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

[Jann Horn]

On Tue, Mar 16, 2021 at 8:26 PM Mickaël Salaün <mic at digikod.net> wrote:
> On 16/03/2021 20:04, Jann Horn wrote:
> > On Tue, Mar 16, 2021 at 6:02 PM Mickaël Salaün <mic at digikod.net> wrote:
> >> 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?

> > 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?