RE: CLONE_NEWNS and bind mounts to make "chroot" jail

[Bodo Eggert <7eggert@gmx.de>]

Leibowitz, Michael <michael.leibowitz@intel.com> wrote:

> If I understand correctly, the following should accomplish what I'm
> looking for.  However, pivot_root gives me EBUSY.  I played around with
> moving the mount --bind /jail /jail to before the unshared, as well as
> making old_root a bind mount to itself.  However, pivot_root always
> seems to fail.  Is there something obvious that I'm doing wrong?  The
> following is my test code (error checking has been removed for clarity,
> except for pivot_root).
> 
>   char *newargv[]= { "sh", NULL };
> 
>   chdir("/jail");
>   unshare(CLONE_NEWNS));
>   mount("/jail", "/jail", NULL, MS_BIND, NULL));
>   mount("/bin", "bin", NULL, MS_BIND, NULL));
>   mount("/usr", "usr", NULL, MS_BIND, NULL));
>   mount("/lib", "lib", NULL, MS_BIND, NULL));
>   if (pivot_root(".", "old_root")) perror("pivot_root . old_root");
>   exec("./bash-static"); /* copied to /jail prior to running */

This works for me:

#include <sys/mount.h>
#include <unistd.h>

#define _GNU_SOURCE
#include <sched.h>

#define MNT_DETACH      2       /* Detach from tree only */

int main()
{
  unshare(CLONE_NEWNS);
  mount("jail", "jail", NULL, MS_BIND, NULL);
  mount("/bin", "jail/bin", NULL, MS_BIND, NULL);
  mount("/usr", "jail/usr", NULL, MS_BIND, NULL);
  mount("/lib", "jail/lib", NULL, MS_BIND, NULL);
  /* abuse bin as the temporary old root directory */
  if (pivot_root("jail", "jail/bin"))
    perror("pivot_root");
  chdir("/");
  umount2("bin", MNT_DETACH);
  execl("./sash", NULL); /* copied to /jail prior to running */
}