Re: [PATCH v4 1/1] fs: Allow no_new_privs tasks to call chroot(2)

On Tue, Mar 16, 2021 at 8:26 PM Mickaël Salaün [off-list ref] wrote:

On 16/03/2021 20:04, Jann Horn wrote:

quoted

On Tue, Mar 16, 2021 at 6:02 PM Mickaël Salaün [off-list ref] wrote:

quoted

One could argue that chroot(2) is useless without a properly populated
root hierarchy (i.e. without /dev and /proc).  However, there are
multiple use cases that don't require the chrooting process to create
file hierarchies with special files nor mount points, e.g.:
* A process sandboxing itself, once all its libraries are loaded, may
  not need files other than regular files, or even no file at all.
* Some pre-populated root hierarchies could be used to chroot into,
  provided for instance by development environments or tailored
  distributions.
* Processes executed in a chroot may not require access to these special
  files (e.g. with minimal runtimes, or by emulating some special files
  with a LD_PRELOADed library or seccomp).

Unprivileged chroot is especially interesting for userspace developers
wishing to harden their applications.  For instance, chroot(2) and Yama
enable to build a capability-based security (i.e. remove filesystem
ambient accesses) by calling chroot/chdir with an empty directory and
accessing data through dedicated file descriptors obtained with
openat2(2) and RESOLVE_BENEATH/RESOLVE_IN_ROOT/RESOLVE_NO_MAGICLINKS.

I don't entirely understand. Are you writing this with the assumption
that a future change will make it possible to set these RESOLVE flags
process-wide, or something like that?

No, this scenario is for applications willing to sandbox themselves and
only use the FDs to access legitimate data.

But if you're chrooted to /proc/self/fdinfo and have an fd to some
directory - let's say /home/user/Downloads - there is nothing that
ensures that you only use that fd with RESOLVE_BENEATH, right? If the
application is compromised, it can do something like openat(fd,
"../.bashrc", O_RDWR), right? Or am I missing something?

quoted

As long as that doesn't exist, I think that to make this safe, you'd
have to do something like the following - let a child process set up a
new mount namespace for you, and then chroot() into that namespace's
root:

struct shared_data {
  int root_fd;
};
int helper_fn(void *args) {
  struct shared_data *shared = args;
  mount("none", "/tmp", "tmpfs", MS_NOSUID|MS_NODEV, "");
  mkdir("/tmp/old_root", 0700);
  pivot_root("/tmp", "/tmp/old_root");
  umount("/tmp/old_root", "");
  shared->root_fd = open("/", O_PATH);
}
void setup_chroot() {
  struct shared_data shared = {};
  prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
  clone(helper_fn, my_stack,
CLONE_VFORK|CLONE_VM|CLONE_FILES|CLONE_NEWUSER|CLONE_NEWNS|SIGCHLD,
NULL);
  fchdir(shared.root_fd);
  chroot(".");
}

What about this?
chdir("/proc/self/fdinfo");
chroot(".");
close(all unnecessary FDs);

That breaks down if you can e.g. get a unix domain socket connected to
a process in a different chroot, right? Isn't that a bit too fragile?