition).

------------------------------------------------------------------------
Proof of concept
------------------------------------------------------------------------

The following proof of concept can be used to exploit this issue. The 
proof of concept tries to exploit this issue by creating hard links in 
the /tmp directory.

pa_race [6]

$ ./pa_race
I: caps.c: Limited capabilities successfully to CAP_SYS_NICE.
I: caps.c: Dropping root privileges.
I: caps.c: Limited capabilities successfully to CAP_SYS_NICE.
N: main.c: Called SUID root and real-time and/or high-priority 
scheduling was requested in the configuration. However, we lack the 
necessary privileges:
N: main.c: We are not in group 'pulse-rt', PolicyKit refuse to
 grant us the requested privileges and we have no increase 
RLIMIT_NICE/RLIMIT_RTPRIO resource limits.
N: main.c: For enabling real-time/high-priority scheduling please 
acquire the appropriate PolicyKit privileges, or become a member of 
'pulse-rt', or increase the RLIMIT_NICE/RLIMIT_RTPRIO resource
 limits for this user.
E: pid.c: Daemon already running.
E: main.c: pa_pid_file_create() failed.
[...]
uid=0(root) gid=0(root) groups=4(adm), 20(dialout), 24(cdrom), 
25(floppy), 29(audio), 30(dip), 44(video), 46(plugdev), 107(fuse), 
109(lpadmin), 115(admin), 1000(yorick)
# 

------------------------------------------------------------------------
References
------------------------------------------------------------------------

[1] http://www.akitasecurity.nl/advisory.php?id=AK20090602
[2] http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2009-1894
[3] http://www.gentoo.org/security/en/glsa/glsa-200907-13.xml
[4] http://www.ubuntu.com/usn/usn-804-1
[5] http://pulseaudio.org/
[6] http://www.akitasecurity.nl/advisory/AK20090602/pa_race

------------------------------------------------------------------------
-- 
------------------------------------------------------------------------
Akita Software Security (Kvk 37144957)
http://www.akitasecurity.nl/
Telephone: +31 6 15 325 647
------------------------------------------------------------------------
Key fingerprint = 5FC0 F50C 8B3A 4A61 7A1F  2BFF 5482 D26E D890 5A65
http://keyserver.pgp.com/vkd/DownloadKey.event?keyid=0x5482D26ED8905A65

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